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2021 Mid-Atlantic Severe Weather - General Discussion


Kmlwx
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Doesn't usually happen around here

Short Term Forecast
National Weather Service Baltimore MD/Washington DC
529 PM EDT Tue Sep 28 2021

VAZ038-039-050-282200-
Greene-Madison-Orange-
529 PM EDT Tue Sep 28 2021

.NOW...

At 529 PM EDT, Doppler radar indicated a strong thunderstorm near
Stanardsville, or 7 miles southwest of Madison, moving southeast at
15 mph.

Gusty winds of 40 to 50 mph and small hail are possible with this
storm. These storms have a history of producing hail that covers the
ground. Be alert for slippery travel conditions.

Locations impacted include...
Orange, Stanardsville, Montpelier Station, Madison Run, Pratts,
Rochelle, Newtown, Aroda, Madison Mills, Montford, Old Somerset,
Shelby, Dawsonville, Twymans Mill, Wolftown, Somerset, Hood, Locust
Dale, Burtonville and Rapidan.
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Severe Weather Statement
National Weather Service Baltimore MD/Washington DC
548 PM EDT Tue Sep 28 2021

VAC003-079-137-282215-
/O.CON.KLWX.SV.W.0531.000000T0000Z-210928T2215Z/
Orange VA-Albemarle VA-Greene VA-
548 PM EDT Tue Sep 28 2021

...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 615 PM EDT
FOR SOUTHWESTERN ORANGE...NORTHEASTERN ALBEMARLE AND SOUTHERN GREENE
COUNTIES...

At 547 PM EDT, a severe thunderstorm with a history of producing
large hail covering the ground was located near Free Union, or 11
miles north of Charlottesville, moving southeast at 20 mph.

HAZARD...Golf ball size hail.

SOURCE...Trained weather spotters.

IMPACT...Hail is large enough to dent and damage vehicles. Some
         damage to roofs, siding, and windows is possible. Hail
         accumulating on roadways may result in very slippery travel
         conditions.

Locations impacted include...
Hollymead, Ruckersville, Boonesville, Eheart, Barboursville, Stony
Point, Advance Mills, Saint George, Earlysville, Amicus and Shady
Grove.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

For your protection move to an interior room on the lowest floor of a
building.

&&

LAT...LON 3825 7863 3827 7855 3820 7822 3809 7835
      3815 7851
TIME...MOT...LOC 2147Z 293DEG 16KT 3820 7846

THUNDERSTORM DAMAGE THREAT...CONSIDERABLE
HAIL THREAT...RADAR INDICATED
MAX HAIL SIZE...1.75 IN
WIND THREAT...RADAR INDICATED
MAX WIND GUST...<50 MPH
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Severe Weather Statement
National Weather Service Baltimore MD/Washington DC
549 PM EDT Tue Sep 28 2021

VAC047-137-282215-
/O.CON.KLWX.SV.W.0532.000000T0000Z-210928T2215Z/
Orange VA-Culpeper VA-
549 PM EDT Tue Sep 28 2021

...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 615 PM EDT
FOR NORTHEASTERN ORANGE AND SOUTHEASTERN CULPEPER COUNTIES...

At 549 PM EDT, a severe thunderstorm was located 11 miles southeast
of Culpeper, moving southeast at 15 mph.

HAZARD...Two inch hail.

SOURCE...Radar indicated.

IMPACT...Hail is large enough to dent and damage vehicles. Some
         damage to roofs, siding, and windows is possible.

Locations impacted include...
Gold Dale, Raccoon Ford, Mine Run, Lake Of The Woods, Burr Hill,
Locust Grove and Flat Run.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

For your protection move to an interior room on the lowest floor of a
building.

&&

LAT...LON 3837 7809 3838 7798 3840 7787 3838 7768
      3824 7782
TIME...MOT...LOC 2149Z 301DEG 15KT 3832 7791

THUNDERSTORM DAMAGE THREAT...CONSIDERABLE
HAIL THREAT...RADAR INDICATED
MAX HAIL SIZE...2.00 IN
WIND THREAT...RADAR INDICATED
MAX WIND GUST...<50 MPH
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  • 2 weeks later...
18 hours ago, yoda said:

Saturday maybe?  High shear low instability event possible 

      Certainly possible.     Looks likely that there will be a line of forced convection along the cold front in the late afternoon.    Deep layer shear will be robust, but as you note, the instability is the question mark.

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Nothing has really changed.    Instability is going to limit the severe threat, but a forced line of convection will sweep across the area during the early to mid afternoon hours on Saturday.    Lightning threat is limited, but I would still not be surprised if some healthy gusts accompany the line.

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This is just NW of our area, but worthy of mention. Last night there was a burst of tornado warned cells in a line. Appears that so far, 6 confirmed tornadoes with another 5 surveys planned by NWS Pittsburgh. 

 

SPC meso'd the activity (https://www.spc.noaa.gov/products/md/md1882.html), but I gotta be honest, it was short sighted to not box this. Nocturnal tornadoes in October are rare in PA and SPC / EMs / NWS et al are lucky no one was killed. Really disagree with the decision here.

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6 hours ago, yoda said:

Monday severe threat... SLGT risk up on Day 3 

        SLGT for the southwest part of the area and MRGL for the rest.     The question will be how warm it can stay after dark.    Clouds and southerly winds should keep temps up through the evening and overnight hours, but it will still be cooling, and instability will be limited.     The better threat is definitely well west and southwest of our area, but a line of storms (or heavy showers) well after dark with some wind potential is probably worth the threats SPC has assigned.

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On 10/23/2021 at 10:11 AM, high risk said:

        SLGT for the southwest part of the area and MRGL for the rest.     The question will be how warm it can stay after dark.    Clouds and southerly winds should keep temps up through the evening and overnight hours, but it will still be cooling, and instability will be limited.     The better threat is definitely well west and southwest of our area, but a line of storms (or heavy showers) well after dark with some wind potential is probably worth the threats SPC has assigned.

       This post didn't age well, as the timing now appears to be much faster.     If we toss the anemic, outlier NAM Nest solution, a large area of convection should be approaching from the southwest in the early evening with perhaps a few lead cells out ahead racing south to north.     Deep layer shear is good but is mainly due to very fast winds at the higher levels;  the combination of weaker shear in lower levels and limited instability will likely mitigate the threat in the DC/Baltimore area, although an earlier arrival would open the door to a slightly higher threat.

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LWX with a great writeup this morning about today's threat

 

NEAR TERM /THROUGH TONIGHT/...
The key players on the synoptic map early this morning are a
warm front arcing west-to-east across southern Pennsylvania. Off
to the west, GOES-16 water vapor imagery shows a potent cyclone
spinning over central Illinois. The attendant cold front moving
through central Indiana and western Kentucky features relatively
cold cloud tops and plenty of lightning activity. This system is
set to embark on the Mid-Atlantic region later this evening and
into the night. Expect locally heavy downpours, gusty to possibly
damaging caliber winds, an isolated tornado, and a marked cool
down into the following day.

Relatively benign weather is being observed at 07Z with a few
high clouds streaming overhead. Otherwise, quiet conditions are
anticipated through the early afternoon. Mild southerly flow is
supporting above average temperatures with mid 50s to low 60s
being rather widespread. Throughout the day, continued warm
advection and rising heights will allow highs to soar into the
mid/upper 70s. With 850-mb temperatures progged to reach 15C, a
couple of low 80s readings are not out of the question. However,
all of this warmth will disappear as a strong cold front looms
upstream across the Ohio Valley.

The scenario which unfolds is somewhat complicated which lends
itself to some uncertainty. First, in addition to the system
ejecting out of the Midwest, a second system lifting out of the
southeastern U.S. will also play a role in the pattern.
Energetics from the southern stream impulse are forecast to move
toward southern Maryland during the late afternoon while
cyclogenesis unfolds off the Carolina coast. The shortwave may
spawn some showers east of I-95 in advance of the main cold
front. Forecast soundings ahead of the expected frontal
convection are somewhat erratic, possibly owing to the pre-
frontal activity. Vertical profiles show more of a veer-to-
back-to-veering wind field which is not terribly conducive to
supercellular structures. However, deep-layer shear of 40 to 50
knots is substantial enough to keep a severe thunderstorm risk
in place. This is despite the limited instability of 250 to 500
J/kg, locally higher in spots.

Based on the recent high-resolution model suite, a north/south
oriented line of showers and thunderstorms should form along the
cold front. These will race eastward from the Shenandoah Valley
toward the I-95 corridor in the 22Z-02Z window before reaching
the Eastern Shore by after midnight. The main threat will be
damaging winds as higher momentum air gets mixed down to the
surface. Cannot rule out an isolated tornado, but such an
occurrence would be brief. Heading into the night, some residual
showers should linger as the shortwave passes overhead. Aside
from the Alleghenies which will be in the upper 30s to low 40s,
mild conditions continue as the colder air will yet to arrive.
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   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Mon Oct 25 2021

   Valid 251300Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
   CENTRAL/SOUTHERN APPALACHIANS AND MID-ATLANTIC STATES...

   ...SUMMARY...
   Thunderstorms capable of damaging winds and possibly a tornado risk
   are expected from the southern and central Appalachians into the
   Carolinas and Mid-Atlantic region, mainly this afternoon and
   evening.

   ...Upper Ohio Valley to the Mid-Atlantic States/Carolinas...
   The remnants of last evening's severe storms across the
   Ozarks/Middle Mississippi Valley grew upscale and persist early this
   morning in a diminished intensity from Ohio south-southwestward into
   eastern portions of Kentucky and middle/eastern Tennessee, parallel
   to but ahead of an eastward-moving cold front.

   The parent shortwave trough will continue eastward over the
   Midwest/Ohio Valley toward the central/northern Appalachians through
   tonight, with height falls and the exit region of a strong mid/upper
   jet overspreading the southern Appalachians and Carolinas/Virginia.
   The aforementioned bands of convection and related cloud cover will
   tend to hinder pre-frontal destabilization across the Upper Ohio
   Valley and windward side of the Appalachians, but a few
   strong/severe low-topped storms could occur pending weak but
   sufficient destabilization this afternoon in those areas.

   A more certain/probable severe-weather risk is expected this
   afternoon east of the Appalachians spine, perhaps initially
   near/just east of the Blue Ridge across Virginia and nearby
   Maryland/North Carolina. This is where cloud breaks ahead of the
   upstream cold front and lingering overnight convection should allow
   for modest destabilization, with upwards of 1000 J/kg MLCAPE
   possible from the North Carolina/Virginia Blue Ridge and Piedmont
   vicinities toward the Delmarva by late afternoon. Strong deep-layer
   shear will support some well-organized storms including linear bands
   aside from the possibility of some initial/embedded supercells,
   particularly across the Piedmont of Virginia and northern North
   Carolina.

   Although low-level shear will not be overly strong and details of
   convective mode are a bit uncertain, the potential for a tornado or
   two could somewhat increase late this afternoon/early evening. This
   would particularly be across southern Virginia/northern North
   Carolina pending weak lee-side low/trough development and a related
   increase in low-level shear/SRH. Damaging winds will otherwise be
   the most probable hazard across the region this afternoon and
   evening, with a few instances of marginally severe hail also a
   possibility.
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