2021 Mid-Atlantic Severe Weather - General Discussion

[yoda]

Lulz... DCA sounding for 21z WED

[CAPE]

From Mount Holly morning AFD-

Meanwhile, the southern portions of the area will likely experience some period of time in the system`s warm sector. Though the main impacts of this system may occur after Wednesday`s peak heating, substantial warm/moist advection beneath very favorable large-scale ascent from a strong and intensifying anticyclonic jet streak in the Northeast will likely keep a modestly unstable thermodynamic profile through the overnight hours within this region. Shear profiles are also appearing favorable, with 0-6 km bulk shear 30-40 kt and low- level SRH exceeding 200 J/kg. Convection in this region will likely be organized and potentially rotating as the low moves through, with isolated strong/severe wind gusts and even a couple of tornadoes possible. The SPC has upgraded the southern half of the CWA to a slight risk for Wednesday night, which is quite reasonable given the evolving pattern.

[CAPE]

Slight risk for much of VA, MD, DE.

[CAPE]

SPC AC 310554 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1254 AM CDT Tue Aug 31 2021 Valid 011200Z - 021200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE MID-ATLANTIC... ...SUMMARY...

A few tornadoes could occur across the Mid-Atlantic States as the remnants of Ida move through the region on Wednesday. Isolated severe thunderstorms are also possible across the northern Plains Wednesday afternoon through Wednesday evening. ...Mid-Atlantic... Tropical Depression Ida is forecast to be centered near the KY/TN/VA border intersection early Wednesday morning. Continued northeastward motion of the system is anticipated, taking it off the Mid-Atlantic Coast Wednesday night. Moist low-level southerly flow ahead of this system will likely bring mid 70s dewpoints into the Mid-Atlantic region, helping to increase instability. Additionally, a more banded storm structure is probable by this stage in its life cycle, result in the potential for modest diurnal heating between the bands. Current guidance suggest afternoon temperatures across much of NC and VA will be in the 80s, with dewpoints in the low to mid 70s. These conditions are expected to result in sufficient instability for deeper, more persistent updrafts. In addition to the favorable thermodynamics, strong low to mid-level flow will persist around the remnants of Ida. In fact, some guidance suggests the system may slightly deepen as it encounters increased baroclinicity and undergoes the transition to an extra-tropical system. All of these factors indicate a tornado threat will persist throughout the eastern quadrant of the system as it moves through the Mid-Atlantic. A few convectively augmented wind gusts are possible as well.

[yoda]

SLGT risk for today as well for VA into MD and DC

[yoda]

LWX gung ho for Wednesday severe

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Wednesday continues to have the potential to be a high impact
weather day for our area. In summary, northwestern portions of
our forecast area need to prepare for significant rainfall and
the threat for flash and river flooding. The southeastern half
may see less rain overall, but will have a greater risk of
tornadoes, likely during the afternoon and evening. River
impacts are discussed in the hydrology section below.

Some guidance is indicating the system comes in two pieces.
During the morning hours, the heaviest and most widespread
rainfall will likely continue to be west of the Blue Ridge in
the zone of isentropic ascent/tropical moisture flux. This could
serve to allow some additional heating to occur especially over
southeastern portions of the area, allowing greater instability
to build. The low center will cross our area during the late
afternoon and evening, potentially strengthening as it
encounters increasing baroclinicity along the stalled front.
While models continue to wobble some, and there is definitely
some spread in exact solutions, the main stories remain in tact.
Precipitable water values will exceed two inches during this
time with deep, tropical warm cloud layers. A sharp mid level
trough and jet streak to the north will provide additional
forcing. Therefore, very high rain rates are possible during
this time, which could be especially hazardous to our
northwestern counties which will have already had a healthy
soaking of rain. Widespread and significant flooding could occur
in these areas if the worst case rain totals verify. However,
anywhere these rain rates persist for any amount of time could
see flash flooding. Meanwhile, low level shear will increase
markedly, especially near the low track and frontal zone, but
shear will be large across the warm sector. Model forecasts of
instability, low level lapse rates, and the shear combine to
suggest tornadoes will be a threat in the warm sector. Initial
CAM depictions of broken/cellular convection also support this
idea. Given the strong winds aloft, storms could also cause
damaging wind gusts. At this time, am not sure we are able to
mix into the strong wind fields aloft enough to warrant wind
advisories, but in general gradient winds will also increase as
the low passes.

Precipitation will taper off from west to east late Wednesday
night into Thursday morning, and clearing should rapidly follow.
Timing of departure is still a little uncertain, but most areas
should be dry by afternoon. Gusty north winds will continue, and
temperatures will remain below normal even where the sun comes
out. Lower humidity levels and cold advection will allow for
temperatures to dip into the 50s and lower 60s Thursday night,
with some potential 40s in the mountains.

[nw baltimore wx]

The thing that excites me the most about that discussion is where they say gusty north winds and below normal temperatures.

[CAPE]

9 minutes ago, nw baltimore wx said:

The thing that excites me the most about that discussion is where they say gusty north winds and below normal temperatures.

Just as I am leaving the country. I am sure it will be warm and humid again when I return.

[yoda]

06z NAM NEST sim radar looks pretty ominous at 22z WED

[yoda]

[mappy]

everyone should expect spin ups tomorrow. always a threat with tropical remnants. 

[Kmlwx]

I'm definitely a little more "pumped" than I was for our last tropical event. Seems we are in a better position for some activity. 

[yoda]

8 minutes ago, Kmlwx said:

I'm definitely a little more "pumped" than I was for our last tropical event. Seems we are in a better position for some activity. 

Now if you can only get @Eskimo Joe to join in... pretty sure @high risk and @MN Transplant are in

[H2O]

12 minutes ago, Kmlwx said:

I'm definitely a little more "pumped" than I was for our last tropical event. Seems we are in a better position for some activity. 

Euro went north with the rain this morning.  Storm cancel.  

[yoda]

2 minutes ago, H2O said:

Euro went north with the rain this morning.  Storm cancel.  

I'm hugging the NAM 

[Kmlwx]

1 minute ago, H2O said:

Euro went north with the rain this morning.  Storm cancel.  

Eh - with the water "incidents" you and I have both had recently...thinking we can pass on the rain if it means having some exciting radar watching for spinnys. Even on the dry days our lawn has been like a saturated sponge when you walk on it. 

[H2O]

Just now, Kmlwx said:

Eh - with the water "incidents" you and I have both had recently...thinking we can pass on the rain if it means having some exciting radar watching for spinnys. Even on the dry days our lawn has been like a saturated sponge when you walk on it. 

I don't need ark floating rain but a weak ass spin thing would be fun

[Eskimo Joe]

34 minutes ago, yoda said:

Now if you can only get @Eskimo Joe to join in... pretty sure @high risk and @MN Transplant are in

Not really sold on this. I'll see what things look like tomorrow morning, but the whole system looks pretty progressive.

[yoda]

31 minutes ago, Eskimo Joe said:

Not really sold on this. I'll see what things look like tomorrow morning, but the whole system looks pretty progressive.

Thought you would be with LWX going pretty much all in in their morning AFD 

[Eskimo Joe]

Just now, yoda said:

Thought you would be with LWX going pretty much all in in their morning AFD 

I've been burned twice this summer. Hopefully I'm wrong and we get slammed, it's been years since we've had a decent remnants system in Baltimore/DC proper. I will say that if I'm from US 15 west in Maryland, or the WV panhandle there could be some decent rainfall totals aided by terrain so someone out west of here could be in for a fun day tomorrow.

[H2O]

Need that scale again.  We are level meh 

[yoda]

Just now, H2O said:

Need that scale again.  We are level meh 

I think we are at Level 3

[yoda]

DAY 1 SPC 1300Z OTLK 

   ...Northern VA to the Delmarva this afternoon/evening...
   Surface heating will be strongest the first half of the day, prior
   to the arrival of the thicker high clouds from the remnants of Ida. 
   At the surface, a weak front sagging southward from PA toward
   northern VA will provide a focus for thunderstorm development this
   afternoon into this evening.  Moderate buoyancy (MLCAPE of 1500-2000
   J/kg) is expected in the warm sector as surface temperatures warm
   into the 80s with dewpoints at or above 70 F.  Though midlevel lapse
   rates will be poor in the moisture plume aloft emanating from Ida,
   low-level lapse rates will be relatively steep with daytime heating,
   and there will be an increase in the 700-500 mb flow into the 40-50
   kt range this afternoon.  These factors will support a threat for
   damaging winds with the stronger cells/clusters this afternoon into
   this evening. 

[high risk]

1 hour ago, yoda said:

Now if you can only get @Eskimo Joe to join in... pretty sure @high risk and @MN Transplant are in

         I'm in on SVR.     I'm not in on widespread flooding rains until you get further north and northwest, although there will be isolated local flooding where recent heavy rains have occurred.    

         Right now, guidance shows most of Wednesday being dry which will allow for some heating for sure.     We then get some bands of convection arrive later in the day, with perhaps some moderate instability and likely strengthening low-level wind fields.     If some of the more aggressive guidance is correct, these won't be little spin-ups in a big shield of rain;  these will be legitimate supercells embedded in intense convective bands.    This UH image from the 12z HRRR would warrant a potential ENH for tornadoes:

 

  

       There are plenty of ways that this could fail, and we often fail here, but this setup has *potential* to be a big deal if we legitimately heat up tomorrow.    As for the rain, there could be some sneaky higher totals tonight, and those bands tomorrow evening will have torrential rainfall, but the heaviest rain will overall be too progressive, and the periods of intense rain will overall be relatively short-lived until you go further north.

 

 

[MN Transplant]

12z HRRR and NAM Nest get surface based CAPE >2000 tomorrow afternoon.

[delgto04]

39 minutes ago, high risk said:

         I'm in on SVR.     I'm not in on widespread flooding rains until you get further north and northwest, although there will be isolated local flooding where recent heavy rains have occurred.    

         Right now, guidance shows most of Wednesday being dry which will allow for some heating for sure.     We then get some bands of convection arrive later in the day, with perhaps some moderate instability and likely strengthening low-level wind fields.     If some of the more aggressive guidance is correct, these won't be little spin-ups in a big shield of rain;  these will be legitimate supercells embedded in intense convective bands.    This UH image from the 12z HRRR would warrant a potential ENH for tornadoes:

 

  

       There are plenty of ways that this could fail, and we often fail here, but this setup has *potential* to be a big deal if we legitimately heat up tomorrow.    As for the rain, there could be some sneaky higher totals tonight, and those bands tomorrow evening will have torrential rainfall, but the heaviest rain will overall be too progressive, and the periods of intense rain will overall be relatively short-lived until you go further north.

 

 

Hope not. Isaias was a little to close for comfort in my neck of the woods.

[mappy]

FYI there is remnant thread, if you wanna move severe chat to that one

 

[yoda]

5/5/15 today from N VA into DC and much of MD

[yoda]

LWX AFD from this morning... sounds like a fun afternoon and evening 

NEAR TERM /THROUGH TONIGHT/...
Latest surface analysis depicts a strong area of low pressure
over central Ontario with its attendant cold front draped across
the Great Lakes region down into Indiana/Ohio. The stationary
front that had been sitting along the MD/PA border has lifted
northward as a warm front this morning as high pressure moved
offshore. Water vapor satellite imagery depicts the large-
scale upper trough, whose axis runs from Ontario into Wisconsin.
Radar/IR satellite depict some ongoing convective activity out
ahead of the aforementioned cold front across the Ohio Valley,
which is starting to taper off.

With high pressure having shifted offshore and a warm front well
to our north, strong warm air advection is beginning to take
shape across the region. The first half of the day will be
rather uneventful, with only a little early morning fog in the
valleys in our far western zones perhaps. Cloud cover will
gradually increase throughout the morning as warm advection
increases.

This afternoon is where things start to get a little more
interesting. High temperatures are expected to rise into the
mid-upper 80s this afternoon, with dewpoints rising into the
mid-upper 60s and perhaps even some near 70. This will result in
a fairly wide swath of moderate CAPE values in the 1500-2000
J/kg range ahead of the approaching cold front (primarily east
of the Blue Ridge). Additionally, hi-res guidance hints at a
corridor of even higher CAPE amounts (closer to 2500 J/kg). A
subtle shortwave ahead of the main trough will move overhead
this afternoon. This combined with 10-15 kts of surface SSW flow
colliding with the bay breeze, should result in thunderstorm
development this afternoon ahead of the main line of storms,
which will come through during the evening. This afternoon round
could pose a threat for an isolated tornado or two as deep
layer shear begins to increase. The primary convective mode with
storms this afternoon will be supercellular. Tornado threat
with this round would stem from backed low level flow near the
bay breeze. Steep low- level lapse rates will yield the threat
for damaging winds as well. Additionally, while not the largest
threat, hail is still a threat with the fist round of storms.
Think it will be mostly on the smaller side given weak mid-level
lapse rates, but can`t rule out some larger hail should we get
a stronger supercell to form.

The second round of convection will come through during the
evening hours. This will be more of a linear structure
associated with the cold front. Primary threat with this round
will be damaging straight-line winds. An isolated tornado also
isn`t out of the question with round 2 either in any properly
oriented evening linear activity as a low level jet increases
ahead of the front. However, low-level winds may be more
southwesterly by that point (limiting SRH). Time of day and less
than ideal low level thermodynamics could be limiting factors.

In addition to the severe threat, as the front becomes more
parallel to the upper flow, forward propagation of linear
features will tend to slow down as well. This could limit
robust, forward- moving downdrafts. This fact may also lead to
some locally heavy rain totals, especially in eastern part of
the CWA, as clusters could train over areas for a time.
Hydrologically sensitive areas due to recent rainfall will be
most susceptible to any flooding, as precipitable water values
will not be excessively high.

[yoda]

1300z OTLK from SPC 

 

Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Wed Sep 08 2021

   Valid 081300Z - 091200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE MID-ATLANTIC REGION AND NORTHEAST...

   ...SUMMARY...
   A few tornadoes and scattered damaging thunderstorm winds are
   expected across portions of the Mid-Atlantic region and Northeast
   this afternoon and evening.

   ...Synopsis...
   A high-amplitude mid/upper-level pattern will prevail over the CONUS
   this period, featuring:
   1.  A strong anticyclone moving slowly east-southeastward over the
   Great Basin and Four Corners region, anchored by a 598-600-dm 500-mb
   high, and
   2.  A well defined trough extending south-southwestward from a
   cyclone now over northeastern ON.  As the cyclone moves
   northeastward to the QC Hudson Bay coastline, a series of shortwaves
   and vorticity maxima will traverse the amplifying trough over the
   Great Lakes, Ohio Valley and southern Appalachians.

   At the surface, 11Z analysis showed a cold front from southwestern
   ON across southwestern IN, southeastern MO, southern OK, northwest
   TX, and southeastern NM.  A dissipating, quasistationary to warm 
   frontal zone was drawn from the northern Chesapeake Bay region
   southwestward across middle/eastern TN to south-central TX.  By 00Z,
   the cold front should extend from VT/eastern NY southwestward across
   south-central PA, middle/eastern TN, northern MS, and central/west-
   central TX.  By the end of the period, the front should reach
   central/southern New England, southeastern VA, western SC,
   south-central AL, and portions of south-central/southwest TX.

   ...Northeastern CONUS to Mid-Atlantic/Carolinas...
   Scattered thunderstorms are forecast to develop during the midday
   through afternoon near the cold front, from northern NY across
   western PA, WV and eastern KY.  Additional activity should form in
   the warm sector over the Mid-Atlantic into the Carolinas.  The most
   intense, well-organized convection in both regimes (which may merge
   over the Susquehanna/Delaware Valley region by late afternoon)
   should be in a corridor of greatest shear and return-flow buoyancy
   from eastern NY and VT southwestward toward the DC vicinity, with
   damaging to severe gusts being common.  A few tornadoes will be
   possible as well, along with isolated large hail.  At least isolated
   severe-wind potential may persist eastward over central/southern New
   England and the southeast NY/Long Island region this evening. 
   Farther south into the Carolinas, activity will be less-organized,
   but still capable of isolated severe hail/wind.  Isolated damaging
   gusts also may occur with the frontal convection in higher terrain
   of the central/southern Appalachians, amidst weaker but still
   marginally supportive instability.

   Strong southwesterly flow and broad height falls will spread over
   the Mid-Atlantic and Northeast today along/ahead of the front and
   east of the slowly progressive upper trough, with largely front-
   parallel winds contributing to a dominant quasi-linear convective
   structure.  However, sufficient deep shear (35-45-kt effective-shear
   magnitudes as far south as MD/northern VA) and low-level SRH (150-
   250 J/kg effective values) will support early-stage discrete
   supercells, as well as line-embedded supercells, bows and
   mesovortices.  Pronounced weaknesses in the 2-4-km hodographs remain
   evident, which further indicate potential for messy storm
   structures.  A returning plume of mid/upper 60s F surface dew points
   is expected east of the Blue Ridge, and into southeastern
   NY/southern New England. This will act in tandem with diurnal
   heating to offset modest lapse rates aloft and yield MLCAPE in the
   500-1500 J/kg range, generally increasing southward.  MLCAPE values
   of 1500-2000 J/kg are possible over southeastern VA and the
   Carolinas, but amidst weaker midlevel winds and deep shear
   supporting multicells and marginal/transient supercells.