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2020 Mid-Atlantic Severe Weather - General Thread


Kmlwx
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Afternoon AFD from LWX sounds good

A cold front associated with the upper-level trough will
approach this afternoon before moving into the region tonight.
A westerly flow around the subtropical high has ushered in hot
and humid conditions. The heat and humidity has led to an
unstable atmosphere. Latest guidance shows around 1500-2500
J/KG of MLCAPE developing overhead.

Shortwave energy moving through Pennsylvania along with a
surface trough out ahead of the cold front will pass through
the area late this afternoon into this evening (generally
between 4 PM and 9 PM from northwest to southeast). A
downsloping westerly flow ahead of this system does diminish
confidence some regarding convection. However, there is a bit
of a theta-e ridge at the low-levels ahead of the cold front
that will be advecting into the area ahead of the surface
trough. Also, weak convergence with the surface trough and
boundaries from convection upstream should be enough to cause
scattered to numerous showers and thunderstorms during this
time. Latest hires guidance has trended for this to occur as
well, and that makes sense given the increasing activity
upstream over Pennsylvania.

There is also a chance for a bowing segment with more
widespread wind damage during this time. This is because DCAPE
will be quite high (1000-1500 J/KG), low-level lapse rates will
be steep, and there will be a stronger flow aloft. The best
chance for this to occur may be where CAPE is highest and there
is less of an influence from the downsloping flow (over the
Washington/Baltimore Metro areas into southern MD and the
Virginia Piedmont).

The flood threat appears to be low due to faster storm motion.
However, should a bowing segment develop, there may be a
localized flash flood threat from training convection on either
side of the line.

Convection should diminish behind this system later this evening
and overnight. Dry air advection should keep any fog limited in
coverage.

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6 minutes ago, JakkelWx said:

Storms firing in SE PA are going to better organize and I'll likely get something this evening

Pretty good stuff here from Mt Holly-

At 250 mb a subtropical jet was noted around KWAL with another ~90 kt speed max located over western Ontario. The subtropical jet is forecast to re-orient and center over the NC/ VA border. This would place the DELMARVA in a Left Front Quadrant (LFQ) or under upper level divergence. The northern stream jet is forecast to propagate east through the base of the trough axis and center over northern NJ. This would place central and southern NJ in a Right Rear Quadrant (RRQ), or upper level divergence. The jet structure isn`t completely coupled, but some interaction appears likely. At 500 mb strong height falls are occurring over central Quebec (~100 m) with mostly weak height falls across central NJ (~30 m). The overall idea is for this wave to continue to amplify as it digs southeast. The primary DCVA will be located over New England, and the northern Mid- Atlantic States. At 700 mb a weak wave was noted across the Great Lakes region and is forecast to continue to dive southeast towards WV and VA. Overall this wave feature appears to track too far south and west of the region to affect our area. Earlier this morning there was some precipitation that tried to move into the region from the west, but quickly fell apart. This has allowed for the entire CWA to be under mostly full sun for the entire day. As a result the entire area has destabilized, with the greatest destabilization being across the DELMARVA with ML CAPE values around 2000 J/kg. A surface cold front is currently located across west/central NY into western PA and is heading southeast. The orientation of the cold front will slowly transition from SW/NE to more of a W/E orientation. Forecast soundings continue to advertise mostly unidirectional tropospheric flow with effective bulk shear values around 25 kts. The latest 18z KWAL sounding was slightly different than initially expected with a descent amount of dry air in the 300/500 MB layer. Lapse rates in the 500 to 700 mb layer were around 6.6 degrees C though, with DCAPE values around 1000 J/kg. The general idea is for convection currently in central PA to continue to head southeast towards MD and the DELMARVA. The anvil storm relative flow vectors with these storms is easterly, and this can clearly be seen on visible. As the anvil debris continues to spread east, cold pool reinforcement and amalgamations appear likely. This means the convection will likely continue to take on a more linear appearance. The primary threat with these storms will be damaging winds.

 

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20 minutes ago, Baltimorewx said:

This watch definitely isn’t warranted for most of the area. Big time bust on that enhanced risk today

I have a hard time getting excited about severe. I've said it many times. I've seen actual severe weather here like twice in my 41 years. We do better with slow moving flooders with lots of lightning. 

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22 minutes ago, Baltimorewx said:

This watch definitely isn’t warranted for most of the area. Big time bust on that enhanced risk today

Storms dropping southward to the east, storms dropping southward to the west... nada in between.

Mostly what the latest runs of the mesos were advertising.

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