Friday, February 12, 2010

February 13th and 14th, 2010 Forecast

Chilly, breezy conditions will dominate the weekend as a storm system passes harmlessly (except if you're in the Carolinas, Georgia, Florida, on back through Texas where snow has fallen the past couple of days in the Deep South) to our south out to sea. The combination of that and a strengthening trough in Eastern Canada will throw additional cloud cover over the region on Saturday but no precipitation is expected until Monday when a clipper will move through with a minor to moderate snow event.

Tonight: Variably cloudy skies overall should be expected, with low temperatures around 20 in Center City, teens in the burbs. Winds from the northwest will be 7-12 mph.

Saturday & Saturday Night: Winds will increase a fair bit during the day, becoming brisk with northwest breezes at 12-22 mph with occasional higher gusts to 30. Skies will be mostly cloudy on the whole and it'll be a colder day as there will be less sunshine. Expect high temperatures around freezing. Saturday Night features variably cloudy skies, breezy conditions, with lows generally held up in the lower 20's because of those northwest breezes (10-18 mph).

Sunday: Keeping up with the west to northwest winds at 12-22 mph, we'll see more sunshine than we did on Saturday and temperatures should respond a bit...nudging higher into the middle 30's in most places as we get some additional melting out there of the glacier...I mean snow pack.



Clipper Talk: We'll discuss the clipper event for Monday this weekend in a couple of posts and provide a snowfall forecast on Sunday morning and again Monday morning for the event. Midday guidance has ramped up the potential with this clipper...and it looks like it could bring a decent snowfall to the region. Probably not two feet of "decent snowfall" but it does have some potential for as much as 6" (possibly more) in some locations...


Thursday, February 11, 2010

So...When Did The Record Actually Break?

This post is not a rip on the National Weather Service -- they do an awesome job and have a very good handle on forecasting overall for this region, given the unique climate which ranges from coastal Delaware up to the Poconos. This last storm was one that the quality control department would rather forget about as there have been a few data issues and some confusion over how much snow fell in Philadelphia and how much snow we're had this winter.

One, the matter of when Philly's snowfall record was broken. I had tweeted that the record fell at 1 PM yesterday and that 9.7" had fallen but a snowfall accumulation report from the NWS said "not so fast" to that 9.7" and stated that 8.8" was the 1 PM snowfall.

Well, it turns out that neither 8.8" or 9.7" was the 1 PM snowfall yesterday...but that it was 10.0."

Snowfall measurements from official spotters, including the Airport/National Park observer, are reported to the NWS every six hours. It seems somewhere along the way the totals got messed up and we ran into some data control issues.

Time 6 hr snow Real Total NWS Statement
1 AM Wed 7.0" 7.0" 7.0"
7 AM Wed 0.6" 7.6" 7.6"
1 PM Wed 2.4" 10.0" 8.8"
7 PM Wed 4.0" 14.0" 14.0"
1 AM Thu 1.8" 15.8" 15.8"

Between 7 AM and 1 PM, 2.4" of snow really fell at Philadelphia International...which bumped the total to 10.0" for the storm. However, only 1.2" was added to the unofficial official total and was reported out to the media at 8.8." A snowfall report that I had looked at during the 1 PM hour initially showed 2.1" falling in that six hour window, bumping the total to 9.7" which I ran with until the snowfall report came out to contradict me...and the National Weather Service themselves. They did correct the total at 7 PM to 14.0" as four inches of snow fell in the 1 PM - 7 PM timeframe. Problem corrected, right?

Data control doesn't end with how much snow fell, but what is being reported on climate reports. While the official total for the storm is 15.8" some of the media are running with a total of 71.6" of snow for the winter. That's not quite correct either.


This is a climate summary report for the month so far. This summary shows the daily high, low, precipitation, and snowfall. If you add up the snowfall for February 9th (6.0") and 10th (9.3") you'll see a total of 15.3" -- which is a half inch lower than what really fell (15.8"). So, if we add this half inch to the snowfall that fell in December (24.1") and January (3.1") along with the other 29.1" that fell before this storm then we get a total of 72.1" -- not 71.6."

With the second major winter storm falling in the region in a five day span these kind of mistakes happen...and I would not be surprised to see this corrected in the next day or two to reflect accurate data. From a stat geek's realm having the details correct (even if it is delayed and not denied) is never a bad thing. So, if you hear a revised snowfall total in the next couple of days for the winter you'll know why...when winter storms are going on and people are working crazy shifts, a shift in the numbers can happen and change things.

To err is human...yours truly can attest to that by running with a 9.7" total for 15 minutes!

So, with all of that aside...a huzzah to having a winter with six feet of total snow!

Clipping Through Monday Night...

After a few days of cold and sun our next weather maker is on the way for Monday night, perhaps into Tuesday morning in the form of a clipper. Clippers are weaker systems, historically, and generally produce lighter accumulations of snows compared to the massive dumpings of snow we have seen in recent days.

This clipper will have some decent upper level energy accompanying it as it dives southeast through the Ohio Valley and cross the Mid Atlantic. While track is still uncertain computer guidance tends to suggest a track down across Virginia and near Washington, DC at this point. The GFS is a bit more to the north than the EURO but at this point both look like the heavier snows would fall across the Delmarva (potentially several inches). The southern track may prevent the "heaviest" snows from reaching the city but a few inches of snow could slip on through. Of course, a nudge northward in the track could result in a bit more snow in our backyards.

The clipper will pull away on Tuesday, intensifying in the North Atlantic and helping reinforce the windy and chilly regime that we're dealing with in the Mid Atlantic into much of next week. The good news is that while temperatures will be cool we'll be able to maintain a slow melt each day, helping to prevent a significant rise in streams and creeks as the snowpack slowly compacts.



February 12th, 2010 Forecast

Now: It's been a breezy and chilly February day, with temperatures generally in the middle 30's this afternoon around Philadelphia. At least the snow has stopped and that's allowing us to dig out from a foot plus of snow in our backyards. A few days of benign but breezy weather are upon us, which should allow for some melting of the dense snowpack in place.

Tonight: Breezy and becoming clear overnight. With the winds generally staying up through the night temperatures will probably not drop like a rock overnight. Expect lows in the teens outside of Philly to around 20 in the city with winds around 10 mph. Factoring in those winds and it will feel like it's about 10 degrees. Watch out for refreeze on your local driveways and side streets as today's melt freezes up after sunset.

Friday: Sunshine and breezy conditions continue. We should see high temperatures in the 31-35 range across the area, allowing for a continued slow melt of the snow. Northwest and west winds will continue to be gusty at 10-20 mph, keeping wind chills in the 20's.



Weekend Sneak Peek: Saturday may feature some clouds from near Philly and south as a storm system passes off of the Georgia and South Carolina coastline and will work east-northeast. While it will avoid throwing precipitation back into the region, it may throw some high clouds nearby. The story this weekend will be chilly weather and breezy conditions each day, with high temperatures around or just above freezing both Saturday and Sunday.



The East Coast Three Step




15.8" of snow fell in Philadelphia over about 24 hours between Tuesday and Wednesday, the second major snowstorm in less than a week in the Delaware Valley. Going back to 1884, there have been just three instances where there have been two snowfalls of at least six inches within a week of each other --1899, 1918, and 1941. None of the prior three instances brought 44.3" of snow, however, to the region or were snowfalls of ten inches or more. Philadelphia's second major storm in a week also brought us a record for snowfall in a winter with over 72" of snow, surpassing the old record 65.5" from 1995-1996. Crazy to note that three storms this winter have brought Philadelphia more snow than what fell in the 95-96 winter (67.5" from the Big 3 storms compared to 65.5").


The storm, the third of the winter, had three distinct phases to it as well. The first moved in between 5 and 8 PM in Philadelphia and was accompanied by light snow initially.

However, as the evening progressed we saw snow explode in coverage across Virginia and Maryland, lifting northeast into the Delaware Valley and South Jersey over the evening hours. This snow burst was heavier than I expected it to be and dropped five to eight inches of snow in a short duration of time across the region. It was accompanied by warm air creeping northward up the coast and as the snow continued to lift northward, so did the warm air.

By early in the morning hours, a mix of sleet, freezing rain, and rain lifted up the coast as temperatures aloft warmed above freezing. This warm layer made it into the Philadelphia region, even up into the northern and western burbs for a time during the early morning hours, as precipitation waned. This slackening in precipitation allowed for the warm layer to shoot northward. You may remember me talking up this potential and how the NAM's depiction of that warm layer was going to make things less snowy during the lull...however, I also noted that it would not have much of an impact on snowfall totals as the lighter precipitation would not have added much in the way of snow to your backyard (maybe an inch or two).

On top of the lull, some dryslotting developed across parts of South Jersey up into Philadelphia and Lower Bucks County during the morning hours. This was accompanied by the transfer of energy from the Manitoba low towards the southern energy that was deepening off of the Delmarva coastline. Anytime these energy transfers take place dryslot issues can develop -- and it did for South Jersey (east of the AC Expressway) into Bucks and the city. For folks in Delaware and the Philadelphia suburbs north and west the bands of precipitation that were spitting sleet and freezing rain transitioned back to snow as colder air wrapped back into the storm aloft and as the bands of snow increased in intensity.

By late morning a convergence zone developed across South Jersey into the Philadelphia suburbs, resulting in not only heavier snowfall but also thundersnow in parts of the Main Line, Western Suburbs, and Northwest Philadelphia. A convergence zone is when two flows of wind come together and converge, resulting in lift in the atmosphere. In this situation, northeast winds from the developing coastal bomb were meeting up with northwest winds in the broad circulation around the coastal. This lift in the atmosphere resulted in additional instability and some thundersnow that nudged slowly eastward into Lower Montgomery County over the 12:30-1 PM timeframe. This convergence zone introduced the third step of the snowstorm across the region, bringing heavy at times snow to the region. In some places, snowfall rates exceeded two inches per hour in the heaviest banding of snow between 3 and 5 PM in the northern and western suburbs.

The winds itself were only a factor to the south of the city in Delaware and South Jersey, with gusts to over 40 mph in Dover, Wilmington, and along the Shore Points. Across Pennsylvania wind gusts were lower around Philadelphia (topping out at 35 mph) and especially to the north of the city where wind gusts did not get much higher than 25 or 30 as a weak trough lingered across the region...this not only enhanced the snowfall with the afternoon wrap around but also prevented the higher winds from fully developing across the region. Power outages, however, were a major issue across the region as heavy snow weighed down trees, causing them to snap and knock out power to over 100,000 homes in PECO's service area. Unlike the first two snowstorms that hit the area, this storm's snowfall content was much heavier and moisture-laden with lower snowfall ratios for much of the area.

This third storm arguably impacted more people in an adverse way with the power outages and road closures around the region compared to the other two storms. While the second storm adversely impacted the Shore with high winds and power outages and the first storm impacted South Jersey and Delaware indirectly with flooding issues a few days later from snow melt, this third storm's combination of heavy snow and wind has made this arguably as notorious, if not more nasty, than the others despite the lower snowfall totals in Philadelphia.

More: Radar Loop | Snowstorm Photography

Reviewing The Blizzard of 2010 Forecasts

The Delaware Valley is beginning its dig out from the Blizzard of 2010, which dumped enough snow for it to feel like a blizzard although the winds were not quite blizzard criteria for many. This was not the easiest snowstorm to pin down from a couple of standpoints and that was reflected in not only the amount of snow that ended up falling, but also the timing element with the different phases of the storm.

I posted my map from Tuesday morning on the immediate left, which didn't do so hot in terms of nailing snowfall amounts in parts of the region but I think accurately depicting the areas that were going to get among the highest snowfall totals. Computer modeling had been relatively accurate on depicting the heaviest snowfall to the west of the city for a couple of days before the storm...and the highest snowfall totals in the region came in west of Philadelphia over Chester County. I vastly underdid snowfall totals across Berks and north towards Pottsville and Central Pennsylvania, but in general the areas that I felt would get the highest snow did just that.

Philadelphia's final snowfall total of 15.8" was slightly above my forecast from Tuesday morning (8-14", which I revised to 9-15" on Tuesday night). I did revise my map on Wednesday to reflect a heavier-than-expected first wave of precipitation that came through and deposited five to eight inches of snow across Philadelphia and immediate vicinity, which put my forecast into relatively serious jeopardy. While the dryslot and lull lingered a bit longer around Philadelphia and points east, the afternoon and evening thumping of snow yesterday did help bring the final snowfall totals in line with my nowcast for the immediate area. Again, I went way too low across the tier of counties to the north of Berks, the Lehigh Valley, and Harrisburg, where many areas received double digit to twenty inch snowfall. However, for the most part the 14-22" range verified or came close to it. I was a little low on snow down across Sussex County in Delaware as they changed over to snow earlier than I anticipated. On my forecasts, I would give the Tuesday morning map (the one up top) a C+ for getting the heaviest snowfall area nailed but coming up too low on totals to the northwest and in the immediate city, with the second map getting a B. You may think otherwise but I felt I did ok with the storm (at least in terms of nailing who was going to get nailed in our immediate areas) and adjusted the forecasts upward on Wednesday morning to reflect the snow burst that I did not expect.

Regarding the TV forecasts for the storm, I think the 18-24" forecast that were being thrown out there for the snowstorm by many forecasters were high but they verified -- just not in the right spots as the city of Philadelphia came up short (15.8" at the Airport, 17" in Northeast Philadelphia). Based on Wednesday morning's forecasts, NBC 10's 16-22" call and FOX 29's 12-24" call were arguably the best if one were forecasting specifically for the Airport. The 18-24 inch forecasts were not a bad upper range to forecast -- arguably, most forecast those totals a nudge too far northeast into New Jersey as snow totals were no higher than 18" and those were confined to Ocean and Mercer Counties over there.

Wednesday, February 10, 2010

February 11th, 2010 Forecast

Now that the Blizzard of 2010 (part two, or three) is in its final throws we're going to be stuck with this snow for a while. Temperatures will be cold for the next several days compared to seasonal averages but for the next few days -- at least through the weekend -- it looks like it will be dry.

Tonight: Windy through the night with snow winding down. North and northwest winds at 15-30 mph will continue through the night causing blowing and drifting snow through the region. Temperatures will bottom out in the mid 20's overall.

Thursday: Sunshine will peek through a variably cloudy sky and winds will continue to blow at 15-25 mph for much of the day. Temperatures will top out around or just above freezing so some melting will take place although it won't be much given the higher moisture content of this snowstorm compared to our past one.


Coming tomorrow: A full recap/postmortem on the snowstorm.

Snowstorm Gallery, February 10th, 2010 Storm



Send in your snowstorm pictures to photos (at) phillyweather.net and we'll post them online.

Snowstorm Primer



Tracking The Storm:
Current Wx Page | Twitter Feed | Blog Updates |
Snowfall Totals | Chat During Storm | Snowtography

Snowfall Forecasts (for Philadelphia) as of Wednesday 6 AM:

CBS 3 -- 18-24"
6ABC -- 18-24"
NBC 10 -- 16-22"
FOX 29 -- 12-24"
NWS -- 18-24"
Phillyweather.net 14-22"



You can send your snowtography to photos (at) phillyweather.net as the storm comes in.


Tuesday, February 09, 2010

Midweek Snowstorm Blog Update

Update, 7 PM: 100,000 are now without power, according to Fox 29. Snow continues at a moderate clip across South Jersey, with light to moderate snow continuing in Philadelphia and points north and west. We should see things wind down after 9 PM in Philadelphia, towards midnight in South Jersey. It may snow lightly past 9 in Philadelphia and the burbs but precipitation should begin to slowly wind down across the immediate city.

Here's a really good satellite loop of the storm over the past few hours, taking its sweet time working east slowly (like a turtle almost).

Update, 6 PM: Wind and snow continues through the region, with about 30,000 without power according to PECO at this time. Wind gusts over 35 mph are occurring in Delaware and South Jersey and will continue for the next several hours. Snow may begin to diminish slowly over the next 2-3 hours across the region but the winds will continue to remain gusty through the night. We're up to 17" here in Royersford and I'll assume Philadelphia will be around 14-16" at 7 PM when the snowfall update is provided. You can continue to get updated (when the NWS can) snowfall totals at this link.

Update, 5 PM: Heavy snow is continuing to work southeast into and through the Philadelphia area. Heavy snow is also breaking out across Central and South Jersey as well. We'll see this continue over the next couple of hours and then slowly begin to wind down from west to east across the region. Snow in the heaviest bands will fall at almost a two inch per hour rate -- I'm at 14.2" as of 4 PM with snow still falling out there.

Winds are picking up at the Airport, along the river into South Jersey, and across Delaware with gusts to over 35 mph. We'll see winds continue to pick up over the next few hours across the city and northern burbs as the storm pulls way and this weak trough responsible for the heavy snow pulls out.


Update, 4 PM: Radar is going to town across the north and western suburbs as a heavy band of snow has fired up and rotated around from Central Jersey. This band will work down through Pottstown and into Chester County over the next couple of hours and then slowly wind its way back into South Jersey.

Across South Jersey, a dryslot is hindering snowfall across your area but the bands to the north will gradually wind back around and through South Jersey. They may not be as heavy as they are now across Bucks and Upper Montgomery Counties but you will see the snow pick up again around or just after 5 PM. Snowfall in these areas is accumulating at an inch or two inches per hour, with some isolated areas getting more than that.


Update, 2:30 PM: Snow is continuing and it is heavy at times in spots. The winds are also gusting and in spots are gusting to over 30 mph. In the last hour, wind gusts are to 32 in Millville and at Philadelphia International, higher than that to the south of Philadelphia. The stronger wind energy with the storm system is working to the south of Philadelphia across Delaware and South Jersey. Around the northern and western burbs, winds will be gusty at times but the blizzard criteria wind speeds may not be reached for a while. We'll see things crank up further as the afternoon progresses -- it is taking a bit longer for winds to pick up around here but as the storm continues to intensify and the remainder of its energy coalesces offshore we'll see the wind energy crank into the region.

1 PM Update: So, yeah, this thundesnow thing is moving through due to a convergence zone that has developed (wind shift line, boundary line) across the city and Main Line. This boundary si slowly working eastward and is allowing for reports of thundersnow across Philadelphia and now into Lower Montgomery County. As it works east, I would not be surprised to see it work into Lower Bucks, Northeast Philly, and then towards Trenton in the next 1-2 hours.

Philadelphia is still getting blitzed with snow but this convergence zone is helping to reduce some of the snow thumping that was taking place across Chester County. When I say some it's not as heavy -- still coming down at a good clip in places.

Philadelphia's snowfall total is now 8.8" for the storm.



Update, Noon: Snow is really starting to work its way back into the region and picking up in intensity. Philadelphia is reporting heavy snow at Noon and the sleety/sloppy mess across the Northeast and across Bucks is now gradually transitioning over to snow as well. Snowfall is coming down at an inch per hour in spots, with some two inch per hour totals occurring in the heaviest bands.

While blizzard like conditions aren't occurring now in Philadelphia, winds will gradually worsen through the afternoon in the city. Winds along the Jersey Shore are gusting to over 35 mph and winds down near Washington have been gusting to over 40 mph. We will see winds continue to pick up through the afternoon, with significant worsening of road conditions and in the weather over the next several hours as snow picks back up in the second phase of the storm. Again, in some spots some locations will see 1-2" per hour rates. If you have not, I would suggest you get out and shovel now while temperatures are around freezing as the snow that has fallen earlier was wet and almost concrete like. As temperatures slowly drop, the heavier and wet snow layer will freeze up and become much harder to shovel and clear up.


Update, 10:30 AM: Snow has really picked up in the past hour or so across the western burbs, as has the wind,. We're getting occasional but not frequent gusts over 20 mph with moderate snow falling. Philly, you will get your snow and it's slowly working its way towards you. A band of more steadier precipitation is lifting northwest along the AC Expressway and that should work its way into Philadelphia around 11:30 or Noon...we should see the sleet/slop finding turn back over to snow and things will pick up from there.

Look at the temperatures down around DC -- currently in the lower and middle 20's with biting wind chills. The storm is bombing out and strengthening rapidly but it takes a little bit of time for the colder air to work in everywhere...it also does not help the dryslotting issues raised their ugly head this morning from Philadelphia on east. However, snow will increase in intensity as we work into the afternoon.

Update, 9:30 AM: Winds are really cranking up down the Shore and to our south. We'll see winds increase as we work into the midday hours. Around Philly, if you're getting sleet and light precipitation, it's due to mild air that's still aloft in the atmosphere. As heavier bands of precipitation work over you, they will mainly fall in the form of snow as the precipitation drags colder air down from aloft to the surface. You may see light sleet/snizzle (lull) and then moderate snow and then light sleet/snizzle (lull) for a time for the remainder of the morning as these waves of heavier precipitation lift northwestward from the developing coastal storm. Pressures with the developing coastal are continuing to drop...generally down into the low 29's (29.15" at Millville at 9 AM, equivalent of a low end Category 1 hurricane). We won't see hurricane force winds on land but as the storm continues to intensify we will see winds increase significantly in the next few hours.

Update, 8:30 AM: Banding of snow is starting to increase to the south of Philadelphia, working northward into the city and western suburbs. This is the first signs of the heavier banding of snow that should break out over the next few hours across the city. As of 8:15 the radar is showing that heavier banding along the NJ Turnpike and near Route 40 in Salem County and other patchy heavier areas of snow in Delco and near Flourtown. You can see a bit of a dryslot working up the coast in Central Jersey and another one down near Cape May. The snow bands should organize more as they work northward into the city and western burbs and I would not be surprised to see the dryslot down south work north up the coast a bit later on this morning.

Update, 7 AM: In my backyard, we're at 4.8" of snow and things have transitioned back to snow from a couple of hours of wintry slop. We'll see that transition continue over the next couple of hours from west to east. I'm also watching lighter areas of rain try to sneak up the Delaware coastline towards South Jersey. I don't expect that to work into Philadelphia but it may sneak up the Jersey coast a ways over the next few hours before the afternoon kicks in and snow picks up at the Shore.


Update, 6:30 AM: I have to admit defeat and when to throw in the towel on lower snowfall totals. I've bumped my totals and think we're going to see 14-22" from Philly on north/west through the suburbs back through Lancaster and east through Central New Jersey once the storm ends. This is not new snow but is total snow including the 3-7 that has fallen across much of the region last night and this morning. In other words, the region should pick up around a foot, maybe a notch more, of snow later today from the incoming snowfall across the region later on this morning. Last night's snow burst was greater than expected and pretty much drove myself into bustogenesis territory. Best to admit defeat, throw in the towel, and fall in line with the rest than go it alone and go down with the ship when you know the ship is sinking.

Anyways, note that snow is breaking out in Maryland (see my earlier update) and that precipitation is slowly transitioning back from a wintry slop across Maryland's Eastern Shore and across the DC area to snow...we'll see that transition continue to work east slowly over the next few hours, reaching the Philly and our part of the I-95 corridor around 9 or 10 AM.



Update, 5:30 AM: Radar depicts a some "lull" going on across the Delaware Valley as warmer air aloft and at the surface has worked on in. We're going to see a wintry slop at times during the morning hours during the lull, with that wintry slop having covered more of the region than anyone had expected. That's ok...the slop won't eat much into your snowfall totals but it will compact your snowcover down a bit.

Between now and around 9 or 10 AM we should see this slop continue in Philadelphia, with the slop continuing towards Noon at the Shore. After those mentioned timeframes the transition to snow will begin to take place, with conditions worsening after midday into the afternoon.

Yes, we are under a blizzard warning but that doesn't mean we'll see blizzard conditions right out of the gate this morning. Blizzard conditions are more likely from midday on through the duration of Wednesday from Philadelphia on north and west and then from the early afternoon on east of the Delaware River. Blizzard conditions are defined as low visibility due to blowing snow and very strong winds in excess of 35 mph sustained or frequent gusts.

Round two of the storm may ultimately bring more damaging impact to the region in the form of power outages and general impact to travel and livelihood as the storm's winds, snow, and slowly falling temperatures will help slicken things back up. The "good" thing from the current slop that is falling is that it is compacting the current snowpack in place and will make that less prone to whip around when winds pick up. Unfortunately, the snow in round two will compensate for that as it will be a fluffier snow and more powdery than what fell last night. With the higher winds around we should see that snow whip around a fair bit.

Watch the areas down by Washington back to Hagerstown over the next few hours on radar and see the snow fill in down there...assuming that takes place, that will be the afternoon main course. That should work through and drop another significant round of snow this afternoon.

Update, 5 AM: Got some sleep and am waking up to see that a blizzard warning is in place for much of the region for the day today. We're in the "lull" part of the storm this morning as milder air as crept into parts of the region, leading to freezing drizzle, sleet, freezing rain, or even rain from Philadelphia on south and east, with some sleet mixing in across the suburbs. Last night's snow burst dropped a general 3-7" across the region and was more impressive than I expected...at this point I admit it's likely my snow totals across the region are going to be too low and really thought the first round was going to be more of a dud than it was. I'm in the process of reviewing the rest of last night's computer modeling and current surface trends to confirm that this will be a bust for me or if there is anything I can hang my hat on lol

I'll be back with an update regarding the rest of this morning and the blizzard warnings we are under in a little bit...

Update, 9 PM: Watching precipitation continue to blossom over Virginia and Maryland with the developing coastal. Snow is falling just about everyone with the exception of the coastal Delmarva, where some sleet and freezing rain are mixing in at Ocean City and Salisbury. I would expect some of the warmer air aloft to work northward but at this point modeling from the NAM is shying away from Philadelphia getting in on the mix...it does approach the NJ Turnpike though but it seems to be trending away. Sleet/mix is still an issue at times later tonight in South Jersey (Millville-Atlantic City down). Philadelphia at 9 PM is reporting heavy snow. The 2-4 for north/west /1-3 for the city discussion for earlier is probably to be more like 2-4 for most...if not more.


Update, 8 PM: Precipitation over the past hour or two has broken out in additional coverage across Virginia and Maryland and is lifting northeast. Snow is being reported throughout parts of the Delaware Valley and we should see light to at times moderate snow continue through the evening and tonight from Philadelphia on north/west. Precipitation will pick up but milder air should work up the coast (at least aloft if not at the surface in some locations), resulting in a sleet mix or rain at the immediate coast. Washington has picked up a couple of inches so far and here in Royersford a dusting of snow has accumulated. We'll see this first batch continue on through overnight before a lull sets in towards morning...with the main course tomorrow.

Update, 7 PM:

I've updated snowfall totals slightly after some significant thought and hemming/hawing over the dryslot/mix issue. I still think it's significant but I have increased totals to 9-15" in Philadelphia and points just south and east of Philadelphia. One thing I did was increase the snowfall totals slightly to reflect higher ratios with the backside of the storm as the heavy band of snow develops (assuming it's as expected). Snow should be powdery with relatively higher ratios (12 to 1 compared to the standard 10 to 1), which is why I've nudged the totals up a notch.






The first part of a one-two pop is starting to work on into the western portions of the region. Tonight's first nudge of precipitation will be minor in nature and will not bring huge accumulations at first as energy between the incoming storm and the southern energy are just beginning to interact.

There are two mitigating factors for snow accumulation tonight to the south and east of Philadelphia. One, this is not bringing a ton of precipitation to begin with and the relatively weak amount of precipitation should allow for milder air to sneak on in aloft from the Delaware River on south and east. To the north and west, snow should be light to moderate overnight.

Precipitation will start over the next couple of hours here in Philadelphia. Accumulations tonight across the region will range from 2-4" north/west, 1-3" in Philadelphia, and an inch or so south and east. Milder air will result in that changeover from snow to sleet, with rain possible along the coast. In Philadelphia, that sleet area may work in in lighter lulls in precipitation. It won't substantially cut down on your snowfall totals but may knock an inch or so off compared to folks to the northwest.

You may wake up in the morning and wonder what's going on -- well, the main part of the storm will just be firing up. The coastal will be firing up, precipitation will be enhancing and increasing, and the storm will crank up big time after about 7 or 8 AM in the region. North and west of Philadelphia will see snow, heavy at times, with that snow band working into Philadelphia and then working across New Jersey during the afternoon hours. There may be a longer lull EAST of Philadelphia than west due to the potential development of a dryslot along the coast or just inland. If that doesn't happen, snowfall accumulations will be higher than what I'm advertising. This will ultimately be a nowcast setup and will be monitored, especially early tomorrow.

Arguably, the worst of the storm will be between 10 AM and 4 PM on Wednesday, when snowfall rates in Philadelphia and north/west will be an inch per hour or more. Combined with increasing winds from the north at 20-30 mph, blizzard like conditions will result.

February 10th, 2010 Forecast

A major winter storm is on the way, with significant accumulation of snow for many in the region. For others to the south/east, a wintry mix of sleet and (at the Shore) rain will limit the potential for big time accumulations...even there accumulations will occur. Winds will increase later tonight, leading to blowing and drifting conditions along with blizzard-like conditions at times...especially east of the city.

Tonight: Snow spreads in after 5 south/west of the city, after 7 in Philadelphia, and after 9 north/east. Snow will mix and then change to rain at the Shore and simply mix with sleet south/east of Philadelphia. Winds will increase to 10-20 mph from the northeast during the night. Low temperatures should be reached early in the mid 20's north to the low 30's south, then temperatures should hold steady or slowly nudge higher south of town.

Wednesday: A brief lull in the precipitation is possible in the city during the morning, with snow becoming heavy at times from mid morning on. Winds will veer to the north, increase to 20-30 mph with higher gusts, leading to blizzard-like conditions in the region at times. Snow will accumulate to 8-14" in the city and along the Delaware River, 10-16" north/west. Temperatures will nudge to around 30 in the morning in the city and then fall back into the 20's in Philadelphia. Temperatures will slowly fall south/east after morning highs in mid and upper 30's, with early mixed precipitation or rain changing back to snow.

Coming Later This Evening: Updated coverage on the storm.

Sleeting....Snowing...Blowing...Drifting...

The "infamous" new information is in regarding the storm from last night's computer modeling and for the most part, the storm held serve regarding placement of the heaviest snows (Lancaster County, congrats if trends hold) and regarding precipitation amounts. There were no real significant changes in those two areas although modeling last night was a bit more moist with this storm. Modeling also continues to show this storm as a two round event with a lull in between, with the two rounds being tonight and from mid-morning Wednesday on and the lull being between 4 and 8 AM or so. The NAM and GFS continue to show potential dryslot issues along coastal sections of New Jersey which may limit potential snows from the second round of the storm.


The one change worth noting is the continued nudging of milder air aloft with the NAM during the end of the first round of the storm and during the lull. This will not impact the north/west crowd but there may be some sleet mixing in with snow even into Philadelphia and along the Delaware River for a time. It does not look like a significant duration of sleet but as precipitation relaxes after the first round of the storm tonight we could see some moderation of temperatures aloft as precipitation eases and prevents cold air from making it all the way through the atmosphere. The impacts should be minimal, overall, but it is worth noting that we could see some sleet pellets in Philly, the I-295 corridor, and across parts of Central Jersey during this lull.
It's important to note that sleet should not impact the main parts of the storm in Philadelphia itself -- the first round should be primarily snow as should the second on Wednesday. North and west of the city will remain all snow. Sleet may be a bit of a bigger issue across South Jersey and Delaware during round one and the dryslot may play a factor in round two...hence your snowfall totals are lower in Millville, Vineland, and Dover. Along the coast, rain will be an issue during the first round of the event. If the rain is heavy enough, ponding of water in spots could be an issue along roadways near the coast.

I have tweaked the snowfall totals slightly to reflect for the sleet potential along the river and brought my forecast (and the one I'm sticking to) down from 9-15" to 8-14" but again, this isn't a huge change in thinking. I've nudged the totals for snow in the north/west up slightly to reflect the slight increase in moisture across the computer models and 10-16" should fall pretty much above Roosevelt Boulevard in the hilly part of the city and on into the burbs. The jackpot is 12-20" and continues to zero in near Lancaster and York.

The timing element and details with the storm outlined last night still hold with no significant changes...just a slight tweaking to reflect the pinging of sleet near the city during the lull that may eat an inch of snow off the totals.

Monday, February 08, 2010

The Record Setter?

Another major snowfall on the way for the Delaware Valley -- although this may not end up quite as significant as the two other big storms we've had. It does, however, have the potential to cause the most problems being this is a weekday storm and that we have existing snowcover in place. It also could ultimately bring blizzard like conditions to parts of the Delaware Valley on Wednesday. This could get ugly.

First, the modeling, which is bouncing around but getting honed in on a solution for the upcoming storm. This is shaping up as a two part storm...the first will move in on tomorrow night with overrunning snows (mixing with sleet and possibly rain south/east of Philadelphia) before a lull in precipitation takes place around daybreak Wednesday. It may still be snowing lightly for parts of the region but we'll see a lessening of precipitation intensity between say 4 and 8 AM. Round two will be a significant snow thump that will work through after 8 AM for a chunk of the region.

One of the concerns with this forecast is the potential for sleet and/or rain slipping northward along the coastal plain in South Jersey and Delaware during the first round. The city should remain all snow as should areas north/west of the New Jersey Turnpike and north of Route 70 in New Jersey. South of that line could see some mixing with sleet and freezing rain...if not transition over to rain altogether along the coast.

Another concern with the storm is the potential for a dryslot to develop along parts of coastal New Jersey into New York City. The Euro, GFS, and NAM all agree on this developing to a varying degree in between rounds one and two...with Jersey being the likely spot as of now for a dryslot of sorts. This would be determined by the position of the developing coastal storm as well as the track of the midlevel feature as it rambles on through. The dryslot would limit accumulating snow and precipitation totals overall somewhat although you will still get snow -- just not as much as the jackpot areas.

Modeling indicates a pretty decent consensus on precipitation -- generally around an inch for most.

The storm's first wave should move in between 7-10 PM in the city and move in from the southwest. We should make it through the evening rush without snow on Tuesday and precipitation will continue in light to moderate fashion overnight, likely accumulating to a few inches.


After the "lull" or slackening of snow (or mix south/east) during the morning hours, the second thump should develop and move through parts of the region. Whoever gets this second thump will likely jackpot in the storm (get the most snow). There is disagreement between the EURO, GFS, and NAM on who jackpots as the Euro places the bullseye near Harrisburg while the GFS places it just north of Baltimore and the NAM places it between Wilmington and Havre de Grace. Determining the jackpot may be an exercise in futility but taking an average of the three bullseyes places the heaviest snowband across Lancaster, Western Chester County, and points west. On top of the snow, winds will increase on Wednesday to 20-30 mph, with potential gusts over 35 mph. This will result in near blizzard conditions...at minimum blowing and drifting snow across much of the area with our existing snowpack in place.

The timing of this event will result in significant impacts on the region -- likely including school closings assuming things play out as expected away from the coast. The Shore may see delays/closings related to the current snowpack in place plus the wintry mix or rain they see from this storm. If they get a fair bit of rain down there flooding may be a pretty significant issue in Atlantic City and along the coast.
The mix zone is arguably the toughest to forecast -- but at this point I have it across Southeast New Jersey, Central and Southern Delaware, back south of Washington. This could shift slightly away or slightly closer but I do think that Philadelphia stays all snow.

In the city, I expect 9-15" of snow from this storm going up to Allentown and down to Wilmington on south and west. 12-18" is possible in the bullseye, which as of now is along the Mason-Dixon line (the PA/MD border). That bullseye may shift around as does the dryslot -- but as of now the dryslot is generally farther north in New Jersey into the New York City area where accumulations should be a bit less...although a significant snowfall looks likely at this point.

Assuming Philadelphia gets 9.3" of snow from this storm, Philadelphia will set a seasonal snowfall record. The old record, 65.5", was set in the winter of 1995-1996.

February 9th, 2010 Forecast

Now: It's a sunny and cold day as we're experiencing the calm before yet another storm system. Temperatures today are working towards or just above 30 degrees in most locations, with a chilly northwest wind at 10-20 mph making it feel like it's in the teens. Winds will slacken off tonight but will clouds will increase as the storm approaches tomorrow.

Tonight: For the most part a clear night with some clouds increasing late. Lows in the teens with winds diminishing to around five mph overnight.

Tuesday: Increasing clouds through the course of the day, with some sunshine in the morning. At this point, snow looks to hold off until evening or late night so we should make it home from work without weather-related issues. Highs tomorrow will be in the low to mid 30's with light and variable winds.


Later Today: A forecast map and updated discussion on the snowfall for the region.

Later This Evening: Weather chat will be up and running!



A Question Of Time

Last night's computer modeling coalesced around two basic camps...and the impact of one feature (our Manitoba Mauler that we outlined yesterday) will determine whether the snowier or not as snowier scenario verifies.

The GFS, along with the Euro and Canadian, are in alignment that the snowier scenario verifies as it rapidly intensifies the coastal low we talked about yesterday near the region. The resulting scenario gives the Delaware Valley a widespread foot plus snowfall, save for southern Delaware and coastal New Jersey where mixing would result in spots on Tuesday night before the coastal low cranked up and transitioned the coastal regions from rain or rain/sleet/snow mix back into snow.

The NAM is less snowy, generally putting the heaviest snowfalls from Philadelphia on east and is in line with yesterday's midday run. It also has a bit less moisture with the storm and lower snowfall totals. It's still a significant storm...just not AS significant.

Both models indicate at least some mix along the coast (from Atlantic City on south) and in Sussex County, so the precipitation outputs up above are not solely reflective of all of that precipitation falling as snow...especially in AC and in Lower Delaware. It does look like -- and should continue to be -- a snowy scenario for the region outside of those coastal regions.

Explaining the two scenarios in more detail -- the difference between the NAM and GFS computer models stems from the interaction with the mid level northern energy...our Manitoba Mauler...with the southern energy and the coastal low that will fire up over the Carolinas. The NAM computer model is a bit slower with the "hook up" between the two pieces of energy -- or phasing if you will. The Mauler is also slower moving and farther to the north. This results in a scenario that brings about less overall snowfall because the coastal low does not bomb out as quickly. You can see on the GFS how the coastal (out in the Atlantic marked with an X) is able to tap into the Mauler's energy more quickly as the main lobe of energy is in Virginia compared to Northwestern Pennsylvania. This quicker connection and phasing allows for explosive development of the coastal low, the more intense precipitation, and higher snowfall totals for all.

I'm always skepitcal -- mostly because of past performance with these types of energy transfers -- that the models are overdoing the energy transfer and are intensifying the coastal storm much more quickly than reality suggests. This is normally the point where I put out a preliminary map but despite the consistency in the Euro and the GFS, that energy transfer has me a bit on the edge that things are firing up a bit too quickly.

Regarding accumulations, a foot plus of snow is a good bet in some locations...I tend to think the best chances for that foot are just north and northeast of Philadelphia (Trenton, Bucks, on north into North Jersey). In Philadelphia, a preliminary 8-14" forecast is what I'm going with at this point. In the Lehigh Valley, 6-12" is my first guess. Farther south, mixing with rain and sleet will lower accumulations from Atlantic City on south along the coast and inland a bit. I'll have my first forecast map for snowfall out this afternoon.


Sunday, February 07, 2010

A Philly & South Winter


This has been one of the more unusual winters in our lifetimes in that not only is Boston running through the winter with merely "seasonal" snowfall but that Philadelphia has nearly twice as much snowfall-to-date as Boston has. Philadelphia is running with 527% of seasonal snowfall through yesterday, which is the second highest total on the list to the right. Only BWI (Baltimore's Airport) is running with a higher seasonal percentage of average...a mind-numbing 555%.

What has fueled the Philly and south winter?

One, the presence of El Nino...which has opened the door to Pacific moisture and energy tracking along the subtropical jet. The southern stream of energy has been alive and kicking nearly unabated since October and has provided for numerous coastal storms going back to Mid October, which have pounded away at the beaches while dropping copious precipitation across the Delmarva and South Jersey.

Two, strong mid and high level blocking that has been over Eastern Canada has prevented storms from lifting north. Yes, the North Atlantic Oscillation (and its partner in crime the Arctic Oscillation) have been significantly negative for much of the winter (when they haven't, we thawed out with a mild stretch of weather in mid and late January). This significant to extreme blocking as prevented storms from lifting too far northeast and has put the breaks on significant storms near our latitude, deflecting them and turning them east into the Atlantic before they could pop Boston and New England. While temperatures themselves have not been extremely cold here -- we are averaging around normal since December 1st -- the blocking is more in tune with supporting the increased chances for storms to avoid going inland.

One could argue it's been a perfect storm of factors to lead to this winter...a persistent negative NAO combined with El Nino to allow for storms for the Mid Atlantic. Regardless of what ends up happening with the upcoming Tuesday night storm, the winter we have seen to date has been historic and likely something we may not see again for quite a while.

Never Let The Snow Starved Down Again...

Hopefully the GFS' midday computer run doesn't let the snow starved (what few of you remain after this winter) down again...firing up the spirit of Depeche Mode (one of my favorite bands), the snow starved are taking a ride with their best friend (computer modeling) and hoping that the midday forecast of the GFS computer model pans out relatively close to fruition.

The GFS...along with the NAM (see below) are firing up some pretty significant snows for parts of the region from the next storm in the pipeline. This storm is earmarked to come in Tuesday night and Wednesday and could deliver another foot or more of snow to parts of the Mid Atlantic. It's too early to say who is going to get hammered as the models are not in agreement over where the heaviest snows are going to be. However, the GFS, NAM, and EURO are all in agreement that we're going to see a pretty significant coastal storm fire up and lift northeast.

The ingredients are starting to move into place. We have two distinct pieces of energy out there and the two will work east to become the players for our storm. The first is Pacific energy that will race eastwards into the Southern US and then work to the Carolina coastline. At the same time, energy is dropping down from Manitoba (a Manitoba Mauler). This mauler will spin through the Northern Plains and then work east towards the Great Lakes. As the Pacific/Southern energy works towards the Carolina coastline. The two will begin to phase together as the Southern energy fires up a low off of the Carolina coastline. The resulting phase will lift northeast as a quickly developing storm in case of bombogenesis (if modeling is correct), which is a rapid deepening of low pressure.

The result will be precipitation that will work in on Tuesday night and then intensify on Wednesday as the coastal storm deepens and lifts northeast. Snowfall could become heavy at times for parts of the Mid Atlantic with a foot plus in spots.

In the GFS map seen above, it is depicting between 12-18" of snow (or more) in an area from Chambersburg to Harrisburg to Allentown to Toms River on south, with 18" plus near Cape May. The NAM (seen immediately below) is suggesting a bit less precipitation, but it generally has the totals a bit further east and north...with 8"+ likely from Trenton on north. The Euro is somewhat in line with this although a different solution (stronger, with the heaviest snows over the Lehigh Valley, Bucks County, and Central Jersey) so the NAM is taking an eastern approach. Even in the NAM's solution it is at least a 6-8" event in Philadelphia if this map is correct (grain of salt needed).


There are some flies in the ointment to worry about, however. One is the potential for mixed precipitation (sleet/freezing rain) for areas east of I-95 on Tuesday night and early Wednesday as warm air aloft tries to work into the Lakes energy before the coastal gets cranking. This may be a mitigating factor for snowfall for a time before the coastal gets going and could work against accumulations right along the coast from Cape May or Atlantic City on down. There's still time to watch that aspect of precipitation. I do think with surface cold air in place we should not see a huge surge of warmth that will melt it but with milder air aloft trying to sneak in we may see some issues with precipitation types other than snow to our south and east. Fly #2 is the coastal low bombing itself. This is one of those scenarios where the energy transfer between the mauler and the southern stream would allow for explosive deepening. If the timing of those two is not right on...and the storm is slower in developing, the bigger snows will likely be muted around here and best served in New England. However, if we look at the NAM (which has been pretty good on the whole this winter) this looks like a storm where slightly northeast of Philadelphia does better than the city itself. New York may finally get in on the action and pick up more snow than us.

There is agreement, however, that the bombing does take place and in time to catch Philly with another good (NAM) to great (GFS/Canadian) snowstorm. With the intensifying that will take place (regardless of in time to nail us or farther north) winds will become an issue and we'll see another round of blowing and drifting snow...perhaps even blizzard conditions or blizzard-like conditions on Wednesday -- even in areas where snow didn't fall much simply because of snow that's on the ground already.

Between now and Tuesday night into Wednesday, we'll have to watch where the bombing sets up on the model and how precipitation will impact us. We'll also have to watch the mix line and see if that sneaks into South Jersey or Delaware or if we can manage to keep it offshore. After this storm, there's potentially ANOTHER storm for next Saturday that as of today is poised to slip off the coast down near Jacksonville on the computer models. That one may slip a bit closer to us but at this point it looks like we should avoid significant impacts from that one.

Weather Rewind, January 31-February 6 '10

Besides some big snowstorm that fell at the end of last week, temperatures on the whole were seasonably cold for the week, averaging 2.6 degrees below average (Saturday contributed half of the week's departure in temperatures all by itself). A couple of not-so-bad days in the middle of the week blunted a chilly start and end.

We also had a smidge of snow during the middle of the week with a Groundhog Night's snowfall that dropped between a half an inch and three inches of snow in the Delaware Valley. Philadelphia picked up 0.6" of snow from the minor event but its impacts were enough to fire up two hour delays in the suburban schools. The minor event became a rather major storm system in the North Atlantic (see below, towards the upper right). The storm had a low pressure the equivalent of a Category 3 hurricane and was a player in preventing our major snowfall from reaching New York City.