2023 Mid-Atlantic Severe Wx Thread (General Discussion)

[yoda]

Afternoon discussion from LWX already talking it up some for Monday 

On Monday, the aforementioned cold front will approach and move
through our area bringing unsettled weather. Confidence continues
to increase for severe weather ahead of the frontal passage Monday
evening. The most recent guidance continues to indicate bulk shear
values between 30-40 knots with CAPE values between 1000-1500 J/kg.
Southerly flow ahead of the frontal passage will usher in ample
moisture with PWAT values nearing and even exceeding 2 inches along
I-95. While the signal for severe weather has persisted for a few
days now, uncertainty remains regarding timing due to the system
being 5 days away. At this time, damaging winds will likely be the
primary threat for Monday. However, large hail and a tornado are
possible as well. Isolated instances of flooding are also possible
with the moist airmass.We will continue to monitor this system as it
approaches.

[Kmlwx]

Not much change to the SPC outlook or the LWX AFD. Too early for the details CIPS looks decent. 

[Eskimo Joe]

41 minutes ago, Kmlwx said:

Not much change to the SPC outlook or the LWX AFD. Too early for the details CIPS looks decent. 

Their latest AFD does not inspire much confidence for Monday east of I-81:

Quote

There are indications in the models for light QPF over the western half of the area early Monday, likely a result of low-level moisture transport and increasing forcing for ascent. However, persistence of clouds cover and/or shower activity into the afternoon will lead to reduced instability over the region, and a much lower severe threat. As a result, there remains considerable uncertainty in Monday's forecast. 

 

[Kmlwx]

13 minutes ago, Eskimo Joe said:

Their latest AFD does not inspire much confidence for Monday east of I-81:

 

It's funny because the earlier part of the same discussion is like "confidence continues increase" 

[yoda]

1 hour ago, Eskimo Joe said:

Their latest AFD does not inspire much confidence for Monday east of I-81:

 

While true, I don't think I've seen LWX in their AFD mention tornado threat 4 days out

The potential for severe weather is still there, though. Model
soundings indicate around 30-40 knots of deep-layer shear will be
present over the area. East of the Blue Ridge, 925mb winds around
35KT or greater are resulting in elongated, curved hodographs in the
low-levels. If the atmosphere can destabilize, the 1500-2000+ J/kg
of SBCAPE is certainly enough to fuel scattered strong to severe
thunderstorms. The main threats look to be damaging wind gusts,
large hail, and a couple of tornadoes.

[Kmlwx]

38 minutes ago, yoda said:

While true, I don't think I've seen LWX in their AFD mention tornado threat 4 days out

The potential for severe weather is still there, though. Model
soundings indicate around 30-40 knots of deep-layer shear will be
present over the area. East of the Blue Ridge, 925mb winds around
35KT or greater are resulting in elongated, curved hodographs in the
low-levels. If the atmosphere can destabilize, the 1500-2000+ J/kg
of SBCAPE is certainly enough to fuel scattered strong to severe
thunderstorms. The main threats look to be damaging wind gusts,
large hail, and a couple of tornadoes.

"couple of tornadoes" or "isolated tornadoes" are such blanket terms. 

It's those super rare times when you see "significant tornadoes" or "isolated strong tornado" that your ears perk up. 

[high risk]

1 hour ago, Kmlwx said:

"couple of tornadoes" or "isolated tornadoes" are such blanket terms. 

It's those super rare times when you see "significant tornadoes" or "isolated strong tornado" that your ears perk up. 

       Yes, but they usually don't throw those terms around beyond day 2.    Their usage of that wording reflects that the progged shear and especially the low-level shear Monday have been progged to be seasonably strong.      I personally don't like them discussing tornado threats at this range, but I do understand what they're seeing.

[yoda]

Afternoon/evening disco for Monday was well-written on the threats and possible uncertainties

.LONG TERM /MONDAY THROUGH FRIDAY/...
Monday continues to be shaping up to be a potential impactful
weather day with increasing confidence for severe thunderstorms in
our forecast area. A strong mid-level trough will eject from the
Midwest into the Ohio Valley by Monday. The trough is expected to
strengthen and become neutral to negatively tilted during the day on
Monday. In response, surface low pressure will develop over Southern
Michigan and track into Southern Ontario during the day. Models
differ on the idea of a prefrontal shortwave ahead of the trough,
which may lead to early convection on Monday out ahead of the main
trough. This has led to uncertainty regarding the amount of
destabilization.

Out ahead of the system, the air mass is expected to become very
moist as a result of strengthening southerly winds. Most of the
guidance shows precipitable water values near or exceeding 2” east
of the Blue Ridge with values over 1.75 west of the Blue Ridge. This
increasing moisture advection may lead to low-level clouds and
precipitation which may also hinder destabilization during the day,
resulting in further uncertainty.

Despite the uncertainty, with the trough becoming negatively tilted,
mid-level flow is expected to increase across the area with most
guidance showing bulk shear values from 30-40 knots. East of the
Blue Ridge, there is a stronger signal for increased low-level flow,
with soundings showing 0-3 km shear on the order of 20-30 knots
which bears watching for potential low-level rotation within some
storms. Given the strong forcing and the 30-40 Knots of bulk shear,
any sunshine in the afternoon may provide ample instability on the
order of around 1000-1500 J/kg of MLCAPE to fuel severe
thunderstorms capable of damaging winds, large hail, and even a few
isolated tornadoes possible. We will continue to monitor this system
as it approaches and uncertainty decreases.

[yoda]

Maybe some sneaky severe Sunday?  MRGL for i95 and west on new SPC Day 2... 5% wind... but 2% TOR in NW VA into C and W MD

[yoda]

Oh....SPC talking dirty in their Day 3 disco this morning 

SPC AC 050740

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0240 AM CDT Sat Aug 05 2023

   Valid 071200Z - 081200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   THE SOUTHERN APPALACHIANS...MID-ATLANTIC AND NORTHEAST U.S....

   ...SUMMARY...
   Severe thunderstorms are possible Monday across portions of the
   southern Appalachians, mid-Atlantic and northeast U.S.

   ...Mid-Atlantic/northeast...
   As an upper-level trough moves slowly east across the region Monday,
   latest deterministic and ensemble guidance suggests a favorably
   sheared environment for organized severe storms including supercells
   will exist over a rather large area from the southern Appalachians
   northeast into the mid-Atlantic/northeast U.S.  Heating of a
   seasonably moist boundary layer, combined with somewhat steeper
   mid-level lapse rates associated with the eastern fringe of an EML
   extending east from the southern Plains, will contribute to
   moderate-strong MLCAPE across the area. 

   Thunderstorms may be ongoing at 12z Monday over portions of PA/NY
   and vicinity with a weak mid-level impulse and warm advection, and
   lingering clouds/precipitation will likely influence the degree of
   destabilization in these areas. Thunderstorms should
   develop/intensify during the afternoon in advance of a cold front
   aided by large-scale ascent, with both supercell and multicell storm
   structures likely. Relatively long hodographs may support splitting
   storms with a risk for large hail, and seasonably strong low-level
   wind fields will favor scattered damaging gusts, especially where
   low-level lapse rates are steepened through stronger heating. A risk
   for a few tornadoes may also exist, especially across portions of
   the mid-Atlantic and northeast where low-level hodograph curvature
   is most pronounced.

   Have opted to continue a broad 15%/Slight Risk area with this
   outlook, however higher severe probabilities may ultimately be
   warranted for portions of the area as mesoscale details come into
   better focus. 

[yoda]

Morning AFD from LWX suggests Sunday and Monday now

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Sunday likely starts out dry as high pressure continue to move
off to the east. The focus then turns to the Ohio Valley where
multiple disturbances are embedded in the mid-level flow. 00Z
guidance has trended upwards in terms of convective coverage
Sunday afternoon/evening with the higher focus generally
along/west of US-15. This correlates to where SPC has a MRGL (1
out of 5) risk for SVR wx. While the storms should be
progressive Sunday during the day, uncertainty arises heading
into Sunday evening/night as the warm front is progged to be
between the Mason Dixon and Potomac River. Guidance continues to
enhance precipitation in this area into PA as the LLJ advects
anomalous moisture to the boundary. WPC has a MRGL ERO for most
of the CWA, though think the greatest risk is in the
aforementioned area. There is a conditional tornado risk Sunday
afternoon/evening wherever the warm front does set up.

Monday could be an impactful weather day across the region given
the anomalous trough moving to the NW and sfc low moving over
the Great Lakes into Canada. Most SVR parameters are favorable
in our area with a residual lee pressure trough forecast just
east of the Blue Ridge to aid the synoptic scale forcing.
Hodographs are concerning from model guidance. However, there
remains considerable uncertainty regarding morning cldcvr,
upstream convective debris, track of the low, etc. Machine
learning continues to show modest probs for SVR wx as well. We
will continue to message Monday as low confidence for a
potentially higher impact SVR wx event. The greater chances for
SVR wx look to be over the northern portions of the area where
there is a better overlap in kinematics and dynamics. PWs are
anomalous (between 1.75-2" for most), so while the storms may be
progressive, cannot rule out some hydro problems near the
metros. Will continue to monitor, so be sure to check back.

 

[high risk]

Looking at the morning CAMs, looks good for widespread Sunday evening convection.    Low-level shear is interesting near the warm front, so the potential is there for a few spin-ups, but lack of instability and lack of discrete cells should temper the threat (maybe higher south and west of DC).  

Monday has good potential to be an ENH day.

[snowfan]

12z high res nam likes both days. 
 

[Kmlwx]

Some nice UH swaths over the area for Monday on that 12z NAM nest run. 

[Eskimo Joe]

I'll put all my chips in on Sunday.

[high risk]

1 hour ago, Eskimo Joe said:

I'll put all my chips in on Sunday.

     Sunday is challenged by very limited instability.    But if we can get more unstable than currently progged, any storm that interacts with the boundary to the southwest could spin for a bit.     I like the chances of evening convection moving southwest to northeast, but we have work to do to have it be severe.

[yoda]

Afternoon AFD from LWX on Monday mentions "volatile potential"

Monday looks to be the most impactful of the two days, at least
if eerything pans out to support severe weather. The
aforementioned trough continues to push through the Ohio Valley
Monday, with a pretty potent cold front pushing through our
reigon during the afternoon/evening hours. This will be
occurring right during peak heating, with temperatures into the
low 90s and dew points well into the 70s. Wind shear will be
substantial across the region of course, even in the lower
levels. With the potential for substantial CAPE, decent mid-
level lapse rates by Mid-Atlantic standards, inverted V
sounding profiles, and plenty of deep-layer shear, all therats
are on the table Monday. Damaging wind gusts will be the primary
threat, but could certainly see some large hail as well. Pretty
decent low-level flow will be present ahead of the cold front as
well, which could even bring an isoalted tornado threat along
with the other threats.

This could be a volitile day/night across the region, however
there remains considerable uncertainty regarding morning
cloudcover, upstream convective debris, track of the low, etc.
Machine learning continues to show modest probs for SVR wx as
well. We will continue to message Monday as low confidence for a
potentially higher impact SVR wx event. The greater chances for
SVR wx look to be over the northern portions of the area where
there is a better overlap in kinematics and dynamics. PWs are
anomalous (between 1.75-2" for most), so while the storms may be
progressive, cannot rule out some hydro problems near the
metros. WPC just upgraded the area to a Slight Risk for
excessive rainfall actually, so that further lends confidence to
at least some flood threat, particularly in the metros

[yoda]

I've never seen such high numbers of STP that the 18z NAM NEST shows across our area 

Granted it's just one model run and it'll probably be different at 00z, but 6 to 8?  Wow

Good amount of PDS TOR tags too on the soundings across the region 

@high risk @Eskimo Joe @Kmlwx

[87storms]

I’m in for tomo night/Monday. At minimum, it’s trackable.

[high risk]

2 hours ago, yoda said:

I've never seen such high numbers of STP that the 18z NAM NEST shows across our area 

Granted it's just one model run and it'll probably be different at 00z, but 6 to 8?  Wow

Good amount of PDS TOR tags too on the soundings across the region 

@high risk @Eskimo Joe @Kmlwx

       It's partially driven by huge CAPE numbers which are being caused by the usual NAM bias of having dew points that are too high.    That said, the shear will be good, and it will still be a very volatile environment on Monday.

[high risk]

     I'm still thinking SLGT for tomorrow for areas west and southwest of DC with the MRGL expanded into the DC-Baltimore corridor.    Maybe we end up SLGT, but instability looks very limited.   There might be more of a flash flood threat, although the dry week we just had might preclude that.

     I'm fully expecting a Day 2 ENH for Monday.    I think that there is enough in the CAMs to pull the trigger.

 

[yoda]

3 hours ago, high risk said:

     I'm still thinking SLGT for tomorrow for areas west and southwest of DC with the MRGL expanded into the DC-Baltimore corridor.    Maybe we end up SLGT, but instability looks very limited.   There might be more of a flash flood threat, although the dry week we just had might preclude that.

     I'm fully expecting a Day 2 ENH for Monday.    I think that there is enough in the CAMs to pull the trigger.

 

ENH risk trigger pulled for Monday - 5/15/30

 

[yoda]

 

 

 

[yoda]

Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0100 AM CDT Sun Aug 06 2023

   Valid 071200Z - 081200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE CENTRAL/SOUTHERN APPALACHIANS AND THE MID-ATLANTIC STATES...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SEPARATELY ACROSS
   THE CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Severe thunderstorms are possible Monday across portions of the
   southern Appalachians, mid-Atlantic and northeast U.S. Scattered
   swaths of damaging gusts are expected with this activity. Additional
   widely scattered thunderstorms are possible across the central High
   Plains, posing a risk for large hail and damaging gusts.

   ...Synopsis...

   A potent mid/upper shortwave trough this time of year will shift
   east from the Mid-MS Valley to the Appalachians on Monday. This will
   bring a band of anomalously strong west/southwesterly flow to the
   region, with the strongest flow focused from southern PA southward
   to northern GA and the Carolinas vicinity. At the surface, low
   pressure will shift east from northern IL into northern OH through
   evening, before lifting northeast across the lower Great Lakes
   overnight. A cold front attendant to the surface low will sweep
   east/southeast across the Midwest, and become positioned from
   western NY/PA into northern AL/MS by Tuesday morning. Mid 60s to mid
   70s F surface dewpoints are forecast across the large warm sector
   ahead of the cold front and ejecting mid/upper trough.

   ...OH Valley to Mid-Atlantic...

   Some areas of showers and cloudiness are possible early in the day
   across parts of the Mid-Atlantic vicinity in association with an
   ejecting lead shortwave impulse, continued warm advection and backed
   low-level flow to the east of a lee surface trough should keep the
   atmosphere primed for severe thunderstorm potential during the
   afternoon/evening. 

   Convection is expected to develop by midday closer to the surface
   front over parts of OH/KY. Multiple bands of convection are then
   expected into the afternoon with eastward extent into WV/southern PA
   and the Delmarva. A mix of semi-discrete cells and clusters are
   possible initially. A risk for all severe hazards is expected given
   effective shear greater than 40 kt, effective SRH greater than 200
   m2/s2 and MLCAPE 1500-3000 J/kg. With time, upscale growth into a
   bowing MCS is possible across parts of southern PA/WV and into the
   Delmarva vicinity, posing an Enhanced risk (level 3 of 5) for
   severe/damaging gusts during the afternoon/evening.

   Instability will weaken with northward extent into NY and southern
   New England. However, isolated strong/severe storms posing a risk
   mainly for strong gusts and perhaps small hail are expected.

 

[yoda]

Well written AFD this morning from LWX about Monday

SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
Monday has the potential to be an impactful SVR wx day across
the region as anomalously strong flow for this time of year
moves into a favorable airmass. Synoptically, a shortwave
trough will be moving through the Ohio Valley during the day on
Monday. At the sfc, an area of low pressure will trek from
northern Ohio into northwestern PA and western NY Monday
afternoon/evening. This area of low pressure is anomalous for
this time of year (~1000-1004 mb depending on model of choice)
as indicated by NAEFS/EPS CFSR climatology. The associated cold
front is forecast to move through the area during peak heating
in the afternoon/early evening.

Let me preface the further detailed environmental analysis that
uncertainty remains regarding how morning cldcvr or convective
debris may impact the SVR potential in the afternoon. Latest
guidance moves clouds out by mid-late morning and quickly
destabilizes the region. Given the dynamics and kinematics at
play, a long period of heating may not be necessary for the
potential of this event to be realized.

From an environmental standpoint, not much has changed. Wind
shear will be anomalous/impressive for early August standards
(40-60 kts) with lesser, but still notable values of low-level
shear. There remains dry air forecast in the mid-levels, which
should at least locally enhance the downburst potential given
inverted V sounding profiles. Low level lapse rates will be
favorable given heating of sfc temps to near 90F. Mid-level
lapse rates will be okay for Mid-Atlantic standards (6-6.5C/km).
The one observation in hodographs over the past ~24 hours is how
the vertical wind profile is not a linear increase in winds with
height. Different jet streaks are forecast at different levels
between 18-00Z, which is showing up on several models now and
causing the hodographs to be wavy in the mid-upper levels.
Should this come to fruition, this is not conducive for long-
lasting mesocyclones. This may result in a linear storm mode
with an embedded TOR threat. Now should the storm initiation be
delayed into the 21-03Z window, the potential for mesocyclones
becomes more favorable ad the LLJ makes the hodographs wider in
the low levels. Will continue to monitor these trends. LCLs are
forecast to be rather low which adds confidence to the TOR
threat given the wind profiles.

In terms of impacts:

1) All modes of SVR wx are possible. While damaging winds are
the greatest threat at this time, there is also a tornado and
large hail threat. The large majority of the FA is under an
Enhanced Risk from SPC west of the Chesapeake Bay. See
spc.noaa.gov for the latest SVR wx outlook and probabilities for
different phenomena.

2) The seasonably moist airmass will result in torrential
rainfall as the storms move through. Rain rates could be high
enough to support flash flooding given the forecast 2" PWATs,
especially across the DC/Baltimore metro into northeast MD where
WPC has a Slight Risk for Excessive Rainfall.

 

[dailylurker]

Where is everyone? Hangovers? Tomorrow looks interesting. I'm going to head out towards Hagerstown tomorrow for work. I'm thinking about doing some storm chasing on my way back east alone 70. That area looks twisty. 

[Kmlwx]

I'm tentatively in - parameters look pretty good for this region...our usual suspect (potential morning spoilage) will of course be in play. Won't know for sure my "in or out" status until that becomes more clear tomorrow. But I like where things stand for some excitement in the region. 

I love how this stuff tends to happen either when I'm out of the area or going to be in the office. Wish Mother Nature would hold off until my WFH days...

Sent my weather email to the interested folks at work...maybe they'll let us WFH tomorrow afternoon but I doubt it. 

[high risk]

13 minutes ago, dailylurker said:

Where is everyone? Hangovers? Tomorrow looks interesting. I'm going to head out towards Hagerstown tomorrow for work. I'm thinking about doing some storm chasing on my way back east alone 70. That area looks twisty. 

     The previous dozen posts discussing the higher end potential for tomorrow aren't enough for you?     

[Kmlwx]

2 minutes ago, high risk said:

     The previous dozen posts discussing the higher end potential for tomorrow aren't enough for you?     

We have to strive to get this thread as active as this place gets 12 hours before a 30+ inch snowstorm.

[Eskimo Joe]

6 minutes ago, Kmlwx said:

We have to strive to get this thread as active as this place gets 12 hours before a 30+ inch snowstorm.

Even 30 inch snow storms have dry slots and mix lines.