Saturday, December 26, 2009

Snowpack Extinction

A general one to two inch rainfall across the Delaware Valley today and combined with temperatures in the low 50's has lead to a near annihilation of the Delaware Valley's snowpack. The only snowpack I could see when I came back from the Poconos was the parking lot snowpiles from last Saturday's storm. More importantly, flooded streams and roadways dot the landscape today and the region sits in flood warnings for the evening and streams sit in flood warnings through much of the night.

Philadelphia received 1.80" of rain on the 26th, which was enough to lead to Philadelphia setting a record for wettest December on record at 8.58" of liquid precipitation. That surpasses the record for 8.47" of rain set in December 1996. With the prospect of another storm on New Years Eve, it's possible we see that total notch even higher before the year turns the page.

It was the wettest December 26th on record in Atlantic City (over two inches of rain today). Other high rainfall totals over the past 24 hours include Doylestown's 1.47", 2.16" in Millville, and 1.87" in Wilmington.

Friday, December 25, 2009

Wintry Advisory Breakdown

Some winter weather advisories are out for parts of the region over the next 24 hours due to the upcoming rain and ice event as it moves in. As of this morning, winter weather advisories are out in Central Pennsylvania until at least 5 PM due to the incoming freezing rain and rain over cold ground.

In our region, the Poconos are under a freezing rain advisory (Carbon and Monroe Counties) for today and tonight, with those advisories being shifted south and east to include the Lehigh Valley (Lehigh and Northampton) and Northwest New Jersey (Warren, Morris, Sussex Counties) for tonight as temperatures in the Lehigh Valley will be close to the freezing mark. Over the Poconos, cold air may be trapped in some of the valleys a bit longer than elsewhere so the icing threat may be localized but this region has the best potential for icy travel for a longer duration than other areas.

It's possible that advisories may be adjusted through the course of this afternoon and evening and the National Weather Service website will have a full update of those advisories as they are issued.

December 26th and 27th, 2009 Forecast

A soggy start to the weekend will give way to a halfway decent Sunday as this current storm system pulls away. Colder air lurks for next week.

Tonight: Occasional showers become a steadier light rain overnight and it will be breezy at times with east winds at 10-20 mph (15-25 at the coast). Temperatures will rise slowly overnight into the 40's from Philadelphia and points south, with mid and upper 30's to the north and west of Philadelphia. There could be some slick spots on the roads during the evening to the north and west but those should gradually diminish over the evening.

Saturday: Occasional rain will continue through the day with east winds continuing to pump in moisture from the Atlantic. About an inch of rain is expected during the course of the day. Temperatures will rise into the 50's from Philadelphia on south and east, with 40's in the north and west suburbs and perhaps only upper 30's in the Lehigh Valley. Due to the temperature rise and the near inch of rain we'll see some street and stream flooding in locations. Fog is also possible due to the milder temperatures working over the snowpack.

Saturday Night: Rain will end from southwest to northeast during the evening, with slackening winds. Temperatures will drop down to the mid and upper 30's by morning.

Sunday: Skies will be generally variably cloudy, with some sun possible at times. Winds will generally be light from the west or west-northwest at under 10 mph. High temperatures will top out in the lower or middle 40's.

There is a chance of snow in the forecast for Monday as an upper level low (the remnant of what is currently firing this storm system up in the Midwest) works east. Snow showers and flurries are possible across Pennsylvania and the Mid Atlantic and we could see a couple of inches of snow on Monday afternoon in some locations -- we'll talk about it in a bit more detail on Sunday. In the wake of the upper low we'll see colder weather for the middle of the week before some moderation before next Friday, when another storm system may be on the horizon.

Thursday, December 24, 2009

Watching The Christmas Night Rainfall

In addition to the threat for some patchy freezing drizzle and some icy issues to the north and west of Philadelphia with the Christmas Storm, the more significant issue is the snowpack and its interaction with milder temperatures and with nearly an inch of rain.

Computer guidance is suggesting around an inch of rain, with the GFS less a bit less moist and the NAM juicing things up for Christmas Night and Saturday with the upcoming storm. Factoring in the NAM's historical bias towards the wet side of things it seems prudent that most locations should average out to between 3/4" and one inch across the region, with more to the southeast closer to a secondary wave of low pressure that will track up from the south along I-95.

The low's track will be critical regarding temperatures and interaction with snowpack. Temperatures on the west side of the secondary low will not rise as much as they will to the east...a track of the secondary low along the coast would keep Philadelphia from seeing a significant temperature rise and would probably keep a significant chunk of the snowpack in place. The farther west the low goes, the farther west the temperature rise will get.

How warm will temperatures get? For anyone to the east of the low, temperatures should climb to 50 or higher. For those to the west, temperatures will probably hold in the 40's or...for those farther northwest, 30's. Determining the exact track of that low is a bit tough to do at this point but it looks to track relatively close to I-95. If temperatures rise much above 40, snow melt increases at a more rapid rate as temperatures increase. Combining that with an inch or so of rain and we could see some street and stream flooding across the more susceptible parts of the area on Saturday and Saturday evening before precipitation shuts off. This temperature rise will be gradual and not all at once -- we won't be seeing temperatures near 40 in Philadelphia until around midnight Christmas Night and temperatures near 50 (if the low tracks overhead or nearby) until late afternoon Saturday. To the north and west, temperature rises will be slower and many places may not rise above 40 until Saturday afternoon.

As I spend Christmas and the 26th with my in-laws over the Poconos, I won't be able to provide an update on the storm as it comes through. I will provide an update on Christmas morning with any drastic changes (should there be any) to the weather picture with the upcoming storm system. Hope everyone has a Merry Christmas and has a peaceful Friday!

Christmas Day 2009 Forecast

Now: It's a seasonably chilly Christmas Eve around here but with a mostly sunny sky. Temperatures this midday are in the low 30's for most, some upper 20's in a few spots, but we should all end up in the mid 30's before the day is out. Clouds will be increasing this afternoon in advance of the Christmas Day storm system that is currently providing the Midwest with some holiday travel "issues."

Christmas Eve: Quiet, tranquil, with increasing and thickening clouds overnight. There could be some patchy freezing drizzle breaking out over Central Pennsylvania in advance of the rain shield later tonight and some of that could impact travel across Southeast Pennsylvania or South Jersey during the morning hours on Friday. Operative word is patchy so it does not look to be widespread. Low temperatures should bottom out in the 20's with winds nudging east-northeast at 5-10 mph.

Christmas Day: Patchy freezing drizzle could slick up travel in some spots tomorrow morning but a more organized area of showers will move in during the afternoon with the storm system. With temperatures slow to rise during the daylight hours, travel could continue to be slick in spots across the area due to the combination of cold ground and temperatures not much above freezing. Icy travel looks more likely the farther north and west one travels during the day. High temperatures will likely be reached at midnight and generally look to be in the 30's from Philly on north, 40's in South Jersey and Delaware. Daytime temperatures should be in the 30's nearly everywhere except right at the Shore and in Southern Delaware. Winds will increase from the east and northeast at 10-16 mph, making for a raw day and even more raw night.

Weekend Sneak Peek: Icy conditions should ease to the north overnight Christmas Night as milder air nudges inland, although temperatures will not be "mild" by any stretch. Warm air (50 and above) looks to be confined generally to South Jersey, Delaware, and perhaps Philadelphia which will cause an issue with melting snow and an inch or more of rain combining for urbanized and stream flooding in some locations. North/west may not warm as much but rain will still knock a dent into the snowcover over the burbs and Lehigh Valley. The storm system pulls away Saturday afternoon and evening, leaving a variably cloudy and breezy at times Sunday in the Delaware Valley.

Later This Afternoon: A brief update on the computer guidance for the storm and how much rain the models are predicting.

Twas Another Low In The Teens

Philadelphia started off today in the teens for the second consecutive morning (low of 19 was a couple notches higher than yesterday's 17) and still a good ten degrees below seasonal averages. Again, the typical cold areas "won" as Millville and York both dropped to 4 degrees this mornings thanks to calm conditions and clear skies, allowing for radiational cooling to take control and see what little "heat" there was escape into the atmosphere overnight.

Neither reading was the coldest in the immediate area, however. Lakehurst Naval Air Station in Ocean County dropped to one degree this morning and was -3 yesterday morning, which along with York (-2, set after 7 AM yesterday) were the first two subzero readings in the Delaware Valley this winter. Lakehurst, Millville, and York are in prime spots to go colder than the rest of the region due to their observation site locations (rural areas) in normal conditions. Add the fact that all three sites have significant snowpack and you have a recipe for some very cold microclimates as the three locations can see temperatures 15-20 degrees colder than other locations in the region.

Wednesday, December 23, 2009

A Stormy Travelling Sandwich

Travel on Christmas Eve will not be a big deal if you are staying around here as that large storm system works northward through the Midwest and impacts everyone from Minnesota to Mississippi with precipitation. On the northern and western flanks of the storm system snow will fly and pile up fast and furious, with an icy wedge in between the snow and rain in Iowa, Wisconsin, and parts of Illinois.

What is shaping up to be "ground zero" for the storm from a snow standpoint is Minneapolis and parts of Central Minnesota, which could end up with over twenty inches of snow in this upcoming storm. Minnesotans are typically a hearty bunch when it comes to winter weather, dealing with cold and snow at high levels of tolerance. However, heaping helpings of snowfall don't necessarily equate well regardless of location. Minneapolis itself has only seen three twenty inch snowstorms since records were first kept there in the 1880's, with an average seasonal snowfall of 45" over a 125 year record (which is close to what Mount Pocono gets). Thanks to a higher latitude and colder temperatures, the snow sticks around up there a good bit longer than it does at our latitude. With this storm coming in over Christmas, needless to say there is high potential for travel impacts throughout the Upper Midwest.

The large scale storm system could also bring thunderstorms and severe weather to parts of Louisiana and Mississippi and flooding rainfall northward to the Midwest.

The rainy spoke of this storm will impact us on Christmas Day...and believe it or not a part of the upper level energy from this storm may rotate through the region next Monday as the mid level support of the storm slowly meanders east after the rain shield shoots through here Saturday evening. That mid level energy could bring some light snow or flurries to parts of the Mid Atlantic and Northeast.

Breaking Down The Christmas Night Event

The upcoming Christmas and Boxing Day (December 26th) storm system that will move through looks to be a rainy affair for the region. The combination of mild air aloft surging out ahead of the storm system and the lack of a deep cold air mass will allow the region to see raindrops and not snowflakes from the upcoming system. That does not mean that travel is going to be fine and lack problems as there are a couple of things that I think should be mentioned in terms of a cautionary standpoint.

One, this is an event where temperatures will be starting pretty close to freezing (if not below in spots to the north) and an event where the ground has been cold for a few days thanks to snowcover. While temperatures will warm over the course of Christmas Day, we could be looking at temperatures in Philadelphia and vicinity around 35 or so at the onset of precipitation in Philadelphia proper, with temperatures rising overnight Christmas Night. Our forecast high of 40 for Christmas Day reflects an evening high temperature.

Untreated surfaces could have some slick spots when rainfall starts simply due to temperatures at the surface being cold the past few days. This does not mean we're getting "freezing rain" in the classic sense of ice building on trees and cars but it does mean that the first couple of hours of the event could feature some slick spots on Christmas Afternoon before temperatures nudge warmer. The NAM (see below) is running a fair bit colder than other guidance with this event, a typical sign of its understanding of cold air lurking close to the surface. It is predicting a high of 38 at midnight with rising temperatures into Saturday.

Timing will be critical in relationship of precipitation arrival versus temperatures. Model guidance is suggesting an arrival of showers into the region between 3 and 7 PM on Christmas Day, with steadier showers overnight into Saturday. By the time we reach Saturday we should see temperatures warm enough throughout the region where icing is not an issue. However, for areas to the north and west there could be lingering slick travel into Christmas Night before moderating temperatures take place.

I think the highest potential for slick travel will be over the Poconos as those areas should start a notch or two below freezing at the start of the event and should see temperatures rise the most slowly. The potential around Philadelphia is lower than the potential to the north/west of the city simply because temperatures should moderate more quickly in and around Philly. However, because of the aforementioned reasons (cold ground, temps near freezing) I think there is at least "some" potential for some slick spots on Christmas Afternoon and evening before things improve weatherwise.

One of the things that we'll have to also watch out for with this storm is the potential for some street and stream flooding. Model guidance is suggesting about an inch of rain with the upcoming storm, with some areas possibly seeing more than that. Given the relatively deep snowpack (and relatively high moisture content trapped within the snow, see the map below) in place combined with temperatures climbing into the 40's and an inch plus of rain, flooding is definitely possible with the usual suspect streams in our region.

Another fun piece of information to throw in will be the potential of fog due to milder air (and milder rain) traversing over the colder snowpack. Fog could be pretty thick late Friday night into Saturday in spots.

EDIT, 9 PM: One of the things the last couple of runs (the midday and early evening) of the GFS has been showing for Friday morning is the potential for some drizzle or freezing drizzle across Southern and Eastern Pennsylvania (along the PA Turnpike and/or I-78). It's something that I'll look at again in the morning to see if it remains on there. Timing on this would be during the morning hours on Christmas if this model is correct. It is not on the NAM (the other model guidance), however.

December 24th, 2009 Forecast

Now: It's a cold but mainly sunny day across the Delaware Valley as a large scale high pressure center over Canada is nudging some chill down into the East. Temperatures today are slowly working into the very low 30's, with a probable high near freezing in the city and in the upper 20's north/west. Today will likely shape up as the coldest day of the week as we begin to moderate slowly as a storm system over the center of the country eventually sends rain (and some ice) towards us for Christmas Night and Saturday.

Tonight: Continued cold and mostly clear. Lows in the teens outside of the city (perhaps colder in Millville and the Pinelands) to around 20 in Center City Philadelphia with light northwest winds.

Christmas Eve: Tranquil, peaceful weather for local travel as skies should be mostly sunny in the morning. High clouds may spread in during the afternoon in advance of the storm in the Midwest but the day will be dry. High temperatures will generally top out in the mid 30's with light northerly winds.

Christmas Afternoon/Evening: The next storm system will move on in and bring rain to Philadelphia and points south. The question remains for areas north of Philadelphia where colder air could linger at the surface for a time and lead to a period of freezing rain or sleet to start this storm. I'll have a more comprehensive post on the upcoming storm late this afternoon that highlights some of the variables with this storm and what may ultimately result.

Cold Morning In Place

Radiational cooling is great thing to watch in the winter, especially with snowpack. Unless you're in Millville or York and are dealing with temperatures just a few degrees above zero to start your day, that is. Temperatures this morning have dropped into the teens across much of the region, except in Pottstown, Dover and south, DC, and Harrisburg. Philadelphia's low of 17 is the coldest it has seen so far this winter season.

South Jersey is the cold spot of the region in general, with Atlantic City and Mount Holly also both starting off in the lower teens.

With some cold sunshine we should see temperatures today warm into the lower 30's this afternoon as a nudge of cooler air has pressed south from Canada. This will hold in place for another day or so before the larger storm out in the Midwest pushes towards us on Christmas afternoon through Saturday.

Tuesday, December 22, 2009

Snowy Pictures

It wasn't just Philadelphia that was impacted by last weekend's major winter storm. High resolution satellite imagery from NOAA shows the extent of snowcover through the Mid Atlantic as the major winter storm blanketed areas from the Carolinas through New England was massive amounts of snow.

A couple of cooler satellite and radar shots from the snowstorm were from Saturday (see below). First, the satellite shot from early afternoon Saturday showing the enhanced precipitation breaking out to the north of the low pressure center (the center of the comma shape on the picture).

The first wave of that enhanced precipitation was lifting into South Jersey and Delaware around that time and making its progression north. It was around 12:30 when steadier snows picking up throughout much of the Philadelphia region and even into the Trenton and Central Jersey areas.

The next radar shot, from 2:30 PM, shows the thundersnow band working across South Jersey and Northern Delaware. Between 2:30 and 3:30 several reports of thundersnow came into the National Weather Service or were commented upon here. The thundersnow was accompanied by low visibilities (1/4 mile or less) and heavy snowfall rates in the two inch per hour range.

It's no coincidence that the areas that were in this heavy band (and subsequent other moderate to heavy bands that set up shop over South Jersey later Saturday afternoon into Saturday Evening) also ended up with the highest snowfall totals from the last storm.

As the storm wrapped up and lifted up the coast, the orientation of that heavy snow band shifted from east-to-west to a northeast-southwest orientation. This allowed the snow to continue to drop at a 1-2" per hour rate across the city and immediate South Jersey. A dryslot appeared across Delaware and South Jersey and enhanced the precipitation just northwest of it. The heavier bands of snow gradually weakened later as the night progressed and as the storm pulled away. The storm also sent a rather heavy band of snow into Long Island and Southern New England, where parts of Long Island were the lucky recipients of nearly two feet of snow as well.

December 23rd, 2009 Forecast

Now: Temperatures are a few notches ahead of where we thought they'd be today thanks to cloud cover last night helping to prevent a bottoming out of temperatures in the region. Skies feature a mix of clouds and sun this afternoon, with temperatures generally in the mid and upper 30's areawide. (Update, 2:30 PM...there are some flurries passing through the northern burbs this afternoon and may slip south into Philadelphia later. They are no big deal but are merely passing on through)

Tonight: I think this should ultimately be the night where we see some clearing in the skies and a bottoming of the temperatures by a decent level. I'm still forecasting temperatures on the warm end of model guidance based on where we are this afternoon. Still, with lighter winds overnight and probably more clearing in the skies temperatures should drop into the teens everywhere, warmest in Philadelphia and at the Shore.

Wednesday: High pressure noses in from Canada and brings mostly sunny skies and temperatures a couple of degrees colder than past days. We should see things top out in the 28-34 range regionwide with west-northwest winds continuing.

Computer guidance generally continues to nudge the arrival of the next storm back a notch -- now poised to move in during Christmas afternoon and evening. We should see temperatures moderate ahead of the front, especially along the coastal plain. The only question is about areas north/west and whether there is a freezing rain or sleet situation at the front end of this storm because of residual cold air at the surface and due to cold ground. We'll have a larger post tomorrow outlining the impacts for Christmas and December 26th travel.

Monday, December 21, 2009

Skating Into Christmas

A few days of benign weather between now and Christmas Eve for the East Coast will give way to a large scale storm system that will organize and lift through the middle of the country. Given the large scale high pressure poised in computer modeling to be centered over Eastern Canada, this low will likely push to our west and draw moisture northeast from the center of the country towards the Northeast for Christmas Day.

The GFS (shown above) shows precipitation breaking out across Pennsylvania early Christmas morning and spreading east through the day across the state. Depending on timing and temperatures at the surface, some freezing rain is certainly possible across northern and western areas of the region on Christmas Day Afternoon and Christmas Night as this storm system spreads in.

What may mitigate some of the potential for icing is the timing of arrival on this storm. Most guidance had earlier brought this storm in overnight Christmas Eve, a lot earlier and at a colder time temperature-wise, which would result in a more likely freezing rain setup. The later arrival may help bring temperatures up just enough due to daytime heating and retreating high pressure over Eastern Canada. However, with a snowpack in place and cold ground due to a relatively cold pattern going into Christmas some slick travel still could certainly result on Christmas from precipitation.

It's too early to say how bad the potential ice threat would be but if a secondary low pressure center develops along the advancing front and cross through the Mid Atlantic, we could see an entrenching of cold air over the Lehigh Valley, Central Pennsylvania, and the Poconos. Modeling until today had been indicating some potential for a secondary low along the upcoming front so it's certainly something to watch. The upcoming storm is also poised to bring a decent slug of rain -- between one and two inches if the GFS and EURO are correct -- so the snowpack may take a healthy dent...and depending on how warm it gets ahead of the front some flooding may be a concern for parts of the region from the upcoming storm.

Going For Forty?

It's a good sign for a snowy winter when you have accomplished more than your seasonal average snowfall (at least in Philadelphia and South Jersey's case) in one storm. Considering Philadelphia has knocked out 23.2" of snow already this winter, how have past snowy Decembers shaped up historically?

Well, if you like "average" overall -- that's about how they have fared. There have been seven instances going back to 1884 where snowfall in December has exceeded ten inches. The overall recap is listed below of how the snow fared but I thought I would provide a yearly recap of how the years did overall. These winters were without regard for Nino state and are simply based on the fact it snowed 10" in the month of December.

1904 -- January and February were both rather snowy (11.6" in January, 7.9" in February), with temperatures in February over eight degrees below our current day average. There was a 8" snowfall near the end of January but the rest of the winter was filled with relatively minor events.

1909 -- January and February were both relatively seasonable but January 1910 featured 7.6" of snow on the 14th-15th.

1910 -- The remainder of this winter featured mostly mild temperatures overall with minor snowfall events.

1945 -- Very little snow, very typical temperatures. Probably the most boring post December weather of the seven years.

1960 -- The best winter of the group. January 19-20 featured 13.2" of snow and February 2-3 featured another 10.3" in a three week stretch that brought 27.9" of snow to the region. January was very cold, with a typical February in 1961.

1966 -- February 6th and 7th featured a near ten inch snowfall with a number of other minor events scattered throughout the winter. January was quite mild with a rather cold February.

2000 -- February 5th brought a surprise snowstorm that snarled roads and dumped over a foot of snow just north of Philadelphia. The city itself received 7" of snow from a snowfall on February 22nd. There was also that "storm" on March 5th, 2001. Temperatures the remainder of this winter were mild overall.

Averaging all of the snow from these seven winters together and one gets 19" of snow from here on out, which is pedestrian on its own but when you combine it with 23.2" you would get a 40" season in Philadelphia. Three of those seven seasons averaged out over 19" (1904-05 and the two winters in the 1960's). To add better news to this...I looked at Nino years (not included the chart) that have featured seven inches of snow or more in December. There have been only three years since 1950 (small data sample) where this has occurred...1957, 1963, and 2002.

The average snowfall the remainder of those winters? How about 32.3" of snow, with all three featuring double digit snowfall in February. The worst of those winters was 1963-64, which brought only 24.9" of snow to Philadelphia down the stretch. Of course this winter could totally dud out on us down the stretch but from a purely climatological standpoint our past suggests a snowy future this winter. Just how snow remains to be seen but the odds favor more.

December 22nd, 2009 Forecast

Now: A mix of sun and clouds are working through the region this afternoon as a weak disturbance passes by to our north. A few flurries have been drifting by in spots to the north of Philadelphia and a few more are possible before the day is through. Temperatures are generally in the mid 30's this afternoon regionwide.

Tonight: Skies will gradually turn back partly cloudy. It will continue to be chilly with low temperatures overnight in the teens or low 20's areawide. Northwest winds will be at 6-12 mph overnight.

Tuesday: Skies will be mostly sunny and we'll continue to experience occasionally brisk conditions as winds from the west and northwest continue at 10-16 mph. High temperatures for the first full day of winter will generally be in the mid 30's regionwide. High pressure will build in for the next couple of days and keep us relatively cool and continue our stretch of below average temperatures, slowing the overall melt of the snowpack.

The coldest night in the upcoming stretch looks like Tuesday night, where low temperatures could be in the lower teens away from Philadelphia thanks to the snowpack in place. The upcoming storm system has been slowed in computer modeling, with the arrival timed for early Friday. This may help limit some of the icing potential that I will discuss in another post later this afternoon.

Sunday, December 20, 2009

How Does One Properly Measure Snow?

Unlike the yardstick on the left (yes, I know it's upside down), measuring snow according to National Weather Service standards is a bit more arduous and not solely just dropping the ol' yardstick in (right side up, of course) to measure how much snow your backyard, patio, or front yard has. These helpful tips have been lifted from the National Weather Service and are provided as a service in case one always wanted to know how the National Weather Service liked snowfall measured.

How To Measure Snow With A Measuring Stick

Find a location where the snow appears to be near its average depth. Avoid drifts or valleys. Look for a flat, somewhat open area away from buildings and trees. Some trees in the distance may be helpful in making a wind break, preventing drifting, and thus providing for a more even distribution of the snow. Measure the depth with the snow measuring stick (aka "the common household ruler") at several locations and use an average. Traditionally ten measurements are made and the average value is the snow depth. When snow has fallen between observation times and has been melting, measure its greatest depth on the ground while it is snowing, or estimate the greatest depth. During heavy snowfall some of the actual total may be lost due to compaction of the column by the weight of the snow, during these times it may be best to estimate a slightly higher value if snow has been falling at a heavy rate for several hours since the last actual measurement. If all snow melted as it fell, you can estimate a total if you think more than a half of inch fell before melting, or report a trace for the snowfall.

Measuring New Snow On Top Of Old Snow

When fresh snow has fallen on old snow, it is necessary to measure the depth of the new snow (in tenths of inches) and the total snowdepth (whole inches). Snow boards provide the best method of taking measurements in this case. Sometimes if the old snow has settled or partially melted enough to develop a crust or to be noticeably denser than the new snow, it may be possible to insert the snow stick until it meets the greater resistance of the crust of old snow, and to use this depth as the amount of new snow having fallen.

Use of a Snow Board

Snow boards (scroll to page 4 for an illustration) are laid on top of the old snow when there is any possibility of new snow falling. Push them into the snow just far enough that the top of the board is nearly level or just above the top of the old snow. After each observation, boards should be cleaned and placed in a new location. Because of evaporation or drifting, they may need adjusting daily to assure that the top of the board remains flush with the old snow. A clean sidewalk or open cement area where there is some protection from the wind and drifting is a good alternative to using a snow board. You still need to clean an area off before the snow starts and between measurements in order to accurately measure the newly fallen snow. A piece of plywood that is approximately two feet by two feet or a flat board that is painted white will work in this capacity.

The ideal rule of measuring is every six hours and if you use a snow board (which is what the NWS likes) you should clean it off. If you use a flat surface like a patio or sidewalk, you can sweep it away as well to get an accurate total every six hours as accumulated snow will compact due to its own weight.

Weather Rewind, December 13-19, 2009

Despite a mild to warm start to the week in Philadelphia temperatures ultimately averaged out below normal this past week in Philly by about a degree. The three day mild reprieve from an overall colder than average past couple of weeks was quickly zapped by the last three days, which have averaged out to nearly ten degrees below average in the city.

We all know what Saturday's event brought upon the region but it's crazy to think just a week prior we were dealing with nearly an inch of rain during the bulk of the day Sunday, with some icing issues occurring across the northern/western suburbs of Philadelphia at the onset of the rain event. December is already the 8th wettest on record in Philadelphia with just a couple of inches to go to break the monthly record. We might not hit it, depending on what happens with the Christmas Eve/Christmas storm that modeling is depicting, but the wet trends of recent months cannot be ignored.
Nationally, milder than average temperatures held serve in the West, Midwest, and Florida during the past week.

December 21st, 2009 Forecast

Now: The sounds of shoveling and digging out are commonplace throughout the Delaware Valley in the wake of yesterday's historic snowstorm. With northwest winds adding chill to the air, it is certainly a cold day in the Delaware Valley. We'll continue to remain cold over the next few days although a storm system for Christmas may bring rain (or a mix to start) to the Delaware Valley.

Tonight: Skies will range from clear to partly cloudy and temperatures will drop into the 15-20 range across the region, perhaps colder in spots thanks to fresh snowcover. With northwest winds at 7-14 mph wind chills will range from the single digits into the low teens by morning. The wind being up tonight may help prevent a total bottoming out of temperatures in the region and be a saving grace from going a good bit colder.

Monday: Skies will be variably cloudy through the course of the day, with some sunshine to start and more clouds in the afternoon. A weak disturbance will track through the Lakes and into Pennsylvania, possibly spreading some flurries into the region during the afternoon and evening hours. I would expect the flurries to not be a big deal overall. Temperatures should top out near freezing, with a general range of 28-32 regionwide with west winds 10-15 mph adding further chill to the air.

The Week Ahead: Temperatures will be chilly compared to average through Thursday, when a storm system is poised to move towards the Great Lakes. Computer guidance as of present is suggesting a track through the Great Lakes with the main storm system although there is uncertainty about the potential for a secondary low to develop along the front as it slices through the Appalachians. Should this secondary low develop, it's possible the forecast impacts could change...right now, computer guidance is suggesting a wintry mix changing to rain on Christmas Eve night into Christmas Day. Temperatures may warm into the 40's on Friday before backing off over the following weekend although this forecast is far from certain.

Tis The Season To Break Records...

An historic snowstorm for Philadelphia and South Jersey. There really is no other way to describe it as 23.2" of snow has fallen at Philadelphia International Airport, which set several records and meaningful milestones.
  • Second largest snowstorm on record.
  • Largest December snowstorm on record.
  • Second largest one day snowfall on record at 22.5" (behind January 7th, 1996). We have had at least 0.7" since midnight so the above graphic might be adjusted upward somewhat for final total snowfall.
  • Snowiest December on record.
  • We have exceeded seasonal average (19.3") in one storm.
I love the last record and last milestone -- we broke an entire monthly total snowfall record in one snowstorm and are already above seasonal averages in snow for the winter in Philadelphia.

Here's my final revised call from yesterday morning -- my earlier calls for the region were quite a bit low and admittedly I came in too low on my previous forecasts -- as did everyone else until Friday night and Saturday morning when Hurricane Schwartz at NBC 10 correctly forecast 15-25." With that said, nobody can claim that all along they knew that you'd get 23"of snow in Philadelphia (even Hurricane was low on his initial forecast)...but many were right in talking about a potentially big storm. Historical storms are historical for a reason -- they don't happen very often and when a computer model is spitting out 17-20" of snow for a particular spot it's hard to take it seriously even within a day or two of the event taking place given past performance of computer guidance. As such, all forecasters are going to be conservative by nature and are going to start with smaller ranges and work up towards the magic number. I think by and large everyone's final snowfall forecasts from Saturday morning were pretty reasonable considering how high the numbers were looking.

A few things stood out on this storm. One, the sharp gradient in snowfall to the northwest that the NAM pretty much nailed throughout the storm. From 11" in Montgomeryville snowfall quickly dropped to 6" in Allentown to very little north of Interstate 80. The axis of heaviest snow generally centered itself on South Jersey and Delaware, which was consistent across the board on computer guidance over the past couple of days. While dry air was eating away at Saturday morning's appetizer event across Central Jersey and Bucks County snowfall did pick up in earnest later in the afternoon hours as heavy band pivoted around with the storm moving slowly up the coast, with parts of Central Jersey (along I 78) picking up nearly a foot of snow during the afternoon.

Another thing that stood out with the storm was the awesome dynamics involved as the upper air feature in the Midwest interacted with the coastal, allowing for thundersnow to blossom over South Jersey at various times during the afternoon and evening. It's rare to get thundersnow and typically allows for a very heavy burst of snowfall across spots that receive it as the dynamics responsible for convective snowfall also enhance precipitation.

Speaking specifically about my forecast, I think I was a little to slow to adjust totals upward but the final call came in pretty much where it needed to be. I was a little low in Bucks, Upper Montco, and in parts of Central Jersey (dry air issue) but came in about advertised for the rest of the region. Thanks to all of you for sending in your photos, your snowfall totals, and your comments about the storm through the day yesterday!

Updating Sunday's Weather

With the tons of snow now pulling away, some residual bands of light snow continue across parts of the region. Morning radar shows two bands of snow generally setting up across the region as of 6 AM. The first band is to the west of Philadelphia from Wilmington northwest to Lancaster. The second is to our north, from Allentown south to near Plymouth Meeting and east to the Delaware River. Both bands are slowly shifting south and southeast.

Light snow will continue off and on for a few hours in spots, possibly adding a little bit more snow to what we have already seen (especially in the band to the north of the city which seems to have a bit more life).

As we work into midday and the afternoon, some sunshine may pop out from time to time as the coastal storm responsible for today's snow pulls away. Winds will be an issue at times through the day, from the northwest at 10-20 mph through the course of the day.

By the end of the day, we should see high temperatures around 30 in what is shaping up as a cold December day.

Snowtography: Solstice Snowstorm of '09

Thank you everyone for submitting your photos for the snowstorm! I really appreciate the great pictures that were taken and thank you for some great shots that were taken during a very awesome snowfall.

All photos were submitted onto a Photobucket show and I credited everyone for their photos with name and location where applicable. Thank you so much for sending them in!

We also had a flickr link sent in from Josh Rosenberg, who took some photos last night in Rittenhouse Square. Feel free to check out his pictures as well. Jay Reed in Winslow, NJ added more photos to the mix as well.