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Kmlwx

2020 Mid-Atlantic Severe Weather - General Thread

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LWX is pretty bullish in their afternoon AFD, and as Yoda mentioned, the MRGL risk now comes into the DC metro area.     I still, however, haven't seen guidance showing surface-based instability here.     I do see good mid-level lapse rates, so there will likely be some elevated cape, and I certainly believe that some lightning is possible.   Until, however, I see some sfc-based instability being progged, I can't buy in to SVR potential until you get well south of the DC Beltway.

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Per 833pm HWO from LWX for the region:

Quote
DAYS TWO THROUGH SEVEN...Wednesday through Monday

A line of storms is expected to develop along a strong cold front
late Wednesday into Wednesday evening. A few damaging wind gusts
are possible, and an isolated tornado cannot be ruled out.

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22 minutes ago, yoda said:

@high risk pretty decent squall line comes roaring through the region between 02z and 05z THU on 00z 3km NAM

  For sure.    The reflectivity looks so impressive!     But it's not surfaced-based.      That said, there is some elevated cape, so while there wouldn't be any severe threat with that (assuming that the NAM nest is correct with the stable sfc layer), there would definitely be some chance of lightning.    I would call that a win!

 

nam3km_2020022600_fh28_sounding_38.59N_76.82W.png

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The latest from LWX:

NEAR TERM /THROUGH TONIGHT/...

Satellite and observational trends show that dense fog is
expanding in areal coverage as high clouds shift to the east.
NSSL-WRF ARW synthetic imagery shows low clouds persisting all
day east of the Blue Ridge, the eastern panhandle of WV, and
northern MD. Based on its output, areas south of Winchester to
Staunton are the only ones that are expected to experience clear
skies. It will likely remain quiet through about 21Z, then
widespread showers and possible thunderstorms are expected as
low pressure forms along the Appalachians and Blue Ridge and
develops north northeastward. The most recent model guidance
showed a trend toward less instability/CAPE available due
low overcast expected to persist all day. Other parameters such
as model flash rate and lifted indices also showed a downward
trend in t-storm potential. The areas of greater risk for any
severe wx seems to be east of the Appalachians to the I-81
corridor where the most sunshine is expected and generally west
of where SPC placed the marginal risk.

Cdfnt will sweep through the area between 03Z-06Z Thu with
showers ending quickly with fropa. Colder air will follow in its
wake with showers turning quickly into snow showers over the
Appalachians where sub-advisory snow accumulations are expected.
A strong pressure surge behind the front will cause strong
winds at higher elevations and have issued a Wind Advisory for
the highest elevs from midnight tonight to 15Z Thu.
 

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shame its happening late night. my old ass can't stay up until 11 waiting for a 15-minute line of storms

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38 minutes ago, mappy said:

shame its happening late night. my old ass can't stay up until 11 waiting for a 15-minute line of storms

LWX did note yesterday that it might come in a bit faster than expected with how dynamic it is. I can barely make it past 9:30pm myself these days. 

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I still don't understand why SPC is now even considering an upgrade to SLGT, but there is no argument that if sfc-based parcels can find a way to be buoyant, the wind fields are amazing.   A faster evolution of the event might allow the line to take advantage of daytime warming for the western part of the outlook, so I understand the outlook expanding west.    I'm setting my goal for getting a torrential downpour and maybe hearing thunder.    Seeing several consecutive HRRR cycles wanting to break up the line as it moves east is giving me some pause, though......

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2 hours ago, Kmlwx said:

LWX did note yesterday that it might come in a bit faster than expected with how dynamic it is. I can barely make it past 9:30pm myself these days. 

i'm not that old lol 

but a rumble of thunder before the cold wind would be cool

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2 hours ago, high risk said:

I still don't understand why SPC is now even considering an upgrade to SLGT, but there is no argument that if sfc-based parcels can find a way to be buoyant, the wind fields are amazing.   A faster evolution of the event might allow the line to take advantage of daytime warming for the western part of the outlook, so I understand the outlook expanding west.    I'm setting my goal for getting a torrential downpour and maybe hearing thunder.    Seeing several consecutive HRRR cycles wanting to break up the line as it moves east is giving me some pause, though......

I wonder if they are a bit gun shy after missing one of the largest winter tornado events in the Mid Atlantic the other week.

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8 minutes ago, Eskimo Joe said:

I wonder if they are a bit gun shy after missing one of the largest winter tornado events in the Mid Atlantic the other week.

They made a reference to that event in the 1630z update.

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11 minutes ago, Eskimo Joe said:

I wonder if they are a bit gun shy after missing one of the largest winter tornado events in the Mid Atlantic the other week.

      This is a very good point!     But that event had some actual sfc-based instability.

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8 minutes ago, Kmlwx said:

HRRR continues to send the most continuous part of the line mostly to the north of Central Maryland. It's more broken in DC proper and very scattered south. 

 

    Several of the morning CAMs show the same thing now.     Seems like some showers break out ahead of the line in those runs and ruin the convergence.

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10 minutes ago, high risk said:

 

    Several of the morning CAMs show the same thing now.     Seems like some showers break out ahead of the line in those runs and ruin the convergence.

Then again - it's the HRRR and could be totally wrong - but with agreement maybe not. 

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It's really gotten bad when the most we can hope for in the climatologically favored snowless February is a potential rumble of thunder and wind shift in the middle of the night with overcast skies and little surface instability.  Maybe a special radio show is in order?  

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Just now, Kmlwx said:

The latest HRRR looks a bit better. Nothing significant but just a more solid line and a bit more on the gusty wind side of things. The weenie in me also sees a tiny bit of that light green shade on the UH maps. 

$20 says we see a quick temp spike pre-front as the low level inversion mixes out and we get a few SVRs / damaging wind reports tonight.

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Just now, Eskimo Joe said:

$20 says we see a quick temp spike pre-front as the low level inversion mixes out and we get a few SVRs / damaging wind reports tonight.

That's about the max potential on this one. Absolute max. My current thinking (not very bold) is for zero TOR reports in the LWX CWA this evening and fewer than 5 reports of wind damage. I'm sure LWX will throw up a few warnings. I'll go max measured gust of 53mph. 

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1 minute ago, Eskimo Joe said:

We had a 5% pity meso last time, but the low was about 7mb stronger and we were in Larko's Triangle.  

Different timing too - though some might argue the timing was even worse for that one. Wasn't it during the morning rush? I remember I was stuck in it driving to work around 7:45 that morning. I guess timing doesn't really mean crap in these low CAPE cold season events. 

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1 minute ago, Kmlwx said:

Different timing too - though some might argue the timing was even worse for that one. Wasn't it during the morning rush? I remember I was stuck in it driving to work around 7:45 that morning. I guess timing doesn't really mean crap in these low CAPE cold season events. 

Correct, it was between 8am - 10am.  That event is a great reminder that if you get the dyanmics/instability right, the time of day doesn't matter.

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LWX seems a bit gungho this evening in their afternoon AFD:

As the aforementioned trough takes on a negative tilt this
evening, surface low pressure will develop on the triple point
between this convergence axis and a warm front stalled over the
region. Low level wind fields will strengthen, forcing this line
of convection to accelerate eastward. The low will quickly begin
to lift northeastward by late this evening as an upper jet
streak strengthens and approaches from the southern Appalachians.
A surface cold front attendant to the upper trough will also
accelerate eastward. The line of showers (and possible isolated
thunderstorms) will trek eastward reaching the Blue Ridge
between 9 and 10 PM, I-95 between 10 PM and midnight, and the
Chesapeake Bay between 11 PM and 1 AM. Residence time of the
line over any one area probably won`t be more than 15 to 30
minutes, but brief downpours and gusty winds (possibly strong
enough to cause isolated wind damage given the ambient low level
wind field) may accompany the line.

As mid/upper forcing and stronger surface convergence pivot
northeastward toward southeastern Pennsylvania through the
evening into the overnight, low CAPE will wane and retreat
southeastward. The best juxtaposition of CAPE, increasing low
level winds and a weaker near-surface stable layer appears to be
over portions of central Virginia (from around Waynesboro to
Charlottesville to Culpeper, generally up US-29 toward Manassas).
Hi-res guidance (namely the HRRR/WRF-ARW) indicate the triple
point/mesolow tracking through this corridor, enhancing low
level shear and vorticity amid backed surface flow. This may
ultimately be where the best chance for damaging winds or an
isolated tornado winds up.

Elsewhere, isolated strong to damaging surface gusts are
possible, especially if the line of convection solidifies more
significantly making downward momentum transfer more probable
despite a near surface stable layer in the cool sector/wedge

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@yoda - don't know if I'd refer to that as "gungho" seems more like they are just saying IF there was an area that had a higher risk - it would be there. The background risk is pretty low still. They just do a great job it seems of fleshing out the discussion and describing the potential. 

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13 minutes ago, MN Transplant said:

HRRR going with gusts in the mid-40s after the line passes.  That might be more interesting than the showery line.

 

       Agreed.    We'll have no problems mixing behind the front.

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