Sunday, August 02, 2009

Updating Sunday's Heavy Rain & Potential Severe

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Update, 12:30: Here are the radar estimates for rainfall since Midnight. Round two, the most recent one, dropped about 1-1.5" of rain in a 30 minute timeframe over the Schuylkill in the flooded areas. However, the first round of thunderstorms that marched through this morning dropped an additional 1.5-2" of rain (there were earlier flood advisories out in case you remember back to this morning's storms). The combination of these two rounds yielded upwards of three inches of rain across Chester, Montgomery, Delaware Counties and parts of Philadelphia...particularly around the flooded areas of I-76. There's a very sharp cutoff in rainfall amounts (literally at City Line Avenue) between the 2-3" of rain and lighter amounts.

Update #2, 12 Noon:
Another shot of the flooded Schuylkill Expressway. Eastbound 76 is the side of the highway that's flooded at the moment.

Update, 11:45 AM: 76 still dealing with backed up traffic from flooding but things should slowly improve later on as the heavy rainfall has moved eastward from this area. However, as of the time posted the road is still closed in spots near the Curve. Severe T-Storm warnings now out for Bucks and Burlington Counties. BTW, thanks PennDOT for linking your traffic cameras up online. Impressive shot of the flooded Schuylkill Expressway just east of the Curve.

Update, 11:35 AM:
If there were any other picture that shows you why you shouldn't drive through a flooded roadway, let this image be forever in your mind.

Around two inches of rain has fallen in this section of the Schuylkill in the last hour. Please, turn around...don't drown.

Update, 11:25 AM:
76 West is now flooded as well. Some real "geniuses" at work going through that high water on the westbound side (right side of the camera shot). Flooding is just east of the Conshohocken Curve (you can see the curve at the background of the PennDOT camera pic).

Update, 11:20 AM:
Issues @ the Conshy Curve on 76 East. Flooding on the eastbound side of the highway results in problems for commuters going into Philadelphia.

Update, 11 AM:
Severe T-Storm Warning for Philadelphia, Delaware County and Camden/Gloucester Counties west of I-295 until Noon. Up in my backyard, it's raining hard with some pretty decent thunder/lightning. About one inch rainfall rates in the last hour with the band that I'm sitting in. The warning that Philadelphia is under is with the main line and in particular a stronger cell down by Newark (DE) that was showing signs of rotation earlier on doppler radar. This one is dropping hail as it moves east northeast. Should work through parts of Delaware County, Philadelphia, and over the river into South Jersey over the next hour to 90 minutes as the main line of storms moves through. Also, storms in Delaware County in front of this line are acting up and doppler is showing signs of hail and some strong winds as they move into North Philadelphia and Lower Montco.

Update, 10:30 AM: A line of heavy rain and thunder is moving in from the west and southwest. Flood warnings are out until 3 PM for Philadelphia and Southeastern PA suburban counties, until 1 PM for Berks and Lehigh Valley. Rainfall estimates have suggested about an inch of rain in the last hour across parts of Lancaster and Berks Counties with this line as it has marched through those areas, with the potential of 1-2" of rain with this line as it moves through.

This line has developed more to the south (into Maryland) and has tracked more east northeast over the last hour compared to its general northeast track earlier this morning. The main thrust of severe potential over the next hour is generally along and south of the PA Turnpike in our region, and along and south of I-195 in New Jersey from midday on. In Philadelphia, the heaviest rainfall looks to fall from 11:30-1:30 with this back line.

In front of this main line are scattered thunderstorms that are developing around the city and lifting northeast. These will impact the city first, with the second line moving in not too long afterwards.

Update, 8:30 AM: It's going to be one of those days -- my initial post below needed several edits as I was caffeine challenged when I wrote it (that's been alleviated) but moreso because of the volume of thunderstorm activity outside in the Mid Atlantic. I'm watching one cell down by Hanover in York County, which is already a pretty nasty storm given it's 8:30 AM. This cell is potentially dropping one inch hail and has some nasty winds with it. This storm is moving northeast and will impact York, Elizabethtown, and track towards Berks County in the next 60 minutes or so. While this cell itself will stay west of Philadelphia -- it's an indication of the amount of energy that this disturbance has...something the computer guidance was not picking up very well in the run up to today.

Also, flood warnings have been out for a few locations this morning, taking various flavors and varieties based on the type of flooding or ponding of water that's taking place.

We're already seeing thunderstorms develop across parts of the region this morning as another storm system moves on in. Thunderstorms are already dropping heavy rains in Chester County -- doppler radar estimates on the left from the last hour suggest two inches of rain falling near Exton, Downingtown, and through Berwyn. Much like the last three events, thunderstorms will be more numerous in the afternoon but unlike the last three events, we will see little if any sunshine, unless one is down near the coast.

Today's atmosphere is not quite as moist as it was on Friday (believe it or not)...however, it is sufficiently moist to support heavy rainfall -- such as Chester County already this morning. We're still dealing with a pretty humid setup even though the moisture level is a bit lower and as such thunderstorms could leave a pretty bad taste in our mouths as rainfall will be heavy at times in places.

As more thunderstorms develop through the day and the main swath of rain that is currently in Western and Central Pennsylvania moves into the region in the next few hours, thunderstorms could drop additional heavy rains in locations. Some areas could see three inches of rain from today and as a result of that a flash flood watch is out for Philadelphia, the I-95 corridor, and points north/west.

We are back under a slight risk for severe weather as upper level dynamics will track relatively close to the region through the course of the day. Given cloud cover in place today, we may not see as much widespread severe weather and I tend to think areas east of I-95 have a better shot at getting nailed compared to areas to the west. New Jersey and Delaware could end up under the gun for severe thunderstorm activity this afternoon.

Temperatures today will not be as warm as our past severe weather event days -- highs should generally be near 80 northwest of Philadelphia to the mid 80's cover and an earlier start time on the thunderstorms will work against any run towards 90 degrees today.