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2018 Mid-Atlantic General Severe Discussion


Kmlwx
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1 hour ago, Kmlwx said:

Did I miss a memo/change - since when did SPC start issuing 2 Day 3 outlooks each day?

I dont think they do... I think they only update the Day 3 outlook if they have confidence in doing so per the 12z model suite?  I have seen it done a handful of times

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3 hours ago, yoda said:

I dont think they do... I think they only update the Day 3 outlook if they have confidence in doing so per the 12z model suite?  I have seen it done a handful of times

      Yoda is spot on.    The day shift has the ability to update the day 3 if they desire, and I remember seeing it at some point this year during the spring.

       I think that this was a good call today, and there is a reasonable chance that we'll wake up Tuesday morning in SLGT for day 2.

  

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  • 1 month later...

Multiple NAM nest cycles show storms initiating well ahead of the main squall line late Wednesday afternoon on some sort of lee trough.     Instability is decent, and deep layer shear is probably sufficient for some svr gusts.    Given how wet the soil is, I suspect that trees could come down even if gusts are only in the 35 kt range.

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40 minutes ago, high risk said:

Multiple NAM nest cycles show storms initiating well ahead of the main squall line late Wednesday afternoon on some sort of lee trough.     Instability is decent, and deep layer shear is probably sufficient for some svr gusts.    Given how wet the soil is, I suspect that trees could come down even if gusts are only in the 35 kt range.

I posted this in the September disco thread:

"00z 3km NAM has some interesting storms around the region from 21z WED to 00z THUR... maybe severe"

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9 hours ago, yoda said:

I posted this in the September disco thread:

"00z 3km NAM has some interesting storms around the region from 21z WED to 00z THUR... maybe severe"

                That sums it up.    Looking more closely, the last 5 (at least) NAM nest cycles show this, along with the 12z HRRR and all last night's Hi-Res Windows.    It's rather impressive agreement.     I'm still concerned about trees toppling easily in the saturated ground.

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  • 1 month later...

marginal risk for areas just south of DC Friday.    With the front now likely to be much slower to move through the area, there is the opportunity for low to mid 60s dew points to return.    Shear will be strong - it's just a matter of how much instability can develop.      It's very possible that much of the area ends up in MRGL (with a non-zero chance of SLGT) on Friday.

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Maybe?  From the 1630 SPC Day 1 OTLK:


   ...Piedmont this afternoon/evening...
   A lead shortwave trough over MS/AL this morning will eject quickly
   northeastward over the Appalachians, in advance of an amplifying
   midlevel trough (now over the MS Valley) that will reach the Mid
   Atlantic by tonight.  Associated surface cyclogenesis is expected
   from the Piedmont this afternoon to southern New England overnight.

   A moist low-level air mass is present east of the Appalachians
   (boundary-layer dewpoints in the 60s), though widespread clouds and
   poor midlevel lapse rates will limit potential destabilization. 
   Low-level mass response to the cyclogenesis will result in
   increasing low-level flow/shear by this evening near and just east
   of the path of the surface wave, across NC/VA/MD later this
   afternoon/evening.  There will be some conditional tornado/damaging
   wind threat, though the depth/intensity of the convection is in
   question given the weak buoyancy/poor lapse rates.  Will opt to
   maintain Marginal categorical risk for tornado/damaging winds, but
   will continue to monitor closely for later evidence of a small
   corridor of somewhat greater severe threat this afternoon along the
   path of the surface cyclone.

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39 minutes ago, Kmlwx said:

@high risk - how much CAPE do you think will be enough to do it? 

      great question.    The issue may not be the amount of total cape, but the distribution of it.    I think in these low instability/high shear events, you can work with 500 or so, but there needs to be some instability just above the sfc.    Tall, skinny cape usually doesn't get it done.     That forecast sounding has lousy low-level lapse rates and likely would not get the job done, assuming it's correct.    History says that these low instability/high shear events rarely work out here, but there are exceptions.

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8 minutes ago, high risk said:

      great question.    The issue may not be the amount of total cape, but the distribution of it.    I think in these low instability/high shear events, you can work with 500 or so, but there needs to be some instability just above the sfc.    Tall, skinny cape usually doesn't get it done.     That forecast sounding has lousy low-level lapse rates and likely would not get the job done, assuming it's correct.    History says that these low instability/high shear events rarely work out here, but there are exceptions.

Looks like SPC agrees with your thinking in their 2000 OTLK:


   ...Mid Atlantic...
   Some low possibility exists for a more focused corridor of locally
   damaging winds from central Virginia to the Delmarva this evening,
   right near the track of a surface cyclone lifting northeast. Despite
   cloud cover significantly limiting heating, surface dew points in
   the mid/upper 60s F are forecast to move north ahead of the low this
   evening, yielding slightly greater buoyancy. Relatively shallow
   convection across the Carolinas (on the nose of stronger forcing for
   ascent) may intensify somewhat as it meets this environment over
   parts of Virginia this evening. Given plentiful storm-relative
   helicity, any organized convection should have an attendant threat
   of locally damaging winds and a brief tornado or two. An upgrade to
   Slight Risk was considered, but concerns regarding adequate
   surface-based buoyancy and convective organization appear too large
   to warrant increased severe probabilities at this time.

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13 minutes ago, high risk said:

MD issued by SPC.    Watch possible as the strengthening line to the west approaches.

mcd1618.gif

Mesoscale Discussion 1618
   NWS Storm Prediction Center Norman OK
   0555 PM CDT Fri Nov 02 2018

   Areas affected...Middle Atlantic

   Concerning...Severe potential...Watch possible 

   Valid 022255Z - 030030Z

   Probability of Watch Issuance...40 percent

   SUMMARY...Strong convection may develop across the Middle Atlantic
   this evening. Damaging winds and perhaps an isolated tornado are
   possible.

   DISCUSSION...Strong mid-level height falls are spreading across the
   Middle Atlantic ahead of pronounced short-wave through. Slow air
   mass modification continues across the Delmarva where lower 60s
   surface dew points are now observed. Over the last hour or so,
   frontal convection has gradually intensified ahead of a surface low
   over south-central VA. Even so, this activity has yet to generate
   lightning which is indicative of the relatively low-topped
   convection. Forecast soundings ahead of this activity appear
   favorable for an organized squall line along the front, and perhaps
   even an isolated supercell. Latest CAMs suggest a gradual upward
   evolving line of convection along the front as it progresses into
   the Delmarva later this evening. There is increasing concern this
   activity could begin to generate locally strong winds, especially if
   bow-type structures evolve. Will continue to monitor this region for
   possible ww.

   ..Darrow/Hart.. 11/02/2018

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...PHI...AKQ...LWX...RAH...RNK...
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