2021 Mid-Atlantic Severe Weather - General Discussion

[Kmlwx]

4 minutes ago, yoda said:

Last chance saloon tomorrow night into Thursday?

We always seem to end up getting a squally threat - or some sort of high shear/low CAPE event later in Oct/Nov as well. Something with a very strong cold front that has a thin tongue of 50-60 dews ahead of it. 

But yeah - perhaps the last of the warm season style severe potential. 

[high risk]

1 hour ago, Kmlwx said:

We always seem to end up getting a squally threat - or some sort of high shear/low CAPE event later in Oct/Nov as well. Something with a very strong cold front that has a thin tongue of 50-60 dews ahead of it. 

But yeah - perhaps the last of the warm season style severe potential. 

    Yeah, I won't assess the end of the season yet, but Thursday (maybe later tomorrow night for western areas) has some sneaky potential, particularly if the slowest solutions are correct.    I still think it's more of a flooding threat, but shear will be strengthening.     If we can get some heating ahead of the main frontal band on Thursday, at least some modest SVR potential is possible. 

[Kmlwx]

18z NAM nest was pretty aggressive with a severe threat in the metro corridor for Thur afternoon. Looks like a small window - but it's definitely there. 

[high risk]

14 hours ago, Kmlwx said:

18z NAM nest was pretty aggressive with a severe threat in the metro corridor for Thur afternoon. Looks like a small window - but it's definitely there. 

       NAM nest (and other guidance) has trended much faster now for tomorrow's activity.    Still perhaps a small threat for an embedded circulation somewhere during the morning, but instability will be limited.

 

[Kmlwx]

Just now, high risk said:

       NAM nest (and other guidance) has trended much faster now for tomorrow's activity.    Still perhaps a small threat for an embedded circulation somewhere during the morning, but instability will be limited.

 

Yep - just looks rainy to me now. 

[A777]

The timing isn't favorable, but models show quite a bit of moisture/warm air advection ahead of the front...

[yoda]

Hmmm

Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0734 PM CDT Wed Sep 22 2021

   Valid 230100Z - 231200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   CENTRAL APPALACHIANS...MID-ATLANTIC AND CAROLINAS...

   ...SUMMARY...
   Thunderstorms associated with a marginal tornado and wind-damage
   threat will be possible this evening into tonight from the central
   Appalachians into the Mid-Atlantic. A few strong wind gusts may also
   occur in the Carolinas this evening.

   ...Central Appalachians/Mid-Atlantic...
   The latest water vapor imagery shows a high-amplitude upper-level
   trough located over the Great Lakes and Ohio Valley with
   south-southwesterly mid-level flow over much of the Northeast. At
   the surface, a 1003 mb low is located in far northwestern
   Pennsylvania. A pre-frontal trough extends southeastward from the
   surface low into west-central Pennsylvania and then southward into
   central Virginia. Thunderstorms are ongoing along and near the
   pre-frontal trough. Ahead of the storms, surface dewpoints are in
   the 65 to 70 F range and weak instability is present. The strongest
   instability is located from southeast Pennsylvania southward into
   far eastern Virginia where the RAP is estimating MLCAPE to be in the
   500 to 750 J/kg range. In addition, the WSR-88D VWP near Washington
   D.C. has strong speed shear and veering winds with height in the
   lowest 1 km. This is resulting in 0-1 km shear in the 35 to 40 kt
   range suggesting that a marginal tornado threat could develop as the
   thunderstorms along the pre-frontal trough move eastward. The
   marginal tornado threat could affect much of Pennsylvania, Maryland
   and central Virginia along and near the axis of a 40 to 55 knot
   low-level jet. Any storm that rotates may also produce strong wind
   gusts.

[high risk]

Confirming the previous two posts, there is some sneaky TOR potential for early Thursday morning, as low-level shear will be very strong.   Instability will be limited, and these super wet early morning environments don't often work out for us, but it's a non-zero threat for sure.

[09-10 analogy]

Friday is the 20th anniversary of the college park F3 and other vortexinal curiosities (e.g., Tidal Basin F1, quick-hitting F4 near Culpepper) that day.  I still remember how strong the winds were at my location (an RFD, perhaps?) as the supercell passed right to the east and the beautiful flanking line I was able to see with it, probably right around the time the tornado was hitting College Park.

[yoda]

MRGL for Tuesday... 0/5/5

[weatherwiz]

Wow...NAM is quite intriguing tomorrow across the Baltimore/Washington corridor. Very steep mid-level lapse rates atop of an unseasonably warm/moist airmass would contribute to moderate levels of instability and steep low-level lapse rates verify this potential. Despite the lack of a stronger more consolidated piece of s/w energy you have a pretty decent amplifying trough with strong height falls. would think we see an upgrade to Slight with Enhanced risk potential if the NAM signal gains support. Would think an area of concentrated and widespread damage is very possible along with the risk for a few tornadoes...though could be mitigated by weaker llvl shear.  

[George BM]

4 minutes ago, weatherwiz said:

Wow...NAM is quite intriguing tomorrow across the Baltimore/Washington corridor. Very steep mid-level lapse rates atop of an unseasonably warm/moist airmass would contribute to moderate levels of instability and steep low-level lapse rates verify this potential. Despite the lack of a stronger more consolidated piece of s/w energy you have a pretty decent amplifying trough with strong height falls. would think we see an upgrade to Slight with Enhanced risk potential if the NAM signal gains support. Would think an area of concentrated and widespread damage is very possible along with the risk for a few tornadoes...though could be mitigated by weaker llvl shear.  

I was just thinking somewhat similar thoughts, at least with respect to some severe wind potential (Not that I'm a red tagger or anything like that).

Also, the near -15C 500mb temps w/ the steep MLLRs you just mentioned would suggest a severe hail risk as well when combined with moderate CAPE (LI -5 to -7).

[weatherwiz]

1 minute ago, George BM said:

I was just thinking somewhat similar thoughts, at least with respect to some severe wind potential (Not that I'm a red tagger or anything like that).

Also, the near -15C 500mb temps w/ the steep MLLRs you just mentioned would suggest a severe hail risk as well when combined with moderate CAPE (LI -5 to -7).

Yeah hail is always a possibility anytime you're dealing with temperatures that cold around H7-H5 and when you have steep mlvl lapse rates. 6z NAM Bufkit at BWI was generating between 500-700 J/KG of CAPE in the -10C to -30C layer so hail is certainly possible. There are some limiting factors though such as storm mode...that could limit large hail potential but pea's/dime's/nickel's are certainly possible with the more robust updrafts.

[Eskimo Joe]

Tomorrow looks sneaky interesting for hail we have a decent fropa and stout mid level lapse rates. Traditionally, that helps sustain updrafts better in these parts so long as we can push 700 - 1000 CAPE.

[yoda]

10 minutes ago, Eskimo Joe said:

Tomorrow looks sneaky interesting for hail we have a decent fropa and stout mid level lapse rates. Traditionally, that helps sustain updrafts better in these parts so long as we can push 700 - 1000 CAPE.

AFD was discussing 1000 CAPE with potential of over 2000

[yoda]

Looking at the 12z NAM soundings... I'd think 2500 SBCAPE with LI in the -7 to -9 range is a bit overdone... but hmmm

ConvT is in the low 80s... should breach that in the afternoon 

[weatherwiz]

I would focus more on MLCAPE over SBCAPE, however, given the presence of mlvl lapse rates approaching 7-7.5 C/KM atop of Td's in the mid-to-upper 60's and sfc temps ranging between upper 70's and lower 80's MLCAPE values 1500-2000 J/KG is not unreasonable. In fact, if mlvl lapse rates are more 7.5-8 C/KM those values would be quite likely. 

[yoda]

10 minutes ago, weatherwiz said:

I would focus more on MLCAPE over SBCAPE, however, given the presence of mlvl lapse rates approaching 7-7.5 C/KM atop of Td's in the mid-to-upper 60's and sfc temps ranging between upper 70's and lower 80's MLCAPE values 1500-2000 J/KG is not unreasonable. In fact, if mlvl lapse rates are more 7.5-8 C/KM those values would be quite likely. 

12z NAM at KDCA for both 18z and 21z TUES shows quite a large amount of SARS loose matches for severe hail... and even supercells 

[weatherwiz]

There is certainly enough directional shear going on to support supercell potential...in fact, perhaps even some right moving supercells. If there are indeed supercells and they become mature enough there probably is golf ball hail potential 

[yoda]

Wouldn't be surprised to see SLGT risk appear on 1730z OTLK from SPC

[SnowenOutThere]

So we getting watermelon sized hail tomorrow right?

[high risk]

The lapse rates and resulting instability tomorrow certainly have my attention.      Deep layer shear looks to be good - will become stronger a bit too late.    I can't help but notice a general consensus for the best threat to from DC to the east and southeast, so it doesn't look for now like those of us north or west of the city have the best chances, but there is certainly still some time to change that.

[Kmlwx]

5 minutes ago, high risk said:

The lapse rates and resulting instability tomorrow certainly have my attention.      Deep layer shear looks to be good - will become stronger a bit too late.    I can't help but notice a general consensus for the best threat to from DC to the east and southeast, so it doesn't look for now like those of us north or west of the city have the best chances, but there is certainly still some time to change that.

Yep - was going to say this as well. HRRR and NAM nest have been pretty consistent with the threat being south and east. The Bay area could be positioned nicely. 

[yoda]

Still MRGL at 1730z... interesting 

[high risk]

8 minutes ago, yoda said:

Still MRGL at 1730z... interesting 

         Might end up with a small SLGT risk area at some point tomorrow, but right now, there are too many questions on magnitude of instability and location and coverage of storms to justify the upgrade, IMHO.

 

[yoda]

3 hours ago, high risk said:

         Might end up with a small SLGT risk area at some point tomorrow, but right now, there are too many questions on magnitude of instability and location and coverage of storms to justify the upgrade, IMHO.

 

LWX seems a bit interested in tomorrow's severe risk with its afternoon AFD 

As the shortwave pulls out by early afternoon, a cold front
associated with a deep upper level trough will start to migrate
through the area. With peak daytime heating in place, this
disturbance may become a bit more organized in nature. MUCAPE values
continue to look fairly strong with 2000+ j/kg possible east of I-81
during the afternoon and evening hours. Bulk shear values continue
to moderately impress with 30 to 35 knots. Guidance continues to
favor the strongest storms along and east of I-95 by the evening
hours. Some strong to severe storms are possible east of I-81 but
especially along and east of the I-95 corridor. Main threats for
these storms will be for damaging winds and large hail. Peak
strength looks to be anywhere from 6 to 10 pm before peak heating
diminishes. With a lack of strong upper level forcing, coverage
looks to remain scattered in nature but will continue to monitor the
latest guidance.

[csnavywx]

Top-heavy nature of the CAPE tomorrow and low-level capping means updrafts might initially struggle in a relatively anemic cumulus field, but once the cap weakens sufficiently, it should take off pretty quickly. Little warm-sector cloudiness to speak of (other than a bit of acu or accas). Abnormally large proportion of the instability is above the freezing level (for these parts). Coming up with 0.5-0.75" on the hail forecasting diagram, though could easily breach 1" with some assist from mid-level rotation in individual cells. Gusts likely to be 35-45kt widespread, especially if a small LEWP or two gets started. Lightning might end up being some of the best and most intense of the season for some areas, especially closer to the Bay.

[SnowenOutThere]

How’s it looking smart people? 6z High Rez nam looks weak wondering if there is still a semi substantial threat

[mattie g]

1 hour ago, SnowenOutThere said:

How’s it looking smart people? 6z High Rez nam looks weak wondering if there is still a semi substantial threat

The smart people at SPC have us in slight risk (as of the overnight outlook), so I'm guessing they think it's looking fairly good.

[Kmlwx]

10z and 11z HRRR looked decent. Again, best chances south and east - but looked a little better perhaps for DC proper. But yeah - that 6z NAM nest run was pretty meh. We'll see.