WW 643 SEVERE TSTM DC MD VA WV CW 021540Z - 022100Z
URGENT - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Watch Number 643
NWS Storm Prediction Center Norman OK
1140 AM EDT Wed Sep 2 2026
The NWS Storm Prediction Center has issued a
* Severe Thunderstorm Watch for portions of
District Of Columbia
Maryland
Northern Virginia
The Eastern West Virginia Panhandle
Coastal Waters
* Effective this Wednesday morning and afternoon from 1140 AM
until 500 PM EDT.
* Primary threats include...
Scattered damaging wind gusts to 70 mph possible
Isolated large hail events to 1.5 inches in diameter possible
SUMMARY...A well-organized thunderstorm cluster should continue to
pose some threat for scattered damaging winds as it moves quickly
southeastward this afternoon. Occasional hail may also occur with
the strongest cores.
The severe thunderstorm watch area is approximately along and 65
statute miles east and west of a line from 5 miles east northeast of
Hagerstown MD to 40 miles south southwest of Washington DC. For a
complete depiction of the watch see the associated watch outline
update (WOUS64 KWNS WOU3).
PRECAUTIONARY/PREPAREDNESS ACTIONS...
REMEMBER...A Severe Thunderstorm Watch means conditions are
favorable for severe thunderstorms in and close to the watch area.
Persons in these areas should be on the lookout for threatening
weather conditions and listen for later statements and possible
warnings. Severe thunderstorms can and occasionally do produce
tornadoes.
&&
OTHER WATCH INFORMATION...CONTINUE...WW 642...
AVIATION...A few severe thunderstorms with hail surface and aloft to
1.5 inches. Extreme turbulence and surface wind gusts to 60 knots. A
few cumulonimbi with maximum tops to 550. Mean storm motion vector
32040.
...Gleason
WW 0643 Status Updates
STATUS REPORT ON WW 643
SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 35 SSW AOO
TO 5 S MRB TO 10 NNW DCA TO 20 S BWI TO 30 SE BWI TO 30 W DOV TO
15 S ILG.
..BENTLEY..09/02/26
ATTN...WFO...LWX...PHI...
STATUS REPORT FOR WS 643
SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS
DCC001-021940-
DC
. DISTRICT OF COLUMBIA COUNTIES INCLUDED ARE
DISTRICT OF COLUMBIA
MDC009-011-017-033-037-041-021940-
MD
. MARYLAND COUNTIES INCLUDED ARE
CALVERT CAROLINE CHARLES
PRINCE GEORGES ST. MARYS TALBOT
VAC013-043-047-059-061-069-079-099-107-113-137-139-153-157-171-
177-179-187-510-600-610-630-683-685-840-021940-
VA
. VIRGINIA COUNTIES INCLUDED ARE
ARLINGTON CLARKE CULPEPER
WW 0642 Status Updates
STATUS REPORT ON WW 642
SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 25 ENE LBE
TO 25 SE AOO TO 30 SW CXY TO 20 S CXY TO 15 ESE CXY TO 25 NE CXY.
..BENTLEY..09/02/26
ATTN...WFO...CTP...
STATUS REPORT FOR WS 642
SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS
PAC001-009-055-057-111-133-021740-
PA
. PENNSYLVANIA COUNTIES INCLUDED ARE
ADAMS BEDFORD FRANKLIN
FULTON SOMERSET YORK
THE WATCH STATUS MESSAGE IS FOR GUIDANCE PURPOSES ONLY. PLEASE
REFER TO WATCH COUNTY NOTIFICATION STATEMENTS FOR OFFICIAL
INFORMATION ON COUNTIES...INDEPENDENT CITIES AND MARINE ZONES
CLEARED FROM SEVERE THUNDERSTORM AND TORNADO WATCHES.
MD 2200 CONCERNING SEVERE POTENTIAL...WATCH LIKELY FOR WESTERN MONTANA
Mesoscale Discussion 2200
NWS Storm Prediction Center Norman OK
1249 PM CDT Wed Sep 02 2026
Areas affected...Western Montana
Concerning...Severe potential...Watch likely
Valid 021749Z - 022015Z
Probability of Watch Issuance...80 percent
SUMMARY...The potential for severe wind gusts should increase
steadily through late afternoon across western and northern Montana.
Watch issuance is expected by 20-21 UTC as thunderstorms begin to
intensify.
DISCUSSION...Latest GOES imagery depicts a pronounced vorticity
maximum traversing central ID and approaching western MT. Ahead of
this feature, weak convection has been percolating within a broad
swath of more stratiform precipitation. Although convective
intensity has been modest so far, steady intensification appears
probable through the afternoon hours as low-level destabilization
occurs ahead of the wave. Based on a consensus of recent guidance,
any lingering inhibition downstream should be largely removed as
temperatures warm into the mid/upper 80s, which will support not
only steep low-level lapse rates, but also MLCAPE values on the
order of 250-500 J/kg. In general, buoyancy magnitudes are expected
to remain fairly modest, and effective shear through the
CAPE-bearing layer is only forecast to reach into the 25-30 knot
range. This combination may support a few organized storms capable
of producing marginally severe hail through late afternoon, though
severe wind should be the predominant convective hazard.
The primary driver of the severe wind potential will more likely be
the strong downslope flow off the terrain of the northern Rockies as
the mid-level wave (and an attendant jet) overspread western MT
through the afternoon. Evidence of this is already noted via 30-35
mph gusts across south-central MT away from ongoing precipitation.
As convection gradually intensifies through the afternoon and
low-level lapse rates steepen, convective augmentation to the
background gradient wind field will likely result in swaths of 60-80
mph winds, even with otherwise anemic convection with poor radar
presentation.
Confidence in the severe wind threat is greatest across portions of
north-central MT where a combination of mostly clear skies should
maximize diurnal heating, and the passage of the mid-level jet axis
will maximize downward momentum transfer within convective cells.
Latest time-lagged CAM guidance has consistently shown the strongest
signal across this corridor during the 22-01 UTC period, though
strong/severe gusts may begin earlier within the higher terrain of
west/southwest MT. Regardless, convective trends are being monitored
and watch issuance is likely as downstream destabilization continues
and ongoing convection begins to intensify.
..Moore/Gleason.. 09/02/2026
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...TFX...MSO...
LAT...LON 45571239 45561291 45741337 46141382 46861430 47851464
48591489 49031454 49030971 48910922 48500887 47950907
47320952 46241087 45941133 45711180 45571239
MOST PROBABLE PEAK WIND GUST...65-80 MPH
MOST PROBABLE PEAK HAIL SIZE...UP TO 1.25 IN
MD 2199 CONCERNING SEVERE THUNDERSTORM WATCH 643... FOR NORTHERN VIRGINIA...AND CENTRAL/EASTERN MARYLAND.
Mesoscale Discussion 2199
NWS Storm Prediction Center Norman OK
1221 PM CDT Wed Sep 02 2026
Areas affected...northern Virginia...and central/eastern Maryland.
Concerning...Severe Thunderstorm Watch 643...
Valid 021721Z - 021845Z
The severe weather threat for Severe Thunderstorm Watch 643
continues.
SUMMARY...Some damaging wind threat will continue through the
afternoon.
DISCUSSION...A well-organized line of storms which produced sporadic
damaging wind gusts across central into southern Pennsylvania is
continuing south across central Maryland and northern Virginia.
Lightning/reflectivity trends suggest this line of storms has
weakened somewhat here recently. As this line of storms continues
southward, it will move into a less sheared environment. Therefore,
the current signs of weakening are unsurprising and expected to
continue through the afternoon. However, a well-established cold
pool and moderate instability ahead of this cluster of storms should
allow for some sporadic damaging wind threat over the next few hours
before it weakens below severe limits.
..Bentley/Gleason.. 09/02/2026
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...PHI...AKQ...LWX...
LAT...LON 39437754 39497672 39637628 39387590 39027587 38667605
38417625 38287672 38297719 38517758 39407787 39437754
MOST PROBABLE PEAK WIND GUST...55-70 MPH
SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1128 AM CDT Wed Sep 02 2026
Valid 021630Z - 031200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
WESTERN NEW YORK...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
MONTANA...
...SUMMARY...
Scattered to numerous severe thunderstorms are likely across parts
of the Great Lakes to the Mid-Atlantic this afternoon through
tonight. Damaging winds are the main threat, though some large hail
and a few tornadoes may also occur. Scattered severe gusts and
isolated large hail appear possible over the northern Rockies/High
Plains this afternoon and evening.
...Lower Michigan into the Mid-Atlantic...
An ongoing cluster across south-central PA has a well established
cold pool and a history of producing damaging winds across
western/central PA. This convection is expected to continue moving
quickly southeastward through the afternoon across parts of
MD/DC/northern VA and vicinity. There is still some uncertainty on
whether this cluster will be able to maintain intensity with
southward extent into the Mid-Atlantic, as it becomes increasingly
removed from the stronger forcing aloft/mid-level flow over the
Great Lakes into NY. Still, a differential heating zone remains
apparent along/near the Chesapeake Bay per recent visible satellite
imagery and surface observations. The cluster should tend to remain
on the warm side of this boundary, with steepening low-level lapse
rates developing as daytime heating continues to destabilize the
downstream airmass. Scattered damaging winds will likely remain a
threat with these thunderstorms as they track across the southern
Mid-Atlantic before eventually weakening.
In the wake of this late morning/early afternoon activity, it
appears likely that robust destabilization will occur across parts
of lower MI into western NY. 12Z observed soundings from APX/DTX/BUF
showed moderate to very strong MUCAPE and modestly steepened
mid-level lapse rates with an EML. This favorable airmass will
spread eastward through the day in tandem with a mid-level shortwave
trough and 50-60 kt mid-level jet also advancing eastward over the
Great Lakes and ON. Even with some cloud cover present, moderate to
strong instability will likely be in place across western NY by late
afternoon, to the west of a differential heating zone/front.
Thunderstorms are expected to develop along/ahead of a cold front
from parts of southeast Lower MI to the lower Great Lakes, with 50+
kt of deep-layer shear supporting organized updrafts. A mix of
supercells and multicells will likely pose a threat for both severe
hail and scattered to numerous damaging winds. Sufficient low-level
shear should also be present to support a few tornadoes, with some
chance for a strong tornado this evening if a supercell mode can be
maintained across western NY and vicinity. Confidence has increased
in a corridor of greater severe/damaging wind potential late this
afternoon/evening across parts of western NY, where an Enhanced Risk
has been introduced.
...Northern Rockies/High Plains...
High-based thunderstorms have already developed over the higher
terrain of central ID this morning. Enhanced winds aloft associated
with a northeastward-ejecting mid-level shortwave trough and steep
low-level lapse rates will support a risk for scattered
severe/damaging winds as this activity spreads northeastward across
MT this afternoon and evening. Occasional 75+ mph gusts may occur,
even though instability should remain weak. Isolated thunderstorms
may also develop later this evening/tonight in a low-level warm
advection regime across eastern MT into western ND, with some hail
threat.
...Eastern Utah into the Central Rockies/High Plains...
Thunderstorms are expected to increase in coverage this afternoon
over eastern UT into the higher terrain of northern/western CO and
southern WY. This activity will spread northeastward into eastern
WY/western NE where a very deep/well-mixed boundary layer should
support locally severe gusts.
..Gleason/Moore.. 09/02/2026
SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1218 PM CDT Wed Sep 02 2026
Valid 031200Z - 041200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
THE NORTH DAKOTA-MINNESOTA RED RIVER VALLEY INTO THE UPPER
MIDWEST...AND IN PARTS OF THE UPPER OHIO VALLEY TO MID-ATLANTIC
COAST...
...SUMMARY...
Severe storms, accompanied by predominantly severe wind gusts and
hail (including 75+ mph gusts and hailstones greater than two
inches), are possible from the North Dakota-Minnesota Red River
Valley into the upper Midwest. At least widely scattered severe
storms capable of mainly damaging wind gusts are also possible from
the upper Ohio Valley to the Mid-Atlantic coastline.
...Synopsis...
A high-latitude omega block will persist through the forecast
period, featuring upper lows along the Pacific Northwest coast and
over QC, which bracket a mid-level ridge centered along the MS
Valley. Several smaller-scale perturbations are expected to move
through the broader-scale flow regime from the northern Plains and
the upper MS Valley through the Great Lakes into the Mid-Atlantic,
aiding in thunderstorm development in those areas. At the surface,
an area of low pressure is forecast to develop through the Dakotas
into MN, while a preceding warm front lifts north through the upper
MS Valley and upper Midwest. The boundary is expected to extend
southeast through the Great Lakes into the Mid-Atlantic, where its
location may be modified from convective outflow stemming from
thunderstorms Wednesday night into Thursday morning.
...Northern Plains into the upper Great Lakes...
A plume of steep mid-level lapse rates associated with an
elevated-mixed layer (EML) will reside atop a moist boundary layer
in the vicinity of the surface low and warm front, yielding a
strongly unstable air mass by afternoon with MLCAPE increasing to
3000-4000+ J/kg. Amidst a backdrop of general height rises, the
progression of weak disturbances through the building ridge will
promote periods of forcing for ascent. The first of these may occur
Thursday morning when elevated thunderstorms appear possible from
portions of the mid-MO Valley into upper Midwest. Given the presence
of the EML and sufficient cloud-bearing shear, isolated occurrences
of large hail may accompany the strongest storms as they spread east
into the upper Great Lakes.
In the wake of the early-day storms, residual capping associated
with the building ridge coupled with the subtle forcing for ascent
results in considerable model variability in the location of
subsequent, surface-based storm development Thursday afternoon into
night. Nonetheless, the degree of instability and up to 40-50 kt of
deep-layer shear will conditionally support higher-order storm modes
including supercells and bowing structures, both of which are
depicted by the suite of various 12Z CAMs. Hail in excess of two
inches, severe wind gusts greater than 75 mph, and a couple of
tornadoes appear possible with the most intense storms.
The 15% hail and accompanying CIG1 contours have been expanded into
the ND-MN Red River Valley and in parts of WI to account for the
large hail potential. Otherwise, the forecast will remain largely
unchanged due to the spatial uncertainty in surface-based storm
development. Once finer-scale details become more clear, an upgrade
to a level 3/Enhanced Risk may become necessary, primarily for
severe winds associated with a cold-pool-driven MCS.
...Upper Ohio Valley to the Mid-Atlantic Coast...
Similar to this morning, clusters of thunderstorms capable of
isolated occurrences of damaging wind may be ongoing at 12Z Thursday
over portions of the lower Great Lakes and upper OH Valley regions.
A subset of the morning storms may intensify by afternoon over the
Delmarva/southern Mid-Atlantic with additional storms forming during
the peak of the diurnal heating cycle in the vicinity of the surface
front and/or residual outflow boundaries over the upper OH Valley.
Despite weakening mid-level lapse rates with eastward extent,
daytime heating combined with dewpoints largely in the low/mid 70s
will contribute to moderate to strong instability by afternoon along
and south of the effective surface front with MLCAPE approaching
2500-3500 J/kg. The frontal zone will coincide with a belt of 40-45
kt west-northwest mid-level flow, yielding sufficient vertical shear
for supercells and organized line segments and/or bowing structures.
The primary hazard will be damaging wind gusts, though isolated
occurrences of large hail are possible with any supercells that may
evolve.
A slight northward shift in the Marginal and Slight Risk
probabilities has been made to account for stronger instability and
an associated severe risk evident in various 12z models.
..Mead.. 09/02/2026
SPC Day 1 Fire Weather Outlook
Day 1 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
1130 AM CDT Wed Sep 02 2026
Valid 021700Z - 031200Z
No changes were made to the Day 1 fire weather outlook as the
forecast remains on track. Please see the previous forecast
discussion for additional details.
..Williams.. 09/02/2026
.PREV DISCUSSION... /ISSUED 1250 AM CDT Wed Sep 02 2026/
...Synopsis...
Southwesterly flow will spread across much of the Intermountain West
today ahead of an approaching mid-level trough near the Pacific
Northwest and an associated shortwave moving through the northern
Great Basin. Ahead of the impending cloudy and cooler conditions
anticipated to impact those regions, an area of warm and dry air
will impact the Southwest to northern High Plains. However, recent
rainfall and therefore less dry fuels over much of the Southwest and
southern Great Basin will preclude any highlights over those
regions. Meanwhile, critically dry fuels due to ongoing drought from
recent drier and warmer than normal conditions are in place over
portions of far western SD through central and southwestern MT. Over
this area, southwesterly surface winds are expected to reach speeds
of 15-20 mph amid low RHs of 15-20%. An area of particularly strong
wind (near 20 mph) and dry RHs (near 10-15%) could promote brief
localized critical fire weather conditions generally over portions
of central/northeast WY. Though not anticipated to alter fuels
receptivity, light rainfall associated with convection is possible
over far northwestern (central MT) and far southeastern (southern
WY) extents of the drawn Elevated fire weather risk area today.
...Please see www.spc.noaa.gov/fire for graphic product...
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