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STORMcast Advisory 2017-07

Issued: 05Apr17

Time:  1400 EDT  

The STORMcasts shown here are intended for the students of UDHS's AGS and AES geoscience programs and are for educational/informational purposes only.  They frequently combine scientific concepts along with a sense of humor that often pokes fun at our behavior during weather events.  While intended to be informative, the STORMcasts should never be used to make life and death decisions during severe weather events.  Follow the appropriate bulletins posted by the National Weather Service and local media outlets for appropriate actions during these events.

 

Current National Weather Service advisory map as of STORMcast issuance...

FLOOD WATCH -
Thurs. thru Fri. morning for widespread 1"-2" of rain with 3" potential in areas and embedded thunderstorms.  Increased flooding potential due to recent rain events and saturated ground. 

HAZARDOUS WEATHER OUTLOOK
- For much of the region Thurs. due to potential of severe weather including severe thunderstorms and flooding rains. 

(Map courtesy of NWS)

DANGEROUS SITUATION DEVELOPING IN THE SOUTH

For the folks at UDHS (and before you ask), I did NOT intentionally plan the severe weather drill the day before we might actually get some.  But, I won't deny I enjoy the irony.

Hi everyone.  It's springtime and that means the threat for severe weather is back.  As we speak, the NWS has issued a Parrticularly Dangerous Situation alert for much of Georgia (including Atlanta) and parts of South Carolina ahead of what could be a major tornado outbreak by day's end.  In the wamr sector of a significant southern storm, ample heat and moisture are combining to destabilize the atmosphere already and it's not even late afternoon when conditions are usually at their worst.  Tomorrow, we're next.

SEVERE POTENTIAL FOR THE MID-ATLANTIC

As the storm system now in the south spins its way up the Appalachians overnight, it is expected to spin up an additional low pressure feature that will feed off of our currently mild temperatures and humidity before exploding tomorrow as the low deepens over PA.  An upper air jet is also situated to support the intensification of the surface system very close to our forecast region.

At this point, models are not exactly convergent as to the focus of the precipitation.  The NAM is focused a bit west of us along an axis through Lancaster and into the Lehigh Valley while the GFS has intense precipitation rolling through the western suburbs much closer to Philly.  NAM rainfall totals paint a swath of 2" through the metro area and wallop Lancaster with close to 3" in already fairly saturated ground.  The GFS hits the western suburbs with about 1.5"-1.75". 

Needless to say, heavy downpours are expected through morning hours with the better than average chance of embedded strong to severe thunderstorms.  These storms could come in waves as large, single rogues early in the morning while we're planted in the warm air followed by a more organized line in the afternoon as the cold front sweeps through.  Instability will not be at its highest levels in our area due mainly to the cooling brought about by the heavy rainfall so it could limit thunderstorm development but around lunchtime when the cold front forces mechiancal lifting of the air, we could see some boomers.  There is also some concern about tornado potential as significant atmospheric shear is also expected.  If they occur, I think they'll be small and short-lived but the potential is still there.  I think things will be much worse in the south within a few hours.  

TIME FRAME

Rain should develop by daybreak and conditions will be noticeably humid and warm.  Things could start of showery but an increase in intensity won't take long.  By 9-10 AM, we should be looking at heavy rain and thunderstorms.  By lunchtime, we should be looking to the west and south for a growing line of heavy thunderstorms that could push theoruhg the region by early afternoon.  Once that occurs, things should calm down but residual storms could be in the area all the way into the evening.

For the moment, I'm still not sold on the severe thunderstorm portion of the forecast but the flooding rain seems to be a near certainty so I see no reason to doubt 1.5"+ of rainfall.  The severe weather threat is focused a bit further south than Philly but it's still a possibility.  Since the Flood Watches are already up, the thing to watch for from the NWS will be the issuance of a Severe Thunderstorm Watch or a Tornado Watch.  Interestingly enough, they will probably go up in the morning instead of later in the afternoon if they go up at all so we'll have to sit and wait on that for now.  If these are posted, we could be in for a wild ride tomorrow during the work day.  

Regardless of what happens severity wise, it'll likely be a messy morning commute so be careful and keep an eye skyward.

rs

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