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2021 Mid-Atlantic Severe Weather - General Discussion


Kmlwx
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Four hundredths in the Young tipper here.

Nice lightning to our NE lasted about 20 minutes with some rumbling.

Came out of nowhere seemingly.  Blew up fast and then out.  Like the city slicker on a camping trip trying to start a campfire with three logs and a pint of white gasoline! :D

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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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17 minutes ago, MN Transplant said:

12z NAM 3k is a bit more frisky than prior runs.  Both the NAM and HRRR indicate that there may be a set of cells out in front of the main line, which is pretty common around here.

      Yeah, the signal for this started showing up yesterday, and even the Hi-Res Windows are buying in, although they disagree a bit on location and timing.   (The HiResW FV3 initiates the lead cells notably further north.)      Seems like several cells will race from south to north out ahead of the more organized line rolling east.    Still unclear how much instability will be available:  the HRRR is much warmer (but drier), while the NAM Nest is more moist (but cooler).   Perhaps there is a temperature/moisture "sweet spot" in there that could increase the potential for some hail and wind.

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1 hour ago, high risk said:

      Yeah, the signal for this started showing up yesterday, and even the Hi-Res Windows are buying in, although they disagree a bit on location and timing.   (The HiResW FV3 initiates the lead cells notably further north.)      Seems like several cells will race from south to north out ahead of the more organized line rolling east.    Still unclear how much instability will be available:  the HRRR is much warmer (but drier), while the NAM Nest is more moist (but cooler).   Perhaps there is a temperature/moisture "sweet spot" in there that could increase the potential for some hail and wind.

HRRR over-mixing coming into play again? BWI is already 77. Even Westminster is 72.

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4 minutes ago, Eskimo Joe said:

HRRR over-mixing coming into play again? BWI is already 77. Even Westminster is 72.

        HRRR seems to have a good handle on the environment.    It's going for roughly 80/58 by mid-afternoon which seems reasonable.   NAM Nest is too cool - it really raises the dew points later this afternoon, and I'm not sure that's going to be right.

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