SPC Sep 4, 2026 1300 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Fri Sep 04 2026

Valid 041300Z - 051200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN
GREAT LAKES INTO THE MID-ATLANTIC STATES...

...SUMMARY...
A couple of organized clusters of thunderstorms may develop across
the southern Great Lakes and mid/upper Ohio Valley today through
tonight, accompanied by the risk for damaging wind gusts. More
widely scattered strong thunderstorms east of the Blue Ridge may
also pose potential for damaging wind gusts this afternoon and
evening.

...Southern Great Lakes into Mid Atlantic...
Early-morning satellite imagery shows a deep cyclone over the
Pacific Northwest and upper ridging covering the majority of the
CONUS east of the Rockies. A belt of enhanced mid-level flow
persists between these two features, arcing anticyclonically from
southern CA through the central Rockies and northern Plains into the
Upper Midwest/Upper Great Lakes and eventually through the
Northeast. Recent surface analysis shows a reservoir of low-level
moisture beneath the upper ridging, to the south and east of this
belt of stronger mid-level flow. Much of this area is void of
notable surface features, but a weak front does extend from southern
New England through southern NY, central Lower MI, and central WI to
a weak low over southern MN. A large cluster of showers and
thunderstorms to the south of this front from southern WI into
southern Lower MI. This cluster, which was initiated by warm-air
advection across MN and WI is forecast to continue southeastward
today, likely encountering a diurnally destabilized airmass.

The steeper mid-level lapse rates will remain displaced west of this
region, but ample low-level moisture (i.e. upper 60s/low 70s
dewpoints) coupled with afternoon temperatures from the mid 80s to
mid 90s will still support moderate to strong buoyancy. As such,
reintensification and/or new development is anticipated as the
ongoing cluster continues southeastward. Much of this region will be
on the southern fringe of the stronger deep-layer shear, with bowing
line segments as the predominant storm mode. The best overlap
between buoyancy and shear is expected across central and eastern
OH, where a corridor of greater potential for damaging gusts exists.

The threat for damaging gusts will extend into the Mid-Atlantic this
afternoon and evening amid a very warm and moist airmass. Vertical
shear will be weak, but ample moisture (i.e. PW values from 1.8" to
2"), strong buoyancy, and steep low-level lapse rates will support
the potential for strong outflow capable of wind damage. Bowing line
segments will be the primary storm mode here as well.

...Central Plains...
Weak lee troughing could contribute to modest low-level convergence
from western KS across western/central NE into southwest SD.
Dewpoints will likely be in the low to mid 60s within this
convergence zone, and steep mid-level lapse rates will contribute to
moderate/strong buoyancy. Even so, warm mid-levels may prevent the
airmass from completely destabilizing. The remaining convective
inhibition, coupled with the presence of large-scale height rises
across the region, casts doubt to whether successful convective
initiation will occur. Any mature updrafts could result in strong
downdraft, but uncertainty is too high to introduce any
probabilities with this outlook.

..Mosier/Lyons.. 09/04/2026


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SPC Sep 4, 2026 0600 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1200Z Day 1 Outlook


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1237 AM CDT Fri Sep 04 2026

Valid 041200Z - 051200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TODAY THROUGH
TONIGHT IN A CORRIDOR ACROSS THE SOUTHERN GREAT LAKES AND
MIDDLE/UPPER OHIO VALLEY INTO MID ATLANTIC...

...SUMMARY...
A couple of organized clusters of thunderstorms may develop across
the southern Great Lakes and mid/upper Ohio Valley today through
tonight, accompanied by the risk for damaging wind gusts. More
widely scattered strong thunderstorms east of the Blue Ridge may
also pose potential for damaging wind gusts this afternoon and
evening.

...Discussion...
Mid/upper flow is forecast to undergo further amplification today
though tonight across southern Canada and the U.S., downstream of
positively tilted large-scale troughing which continues to evolve
offshore of the Pacific coast through Intermountain West. It
appears that this will include building ridging across and north of
the international border, from the Rockies through upper Great
Lakes, to the north of a high centered over the central Great
Plains. Downstream, large-scale troughing is forecast to continue
slowly digging across the northern and middle Atlantic Seaboard. As
it does, it likely will suppress a prominent high initially centered
near the southern Appalachians.

Accompanying the eastern trough, reinforcing surges of cooler
low-level air will continue to overspread portions of the Great
Lakes and Northeast today through tonight. It appears that this
will contribute to a slow south and southwestward progression of the
primary surface front across the Mid Atlantic and through the upper
into middle Ohio Valley.

...Southern Great Lakes into Mid Atlantic...
A moist boundary layer, and remnant plume of relatively steep
lower/mid-tropospheric lapse rates, particularly across the Upper
Midwest and southern Great Lakes vicinity into the western slopes of
the Appalachians, may continue to support moderate to strong
potential instability along the quasi-stationary to slow moving
frontal zone. Latest convection allowing guidance suggests that
this front may already be providing a focus for organizing
thunderstorm development across southern Lower Michigan by the
beginning of the period, beneath the favorably sheared southwestern
periphery of the stronger westerlies. As the boundary-layer
destabilizes ahead of it with insolation, it appears that this
activity may intensify further, accompanied by increasing potential
to produce strong to severe wind gusts while propagating
southeastward across parts of northern/eastern Ohio into the
Allegheny Plateau by this afternoon.

It remains unclear whether forcing for ascent and destabilization to
the immediate lee of the Allegheny Mountains and Blue Ridge will
maintain this cluster. However, convection allowing guidance
suggests that the initiation of at least scattered vigorous diurnal
thunderstorm activity is probable downstream, closer to and across
the coastal plain, where the environment is likely to be supportive
of a few downbursts with strong to severe surface gusts late this
afternoon into evening.

Late this evening into the overnight hours, downstream of another
short wave perturbation digging to the east of the amplifying
mid-level ridging, there is a notable signal within
convection-allowing and related guidance that forcing for ascent may
support another evolving cluster of storms. This may track close
to, but probably at least a bit to the southwest of, the early day
cluster, from the southern Lower Michigan through Ohio and West
Virginia vicinity.

..Kerr/Chalmers.. 09/04/2026


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SPC Sep 4, 2026 0100 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 0100Z Day 1 Outlook


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0800 PM CDT Thu Sep 03 2026

Valid 040100Z - 041200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING
ACROSS PART OF THE MID ATLANTIC INTO MIDDLE OHIO VALLEY...AND ACROSS
PARTS OF THE UPPER MIDWEST...

...SUMMARY...
Scattered strong thunderstorm development, posing mainly a risk for
severe wind and hail, remains possible into the overnight hours
across parts of the Mid Atlantic and Upper Midwest into the Great
Lakes.

...01Z Update...

...Mid Atlantic...
A convectively generated or augmented mid-level perturbation
continues to migrate east-southeastward across the upper Ohio
Valley, toward the northern Mid Atlantic, along the southern fringe
of broadly cyclonic flow overspreading the Northeast. This
continues to provide support for a clustering of stronger convective
development currently in a corridor across east central Ohio into
south central Pennsylvania. Although this activity appears to have
waned some in intensity from earlier, updraft inflow emanating from
a more moist environment, along and south of a diffuse remnant
outflow boundary across the central Maryland through Delmarva
Peninsula vicinity, might reinvigorate activity later this evening
into the overnight hours.

...Upper Midwest into Great Lakes region...
Convective evolution into and through the overnight hours remains a
bit unclear. An initial cluster of convection, supported by a short
wave impulse now progressing toward the Minnesota/Ontario border
vicinity and Lake Superior, is undergoing substantial weakening as
inflow becomes more stable to the cool side of a quasi-stationary
frontal zone, which roughly extends from the Red River Valley
through southern Great Lakes region.

Upstream, near the crest of building mid/upper ridging,
lower/mid-tropospheric warm advection is supporting scattered
renewed thunderstorm development across northwestern and west
central Minnesota. Closer to the frontal zone, where the inflow
environment is characterized by moderate to strong instability,
there still appears potential for the evolution of a small
organizing cluster across the Brainerd MN vicinity through mid to
late evening. As this progresses eastward, it is not clear that
this will be maintained overnight. A strengthening southwesterly
low-level jet may focus additional upscale growing thunderstorm
development downstream, across parts of northeastern Wisconsin into
Lower Michigan, but, elevated to the cool side of the frontal zone,
it is not clear that the environment will support more than very
limited potential for severe hail and wind.

..Kerr.. 09/04/2026


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SPC Sep 3, 2026 2000 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 2000Z Day 1 Outlook


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0255 PM CDT Thu Sep 03 2026

Valid 032000Z - 041200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS EASTERN
OHIO AND WESTERN PENNSYLVANIA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER
MIDWEST AND GREAT LAKES TO MID-ATLANTIC...

...SUMMARY...
Scattered strong/severe thunderstorms are expected through tonight,
including a few organizing clusters, posing a risk for severe wind
and hail across parts of the southern Great Lakes into Mid Atlantic
and portions of the northern Great Plains to the Upper Midwest.

...20Z Update...

Minor adjustments have been made to the outlook to account for
recent convective trends and the latest high-resolution forecast
guidance. Portions of the Slight Risk in North Dakota have been
trimmed behind the ongoing convective complex that has since moved
into northern Minnesota, given the trailing airmass has stabilized
and is less likely to see additional convective development.
Additionally, the Marginal Risk was expanded northward from
Wisconsin into portions of Upper Michigan to account for recent
guidance trending northward with the overnight convective complex
and its associated damaging-wind threat. Otherwise, the previous
forecast remains unchanged and on-track for this evening.

..Halbert/Weinman.. 09/03/2026

.PREV DISCUSSION... /ISSUED 1140 AM CDT Thu Sep 03 2026/

...Lake Michigan/upper Ohio River Valley/Northeast...
Several clusters of storms are ongoing at midday across the Lake
Michigan vicinity and lower Michigan, including a smaller lead
cluster entering Ontario, while other convection farther
east-southeast has weakened/diminished across southern
Pennsylvania/Maryland. Regarding the easternmost activity, steady
air mass recovery/diminishing cloud cover appears likely, with ample
insolation across the Northeast along/south of the primary synoptic
front including much of Pennsylvania.

Available regional 12z observed soundings/RAOBs sampled a belt of
moderately strong northwesterly flow aloft that overlies the primary
frontal zone, with moderate to locally strong destabilization
expected outside of the ongoing convection. Some initial supercells
are possible, but more prevalent will be the possibility of
upscale-growing linear bands/clusters, potentially with multiple
rounds of strong/severe convection this afternoon into tonight with
wind damage. This currently appears most probable across northern
Ohio into Pennsylvania.

...Upper Midwest - eastern North Dakota/northern Minnesota...
A slow-moving linear cluster of occasionally severe storms may
persist and gradually accelerate east and maybe eventually
east-southeastward today, potentially becoming more surface based by
later in the afternoon as it parallels the immediate north side of a
surface frontal zone. This development is being influenced by a
northeastward-transitioning shortwave trough along the international
border, with a belt of strong mid/high-level winds extending
eastward across the northern half of Minnesota toward the Lake
Superior vicinity.

A mixed convective mode seems probable, with a prominent
quasi-linear mode but also the possibility of south-end
semi-discrete development including supercells. Large hail and
damaging winds are the primary hazards, but some tornado risk may
exist as well. One or more clusters may persist east-southeastward
across the region this evening.


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