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2020 Mid-Atlantic Severe Weather - General Thread


Kmlwx
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Mount Holly thoughts this morning-

CAMs continue to indicate scattered, fragmented, multi-mode convection developing across the forecast area. Any robust convection that can get going and maintain itself will have the potential to be of the "power shower" variety. These will have the potential to bring down the stronger winds from just above the surface in the form of damaging straight line wind gusts. If the best instability is realized (>500 J/kg), I wouldn`t be surprised to see a low-topped supercell or two across the coastal plain. In addition to the convective wind potential, low level shear values will be high (40+ kts) with curved low level hodographs supportive of surface based rotation. The tornado potential is certainly non- zero with this environment, but given questionable convective mode the confidence is not particularly high on this potential.

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4 hours ago, CAPE said:

Mount Holly thoughts this morning-

CAMs continue to indicate scattered, fragmented, multi-mode convection developing across the forecast area. Any robust convection that can get going and maintain itself will have the potential to be of the "power shower" variety. These will have the potential to bring down the stronger winds from just above the surface in the form of damaging straight line wind gusts. If the best instability is realized (>500 J/kg), I wouldn`t be surprised to see a low-topped supercell or two across the coastal plain. In addition to the convective wind potential, low level shear values will be high (40+ kts) with curved low level hodographs supportive of surface based rotation. The tornado potential is certainly non- zero with this environment, but given questionable convective mode the confidence is not particularly high on this potential.

      That Mt Holly discussion works very well for most of our area too.     Interestingly, progged parameters for early this afternoon have improved, but overall, the CAM simulated reflectivity forecasts for our area have gotten worse.      12z NAM nest stops the bleeding and actually has a nice convective feature and a secondary low center - we'll see if it has the right idea.....

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Cross-posting:

URGENT - IMMEDIATE BROADCAST REQUESTED
   Tornado Watch Number 515
   NWS Storm Prediction Center Norman OK
   105 PM EST Mon Nov 30 2020

   The NWS Storm Prediction Center has issued a

   * Tornado Watch for portions of 
     Delaware
     Eastern Maryland
     Central and Southern New Jersey
     Southeast Pennsylvania
     Eastern Virginia
     Coastal Waters

   * Effective this Monday afternoon and evening from 105 PM until
     700 PM EST.

   * Primary threats include...
     A couple tornadoes possible
     Scattered damaging wind gusts to 70 mph possible

 

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Just now, Eskimo Joe said:

New TOR coming for Cecil shortly.

Quote
BULLETIN - EAS ACTIVATION REQUESTED
TORNADO WARNING
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
250 PM EST MON NOV 30 2020

THE NATIONAL WEATHER SERVICE IN STERLING VIRGINIA HAS ISSUED A

* TORNADO WARNING FOR...
  SOUTHEASTERN CECIL COUNTY IN NORTHEASTERN MARYLAND...

* UNTIL 315 PM EST.

* AT 250 PM EST, A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO
  WAS LOCATED NEAR HOLLYWOOD BEACH, OR 10 MILES WEST OF MIDDLETOWN,
  MOVING NORTHEAST AT 60 MPH.

  HAZARD...TORNADO.

  SOURCE...RADAR INDICATED ROTATION.

  IMPACT...FOR THOSE IN THE DIRECT PATH OF A TORNADO TOUCHDOWN, 
           FLYING DEBRIS WILL BE DANGEROUS TO THOSE CAUGHT WITHOUT 
           SHELTER. DAMAGE TO ROOFS, SIDING, AND WINDOWS MAY OCCUR. 
           MOBILE HOMES MAY BE DAMAGED OR DESTROYED. TREE DAMAGE IS 
           LIKELY.

* THIS DANGEROUS STORM WILL BE NEAR...
  BOHEMIAS MILLS AROUND 255 PM EST.
  BRANTWOOD AROUND 300 PM EST.

OTHER LOCATIONS IMPACTED BY THIS TORNADIC THUNDERSTORM INCLUDE
CHESAPEAKE CITY.

 

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So... December severe then :lol:

Day 4-8 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 AM CST Tue Dec 01 2020

   Valid 041200Z - 091200Z

   ...DISCUSSION...
   Most recent guidance from several deterministic models suggests some
   severe potential may materialize across the Southeast D4/Friday and
   Mid-Atlantic on D5/Saturday. These models forecast a maturing
   mid-latitude cyclone moving through those regions. However, previous
   runs do not show a mature system, with most favoring a weaker, more
   progressive evolution. Additionally, the ECMWF, which has shown more
   run-to-run consistency, is forecasting mature low over the
   Southwest/northern Mexico which slowly drifts eastward toward the
   southern Plains during the weekend. All of these factor lead to a
   low forecast confidence for the weekend. Even with these
   differences, mostly stable conditions appear likely beginning
   D6/Sunday and continuing through early next week.

   ..Mosier.. 12/01/2020

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  • 3 weeks later...

The 18z NAM at range (FWIW) has some crazy 925mb and 850mb winds depicted for the Christmas Eve system. Has 925mb winds over the Bay of like 75kts around 0z that night. 

Big swath of near 100kt winds at the 850mb level at 3z that night as well (over the Bay again). But the entire area at one point or another gets very strong winds not very far off the surface (few thousand feet). That same NAM run has a really well defined line with the front...would think that with the strong storm system, at least some of those winds could be brought down to the surface. 

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16 hours ago, Kmlwx said:

The 18z NAM at range (FWIW) has some crazy 925mb and 850mb winds depicted for the Christmas Eve system. Has 925mb winds over the Bay of like 75kts around 0z that night. 

Big swath of near 100kt winds at the 850mb level at 3z that night as well (over the Bay again). But the entire area at one point or another gets very strong winds not very far off the surface (few thousand feet). That same NAM run has a really well defined line with the front...would think that with the strong storm system, at least some of those winds could be brought down to the surface. 

Over/under on a surprise watch box? I'm going 40%.

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There is no strong inversion on this sounding near DC as per the 00z NAM as seen below. This should allow for there to be a greater chance of the strong winds at 850mb to be able to mix down. The squall line as predicted by the NAM should further help to mix down the winds from the LLJ.

 

nam_2020122300_051_38.89--77_01.thumb.png.daf179dbc97be108d6ed710087040f73.png

The second image below shows the predicted  850mb winds

 

850wh.us_ne.thumb.png.f1fd85bf7b13d33da606cc261d8c119b.png

 

Lastly, there is PVA present so I am by no means in expert but that could be a source to lift up the air parcels absent CAPE which is maybe why the NAM is forecasting a squall line?

 

namconus_z500_vort_neus_42.thumb.png.935f87639a89e431866307b443275a4c.png

 

 

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9 hours ago, SnowLover22 said:

There is no strong inversion on this sounding near DC as per the 00z NAM as seen below. This should allow for there to be a greater chance of the strong winds at 850mb to be able to mix down. The squall line as predicted by the NAM should further help to mix down the winds from the LLJ.

 

nam_2020122300_051_38.89--77_01.thumb.png.daf179dbc97be108d6ed710087040f73.png

The second image below shows the predicted  850mb winds

 

Lastly, there is PVA present so I am by no means in expert but that could be a source to lift up the air parcels absent CAPE which is maybe why the NAM is forecasting a squall line?

 

 

              You're absolutely correct about the very strong winds above the surface and the intense forcing that will be present.    But that sounding is not one that would promote strong winds mixing to the ground.     There is the slightest hint of mixing in a very shallow layer at the ground, but you do have an inversion above that, and the box below the sounding explicitly shows 0 sfc-based CAPE.      The reason you see some impressive convective signatures in the guidance is the strong lift with the PVA and the front.      There is also a little bit of elevated instability (the most unstable CAPE is listed as around 430) to assist.   

                 To be clear, though, if we can end up a few degrees higher on both the temperature and dew point, we might then have a legitimate threat to have sfc-based storms that are capable of strong wind gusts.

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16 minutes ago, high risk said:

              You're absolutely correct about the very strong winds above the surface and the intense forcing that will be present.    But that sounding is not one that would promote strong winds mixing to the ground.     There is the slightest hint of mixing in a very shallow layer at the ground, but you do have an inversion above that, and the box below the sounding explicitly shows 0 sfc-based CAPE.      The reason you see some impressive convective signatures in the guidance is the strong lift with the PVA and the front.      There is also a little bit of elevated instability (the most unstable CAPE is listed as around 430) to assist.   

                 To be clear, though, if we can end up a few degrees higher on both the temperature and dew point, we might then have a legitimate threat to have sfc-based storms that are capable of strong wind gusts.

Also I assume the area being in the right entrance region of the jet-streak is contributing as well promoting rising air through upper-level divergence. 

 

 

 

 

namconus_uv250_neus_40.png

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