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Kmlwx

2020 Mid-Atlantic Severe Weather - General Thread

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After the surprise February tornado outbreak, it would be fitting to bookend 2020 with another surprise event.

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5% wind

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...NC/VA... Water vapor imagery shows a subtle shortwave trough over middle TN/northern AL moving across the southern Appalachians. This feature will begin to affect parts of VA/NC later this afternoon with slightly strengthening low/mid level winds and vertical shear. Scattered thunderstorms are expected to affect this region through the day, with the increasing shear likely resulting in a few weakly rotating storms. Forecast hodographs are not particularly favorable for tornadoes, but one or two of the storms could become sufficiently organized to produce locally gusty/damaging winds.


 

 

 

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image.png.9cc965946d4034cc227c08cabd487c6a.png                                                  Another high-shear low cape event...  This is the 14z HRRR

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We've been Slighted in much of the region east of the mountains.

..LOWER GREAT LAKES TO MID-ATLANTIC/NORTHEAST    

SCATTERED THUNDERSTORMS ARE ONGOING ACROSS SOUTHEAST LOWER MI INTO   WESTERN OH WITH A HISTORY OF WIDESPREAD STRONG TO ISOLATED SEVERE   WIND GUSTS. THIS INITIAL ACTIVITY MAY STRUGGLE TO FURTHER INTENSIFY   OWING TO BOUNDARY-LAYER DEW POINTS IN THE LOWER 50S WITHIN A NARROW   SPATIAL CORRIDOR AHEAD OF THIS CONVECTION. IT SHOULD EVENTUALLY   OUTPACE THIS NARROW WEDGE OF SURFACE-BASED INSTABILITY AS IT SPREADS   EAST/NORTH OF LAKE ERIE LATER THIS AFTERNOON.    

A MORE PRONOUNCED SQUALL LINE IS EXPECTED TO DEVELOP AROUND 21-23Z   FROM CENTRAL NY TO CENTRAL VA AND SWEEP EAST-NORTHEAST ACROSS THE   NORTHEAST AND NEW ENGLAND COAST THROUGH EARLY TONIGHT WITHIN AN   INTENSE SHEAR/MINIMALLY BUOYANT ENVIRONMENT. THIS SQUALL SHOULD   DEVELOP AS THE ROBUST COLD FRONT IMPINGES ON LOW-LEVEL MOISTURE   ADVECTING NORTH FROM THE CAROLINAS/SOUTH ATLANTIC COAST. BETWEEN   00-03Z, MID 50S BOUNDARY-LAYER DEW POINTS SHOULD REACH AS FAR NORTH   AS EASTERN PA INTO SOUTHERN NEW ENGLAND ALONG THE TRACK OF SECONDARY   CYCLOGENESIS. WHILE LOW-LEVEL LAPSE RATES WILL PROBABLY REMAIN   WEAKER RELATIVE TO THE LOWER GREAT LAKES REGION, THE COMPARATIVELY   RICHER LOW-LEVEL MOISTURE SHOULD COMPENSATE. IT IS PLAUSIBLE THAT   LIGHTNING GENERATION MAY BE CONFINED TO AREAS FARTHER NORTH AMID   SCANT ELEVATED BUOYANCY AND GREATER LARGE-SCALE ASCENT.   NEVERTHELESS, THE PRESENCE OF A LOW-TOPPED SQUALL LINE AMID 50-60 KT   925-MB WINDS SUGGEST STRONG TO SEVERE WIND GUSTS ARE POSSIBLE EVEN   IN THE ABSENCE OF LIGHTNING. AS SUCH, THIS REGION HAS BEEN UPGRADED   TO SLIGHT RISK.    

..GRAMS/DEAN.. 11/15/2020  

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There is zero doubt that a convective line will move through the region later today.    I've been skeptical about the potential for winds to mix down, but given the reports coming out of Ohio, I guess I can't argue with the SLGT.

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Latest from Mount Holly-

A cold front is surging through the Ohio Valley, where a convective band has developed. With 50-70 kt low-level winds, the convection has easily mixed severe-caliber wind gusts to the surface in portions of Indiana and Ohio late this morning, and convection-allowing models (CAMs) indicate maintenance of this convective band all the way to the East Coast through the evening hours. Meanwhile, in advance of the front, strong southerly flow has developed, with diabatic heating sufficient to mix these winds to the surface. Widespread gusts of 25 to 40 mph should be expected during the afternoon across the area, with locally stronger gusts at the coast. Current wind advisory looks sufficient in accordance with this threat (and with the subsequent post-frontal winds during the late evening; see below). There should be a relative lull in wind gusts after 4 pm as diabatic heating wanes (and before the front arrives). Based on CAM guidance, the convective band should generally reach the area in the 22z to 03z time frame. Strong wind gusts are likely in association with downward momentum transfer via the precipitation-cooled downdrafts. With winds aloft in the 40-60 kt range, expect decent coverage of strong to damaging gusts. SPC has increased the Day-1 convective outlook to a slight risk, which appears quite reasonable. Given the impressive dynamics of the system, shear is off the charts (0-6 km bulk wind difference 80+ kt). Combined with strong lift along the front (in the virtual absence of ambient instability), convective organization is likely to be maintained through the CWA during the evening hours. Little or no lightning is expected with the convection given the unfavorable buoyancy profiles, but this most certainly will not limit the wind threat posed by the convection.

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6 minutes ago, losetoa6 said:

Squall line looks decent atm nearing HGR

Hrrr intensifies the line as it comes east 

It has the precip max over my yard ofc lol.

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Big polygons showing up - typical with this kind of a setup to warn essentially the entire line - regardless of radar velocity it seems. Line looking nice - especially the area near the Mason-Dixon line. It's too bad we don't go right from severe season into blizzards. Imagine having something to track almost every week all year. 

Dec-Mar = Winter storms
Apr-Aug = Severe thunderstorms/wedges
Sept-Oct = Less severe but still some and tropical
Nov-Dec = Synoptic wind rippage

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1 minute ago, MN Transplant said:

Interesting text on the SVR behind the line:

At 519 PM EST, gradient winds from a passing cold front will 
continue to produce sporadic gusts around and above 60 mph.

Yeah the storm itself sure wasn’t severe. I was thinking if it’s Orange it must be severe these days. Lasted less than two minutes

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6 hours ago, A777 said:

image.png.9cc965946d4034cc227c08cabd487c6a.png                                                  Another high-shear low cape event...  This is the 14z HRRR

Hmmm looking pretty accurate right now.

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SEL1

   URGENT - IMMEDIATE BROADCAST REQUESTED
   Severe Thunderstorm Watch Number 511
   NWS Storm Prediction Center Norman OK
   530 PM EST Sun Nov 15 2020

   The NWS Storm Prediction Center has issued a

   * Severe Thunderstorm Watch for portions of 
     District Of Columbia
     Delaware
     Central and eastern Maryland
     New Jersey
     Southeastern Pennsylvania
     Northeastern Virginia
     Coastal Waters

   * Effective this Sunday afternoon and evening from 530 PM until
   1100 PM EST.

   THUNDERSTORM WIND GUSTS TO 70 MPH...and DANGEROUS LIGHTNING ARE
   POSSIBLE IN THESE AREAS.

   The severe thunderstorm watch area is approximately along and 65
   statute miles east and west of a line from 25 miles north northeast
   of Allentown PA to 50 miles southeast of Washington DC. For a
   complete depiction of the watch see the associated watch outline
   update (WOUS64 KWNS WOU1).

   PRECAUTIONARY/PREPAREDNESS ACTIONS...

   REMEMBER...A Severe Thunderstorm Watch means conditions are
   favorable for severe thunderstorms in and close to the watch area.
   Persons in these areas should be on the lookout for threatening
   weather conditions and listen for later statements and possible
   warnings. Severe thunderstorms can and occasionally do produce
   tornadoes.

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