This Afternoon's Severe Discussion

Update, 5:30 PM:   The "line" of thunderstorms is now just two thunderstorm cells -- one across Central Jersey and about to hit the Atlantic, with the second cell down near Wilmington in Delaware.   The Central Jersey cell did work through the Trenton area and had a severe warning with it for a time, with the cell in Wilmington also warned across Chester County earlier on this afternoon.   At this point, the storm threat is generally down to the Wilmington cell as dry air is beginning to approach the region.  Dewpoints drop from 71 in Reading to 64 in Harrisburg to the 40's in Western Pennsylvania, with the "trough" that was going to be the trigger for thunderstorm development now generally overhead and working east.

Update, 4:30 PM:   We're seeing a bit of a regenesis in the storm activity, especially in Bucks into Mercer Counties where a severe warning is now out.   Scattered activity is continued on back to the west into Chester County but the storms farther west, at this point, aren't as strong.

Update, 3:30 PM:   The "line" I talked about earlier has fallen apart a good bit -- looking at some of the atmospheric data one would think the potential would really exist -- there is a lot of atmospheric juice and instability.  Dynamics aloft, however, aren't as favorable.  The upper level jet that was modeled to be overhead today is instead a bit farther north, with lighter winds aloft at the surface and a less optimal setup for severe around here.  

Radar is showing a bit of a weak trough/boundary line across the northern/western suburbs.   Nothing has popped from it yet but if anything does fire development could be quick as it may be a focal point for localized thunderstorm development later on.   Unfortunately, things are "stuck" because of a less than favorable upper level of the atmosphere as winds aloft may not be strong enough to help aid in storm development.

Update, 2:45 PM: A couple of smaller storms have started to pop around the city and suburbs. Nothing severe at this point in the city or suburban counties (not including Berks/Lehigh, which are both under warnings until 3:45 PM) but the earlier mentioned line of storms that developed along the lee trough have gotten a bit better organized and now generally extend from Stroudsburg to Allentown to Reading and farther south and then west. We'll probably see that line continue to fill in as it moves east and more storms continue to pop ahead of the line in the next hour.

Lots of stuff to talk about this afternoon!   Not only do we have the heat warning but we have two severe weather watches out there.   First, a Severe Thunderstorm Watch until 5 PM for Central into Northeast Pennsylvania, with a Tornado Watch out until 8 PM for the Philadelphia area on north.

A supercell thunderstorm fired up across Central Pennsylvania and zipped east into Northeast PA early this afternoon.  That cell will pass through North Jersey and towards the northern NYC suburbs over the next couple of hours.   Our development is setting up out near Harrisburg, along the "lee" trough that develops along and just east of the Appalachian Mountains, where you get a bit of a dry line between slightly cooler/less humid mountain air and the moist, humid air that can pool across Central and Southeastern Pennsylvania.   Thunderstorms are starting to fire up and will gravitate east over the next few hours, with the Philadelphia region's best chance for thunder generally between 3 and 6 PM, possibly a bit later than that to our east.

Damaging winds are more likely than tornadoes but a tornado can't be ruled out -- the Storm Prediction Center has Philly and points north in a 5% chance area whereas damaging wind has a 30% chance of verifying within 25 miles of one's spot.   Regardless, expect storms to continue to pop over the coming hours and work towards the region, with our best chances of getting hit after 3 PM.

It also would not surprise me if some sort of watch gets issued for Delaware, Maryland, and the rest of South Jersey within the next hour or two.

Update, 2:30 PM:   Severe Thunderstorm Watch now out for South Jersey and Delaware (basically the rest of the region not under the Tornado Watch) until 10 PM.

We'll have updates within this post as the afternoon progresses.

More:   Current Weather Page

2 Areas of Ongoing Severe Weather

10:40 p.m. UPDATE:

The Tornado Watch has been cancelled a few minutes early for the region that was under it.

At this hour, nearly every county in our coverage area has been placed under a flood warning due to the excessive rainfall. Flooding is especially dangerous at night...turn around and don't drown. The very heavy thunderstorms have just about moved off the coast.

For the next two hours, I will be watching the few line-segments that have developed ahead of the actual cold front for any potential t-storms with at least gusty winds--if not spotty damaging winds.






I am watching 2 bands of convection at the moment.

One across the eastern half of our region and one in the western suburbs. Both areas have prompted numerous severe thunderstorm and flood warnings.

In the line in the western suburbs, a tornado warning was issued for both York and Lancaster Counties. This line has produced numerous reports of wind damage in Central Pennsylvania and will continue to move into portions of our region.

The tornado watch for the southern portion of our region remains valid until at least 11:00 p.m.

Tornado Watch Issued for Parts of the Region

9:00 p.m. UPDATE: Tornado warning will expire for Lancaster County...line moving east-northeast though has been responsible for several areas of wind damage in Lancaster and York Counties. Another tornado could develop at anytime. Berks County area is next in line!

Numerous flood warnings issued...more details shortly...watching tornadic cell approachingYorkCounty .......update at 825....The tornado watch continues (6:45 p.m. update)



In a nut shell, the latest analysis shows an area of low pressure over Virginia with the pre-frontal convection moving northeast ahead of the cold front. While current thunderstorms are reinforcing the cold air at the moment, strong ascent and combined pressure falls will allow the low to eject towards our region overturning the atmosphere to some degree and adding additional instability. Moisture trajectories will increase in response allowing for the deep-layer shear to increase further which will result in better updraft potential. Damaging winds will remain the primary threat, but the isolated tornado threat remains very possible with the low-level shear in place and only increasing for the next several hours.



Thunderstorm Wind Gusts to 70 MPH, Isolated Tornadoes, and Hail to 0.50” in diameter are possible in the watch area.

A tornado watch means conditions are favorable for the development of tornadoes, damaging winds, and or large hail.

Strong pressure falls are becoming clearly visible over Eastern Pennsylvania and Western New Jersey. I wouldn’t be surprised to see this shift northeastward throughout the evening. Without sunshine today, instability is modest at best. However, the jet setting up over the region and heavy thunderstorms will move in from the west and drag the strong winds to the surface with an increased risk for damaging wind gusts. In addition, vertical shear is quite impressive and the strongest thunderstorms may cause some spin-ups which may result in isolated tornado development in our region. Numerous severe thunderstorm warnings and tornado warnings have occurred to our south and west.

The End of This Warmth

Update, 7 AM:   The watch in Pennsylvania have been dropped as the band of rain and wind have moved out of the state and into New Jersey.   No warnings are out at 7 AM but the threat for a brief strong wind gust and a 15 minute shot of heavier rain remain.   Some of the stronger dynamics associated with the storm line have weakened and the line is not quite as potent as it was a couple of hours ago.

Update, 5:05 AM: Severe Thunderstorm Watch issued until 10 AM for the Philadelphia region (counties in the "red" color on the map below).   The highest potential for gusty winds are in the Philadelphia region, Delaware (under a tornado watch until 10 AM), and New Jersey over the next two to three hours as the storms begin to march east.   The orange shading down by Wilmington and in Chester County are for severe thunderstorm warnings that are out until 6:15 AM.

Tornado Watches to our south, temperatures near 70 in the Delaware Valley at 5 AM, and a squall line to end our recent Spring Fling.   Aw, volatile weather in Spring...there's nothing like it in Philadelphia.   The southern parts of the region are waking up to a tornado watch until 10 AM (Delaware), with a gusty line of rain and thunder marching steadily east and northeast through the Mid Atlantic this morning.   This line should begin to work into the western suburbs over the next 30-45 minutes and hitting Philadelphia in the 6 AM hour, with South Jersey getting hit by 7:30.   Storms could gust to over 50 mph and a weak tornado could spin up due to some limited wind shear within the thunderstorms.   While, for now, the Philadelphia area is not under a watch we do run the risk of getting in on some of the stronger wind gusts with these storms as they are nudging northeast along the eastward moving front.

These storms will impact the region this morning and will slap the region with a return to reality as temperatures will fall from near 70 to the upper 40's to around 50 degrees by around midday, with temperatures only nudging slightly higher late this afternoon with some sunshine possibly returning towards dinner.  After the squall line crosses the region, a period of rain will persist through midday before ending.  The region should see about a half inch, with some isolated higher rainfall totals that could approach an inch of rain, from this morning's storms.

You can track the progress of the storms across the region on our current weather page.