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2020 Mid-Atlantic Severe Weather - General Thread


Kmlwx
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Among the CAMs, the NAM nest is by far the best in terms of reflectivity, although its updraft helicity signals are modest.    The other CAMs overall aren't as good.    But the environment still looks like questionable instability but fantastic low level shear, so we're still very much in the game.

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https://weather.cod.edu/text/

 

DAY 2 CONVECTIVE OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK  
1221 AM CDT SUN APR 12 2020  
 
VALID 131200Z - 141200Z  

 
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF  
EASTERN GEORGIA...THE CENTRAL AND EASTERN CAROLINAS AND VIRGINIA...  
   
..SUMMARY
 
 
SEVERE THUNDERSTORMS WITH TORNADOES, DAMAGING WIND GUSTS AND HAIL  
WILL BE LIKELY ACROSS PARTS OF THE ATLANTIC COASTAL PLAINS INTO THE  
CENTRAL APPALACHIANS ON MONDAY.  
   
..ATLANTIC COASTAL PLAINS/CENTRAL APPALACHIANS
 
 
A POTENT SHORTWAVE TROUGH WILL MOVE QUICKLY NORTHEASTWARD ACROSS THE  
OHIO AND TENNESSEE VALLEYS TONIGHT. THE TROUGH SHOULD BE POSITIONED  
OVER THE CENTRAL APPALACHIANS AT THE START OF THE PERIOD. AT THAT  
TIME, A 70 TO 80 KT LOW-LEVEL JET WILL BE LOCATED ACROSS THE  
CAROLINAS AND VIRGINIA WITH AN AXIS OF MODERATE INSTABILITY  
CO-LOCATED WITH THE JET. A BAND OF CONVECTION, POTENTIALLY SEVERE,  
IS FORECAST TO BE IN NORTHEAST GEORGIA, THE WESTERN CAROLINAS AND  
WESTERN VIRGINIA AT 12Z. OTHER STRONG THUNDERSTORMS MAY BE ONGOING  
IN THE UPPER OHIO VALLEY. AN 80 TO 100 KT MID-LEVEL JET ORIENTED  
ALONG THE WESTERN EDGE OF THE LOW-LEVEL JET WILL CREATE VERY STRONG  
DEEP-LAYER SHEAR FAVORABLE FOR SEVERE THUNDERSTORMS. THIS BAND OF  
CONVECTION IS FORECAST TO MOVE QUICKLY EASTWARD ACROSS THE ATLANTIC  
COASTAL PLAINS DURING THE MORNING AND EARLY AFTERNOON.  
 
THE MAIN QUESTION WITH THIS OUTLOOK IS CONVECTIVE MODE. THERE ARE  
TWO POSSIBILITIES. THE FIRST IS THAT THE BAND OF THUNDERSTORMS WILL  
HAVE DISCRETE ELEMENTS AND GAPS THAT WILL BE FAVORABLE FOR  
SUPERCELLS AND TORNADOES. FORECAST SOUNDINGS AT 15Z FROM SOUTHERN  
VIRGINIA SOUTHWARD INTO NORTHERN SOUTH CAROLINA SHOW LOOPED  
HODOGRAPHS WITH 0-3 KM STORM RELATIVE HELICITIES IN THE EXTREME  
CATOGORY, GENERALLY NEAR OR ABOVE 600 M2/S2. THIS WOULD SUPPORT A  
THREAT FOR STRONG TORNADOES ASSOCIATED WITH THE MORE DOMINANT  
SUPERCELLS. A WIND DAMAGE AND HAIL THREAT WOULD BE LIKELY WITH  
SUPERCELLS AND WITH THE MORE ORGANIZED LINE SEGMENTS.  
 
THE SECOND SCENARIO WOULD BE THAT A SQUALL-LINE IS ONGOING AT 12Z  
FROM EASTERN GEORGIA NORTHWARD INTO THE WESTERN CAROLINAS AND  
VIRGINIA. IN THAT CASE, WIDESPREAD WIND DAMAGE WOULD BE LIKELY. A  
QLCS TORNADO THREAT WOULD EXIST WITH THE STRONGER BOWING LINE  
SEGMENTS. HAIL WOULD ALSO BE POSSIBLE WITH THE MORE INTENSE PARTS OF  
THE LINE.  
 
AT THIS TIME, THE OUTLOOK REFLECTS THE FIRST SCENARIO IN WHICH MORE  
DISCRETE CELLS ARE PRESENT ACROSS THE WARM SECTOR AT 12Z. IN THIS  
CASE, A CLUSTER OUTBREAK OF TORNADOES WOULD BE POSSIBLE AND A STRONG  
LONG-TRACK TORNADO OR TWO CAN NOT BE RULED OUT.  
 
..BROYLES.. 04/12/2020  

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2 minutes ago, Kmlwx said:

0z HREF has ensemble mean 850mb winds of 75 knots around the area on Mon.

Even the long-range 6z hrrr (Yes, all caveats apply... end of range) shows 45-50 kt post-frontal gusts across the region. I usually find the hrrr more in line with what happens during synoptic wind events during the daylight hours. All long-range caveats aside... it looks windy tomorrow.

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LWX is suggesting some may need a run to the battery store...

(snip)
The dry slot will work its way into the area later Monday
morning and the cold front will pass through the area Monday
afternoon into Monday evening. On the edge of the dry slot and
ahead of the cold front is where stronger thunderstorms are most
likely Monday morning and afternoon. Long and curved hodographs
are seen in forecast soundings to the very strong winds aloft.
The very strong wind flow, along with strong warm and moist
advection, combined with forcing from the front and somewhat of
an enhancement in instability on the edge of the dry slot,
suggests that a broken line of showers and thunderstorms will
develop Monday morning and progress east and northeast into
Monday afternoon. Embedded supercells are possible withing the
line along with a possible QLCS feature. Therefore, damaging
winds are a concern with convection Monday along with an
increased threat for a tornado and large hail
. Localized flash
flooding cannot be ruled out, but the storms should have enough
motion to keep that threat localized.

Another threat for overnight and Monday will be gradient wind
outside of heavier showers and thunderstorms. For the
overnight, a very strong low-level jet will develop but with
warm advection and showers overhead it will be tough to mix
down. It will still turn out quite breezy with frequent gusts
around 20-35 mph for most areas. There could be gusts 40-55
knots along the highest ridge tops above 3000 feet, but did not
include these in a wind headline at this time since it will be
very isolated. However, with strong warm advection Monday
morning, the mixing layer will begin to increase, causing winds
to increase. For Monday afternoon, there will be dry advection
and weak cold advection behind the dry slot. This will enhance
the mixing layer even more. As of now, it appears that wind
gusts will be around 45-55 mph for most areas late Monday
morning through most of Monday afternoon. A Wind Advisory will
likely be needed
. Did issue a High Wind Watch for the ridges of
the Allegheny and Potomac Highlands as well as the Blue Ridge
Mountains where stronger winds are likely over the higher
elevations. Also included central Mineral, Grant, and Allegany
Counties due to a downsloping flow off the mountains that may
enhance winds in those areas. Gusts around 60 mph are possible.

One thing to note is that there may be numerous fallen trees
if these strong winds do occur, and that is because the ground
will be much more saturated due to recent rainfall
, compared to
when the ground is mainly dry during these types of gradient
wind events. (snip)
 

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Have to say - the big gap in the high wind watch products from WFOs to the west of LWX and WFOs east of LWX doesn't look great. Don't see a ton of harm going with a WATCH and ultimately going with an advisory if the higher wind progs don't end up verifying. But the whole purpose of a watch is for this reason. Could easily see gusts to the 58mph criteria. I wonder if their plan is to go with an advisory and then cover any HWW criteria winds with SVRs

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18 minutes ago, Kmlwx said:

Have to say - the big gap in the high wind watch products from WFOs to the west of LWX and WFOs east of LWX doesn't look great. Don't see a ton of harm going with a WATCH and ultimately going with an advisory if the higher wind progs don't end up verifying. But the whole purpose of a watch is for this reason. Could easily see gusts to the 58mph criteria. I wonder if their plan is to go with an advisory and then cover any HWW criteria winds with SVRs

I have a feeling the watch here will end up being an advisory, with a warning along the coasts of DE and NJ, and also the western highlands.

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The verbiage on the Day 2 synopsis is concerning considering they go with possible strong tornadoes and an outbreak and mention southern va and areas around. Not sure I’ve ever seen that in the 10 years I’ve lived here myself except maybe one time but the ingredients never came together. 

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1 minute ago, Kmlwx said:

Not sure how useful the RAP is anymore - but at range it has 80-90kts at the 850mb level at 15z tomorrow.

I know that the post-frontal winds may be strong but... though we are not expecting much of this atm pre-cold front can you imagine if we somehow got a break in the clouds late tomorrow morning with those kind of low-level winds overhead?... Of course this is more speculative than me predicting it.

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