SPC Sep 28, 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
1151 PM CDT Sun Sep 27 2026

Valid 281200Z - 291200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Severe thunderstorm probability remains low on Monday.

... Synopsis ...

A broad mid/upper-level trough will move inland over California and
begin to close off over the lower Colorado River Valley, while a
weaker shortwave trough moves through the northern Rockies.
Large-scale flow around the Southwest US trough will draw deep
tropical moisture tied to Hurricane Polo and remnants of Odalys
northward into the Southwest. Downstream of this, a weaker mid-level
short-wave trough will move across the northern Rockies, mid-level
ridging will persist across the central US and a weakening
vertically stacked cyclone will begin to move northeast along the
southern New England coast.

At the surface, a weak trough will sharpen along the lee of the
northern Rockies ahead of a surface cold front that will move
southeast into the northern and central Dakotas overnight.

... Northern High Plains ...

Persistent southerly low-level flow will keep modest moisture over
the northern High Plains. Warm-advection showers and thunderstorms
along the low-level jet axis Monday morning, followed by thickening
mid/high-level clouds, will likely limit destabilization. Where
filtered sunshine is able to promote stronger surface heating, a
corridor of MUCAPE between 500-1500 J/kg (per mean HREF values) may
develop by late afternoon near the surface trough from northeast
Wyoming, western South Dakota, and southwest/south-central North
Dakota.

Isolated storms may form near the trough in the late afternoon or
early evening. Increasing mid-level flow ahead of the northern
Rockies short-wave trough will provide sufficient vertical shear for
a few thunderstorms capable of producing small to marginally severe
hail. Additionally, locally strong thunderstorm wind gusts will be
possible where the boundary layer becomes well mixed. However,
modest mid-level warming, weak large-scale ascent, and uncertainty
about whether intense storms form at all should keep any potential
severe window narrow in space and time.

More elevated storms are possible Monday night as the low-level jet
strengthens and the cold front moves into the northern and central
Dakotas. Buoyancy will weaken overnight and become increasingly
elevated, which should limit any severe wind threat after dark.

... Southwest ...

Numerous showers and thunderstorms are expected as an anomalously
moist airmass spreads north ahead of the western trough. Widespread
clouds and weak midlevel lapse rates should keep instability
generally weak and storm organization limited. An isolated wet
downburst is possible from heavier cores in any pockets of strong
heating, but the severe threat appears too low to warrant
unconditional probabilities.

... Northeast ...

Showers and occasional embedded thunder -- mainly near the coast --
may continue with the slowly weakening cyclone. Instability will be
weak, so severe thunderstorm potential is extremely low. However,
strong onshore gradient winds may cause isolated/sporadic damage.

..Marsh/Weinman.. 09/28/2026


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SPC Sep 28, 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
0746 PM CDT Sun Sep 27 2026

Valid 280100Z - 281200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Organized severe storms are not expected tonight.

... Synopsis ...

Several different regimes of thunderstorms are ongoing this evening
across the CONUS and others may develop overnight.

The first of these is across portions of Texas and far southern
Oklahoma. Here, a moist airmass combined with strong diurnal heating
and weak surface convergence have supported scattered
strong-to-marginally severe thunderstorms. The strongest of these
storms will continue to pose a gusty wind threat for another couple
of hours before nocturnal cooling weakens low-level lapse rates and
decreases the greater wind potential.

A second area is in and near western portions of Cherry County,
Nebraska. Here, a couple of intense supercells have developed this
afternoon, with MRMS MESH suggesting hail perhaps in excess of 3" in
diameter at various times this afternoon. These nearly stationary
storms will persist for another couple of hours before weakening
with the loss of diurnal heating.

A third area is located across the central Rockies where large-scale
ascent associated with a subtle short-wave trough embedded in the
southwest mid-level flow has combined with modest instability. These
thunderstorms should progress eastward during the evening hours.
Additional thunderstorms are likely to form ahead of these across
the northern Plains this evening and overnight as low-level warm-air
advection strengthens.

Elsewhere, a couple of ongoing thunderstorms will persist for a few
more hours across northern California and southern Oregon, and a
couple of lightning strikes cannot be ruled out overnight across
portions of southern and eastern New England as the intense, nearly
stationary east coast cyclone begins to slowly move northward toward
the southern New England coast.

..Marsh.. 09/28/2026


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SPC Sep 27, 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
0257 PM CDT Sun Sep 27 2026

Valid 272000Z - 281200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Organized severe storms are not expected through early Monday.

...20z Update...
No changes were made to the existing outlook with this update cycle.
Isolated convective development is ongoing across portions of
central/north Texas as of 1945 UTC. While an isolated
strong/damaging wind gust cannot be ruled out with this activity
owing to steep low-level lapse rates and moderate buoyancy, weak
lower tropospheric flow (as depicted by regional VWPs), limited
effective shear, and poor mid-level lapse rates should largely
temper overall convective intensity. Thus, the severe threat
continues to appear too low (less than 5%) to warrant the inclusion
of severe probabilities. See the discussion below for additional
details.

..Chalmers/Guyer.. 09/27/2026

.PREV DISCUSSION... /ISSUED 1137 AM CDT Sun Sep 27 2026/

...Synopsis...
An occluded cyclone will slowly weaken near the northern
Mid-Atlantic coast, with lingering potential for elevated convection
and isolated lightning flashes to its northeast across southeast New
England. Farther west, weak ridging aloft will extend from the
southern High Plains toward MN. A few thunderstorms will be
possible late this afternoon/evening along a diffuse surface
boundary from north TX into southern OK. The strongest storms may
produce gusty outflow winds given steep low-level lapse rates with
afternoon mixing, but the severe threat appears too low to justify
adding severe-wind probabilities.

Slightly elevated convection is ongoing across central SD in a zone
of weak low-level frontogenesis, coincident with midlevel lapse
rates near 8 C/km and MUCAPE close to 1000 J/kg. Deep-layer
vertical shear is on the lower margins for storm organization, but
it appears this convection has already peaked. Additional elevated
thunderstorms are expected overnight from western SD into ND as
low-level warm advection increases in association with embedded
speed maxima downstream from a midlevel trough moving across CA/NV.
While an isolated hail report could occur overnight as far northeast
as southwest ND, the overall threat is low and when combined with
the questionable threat for diurnal convection across the central
High Plains, the MRGL area has been removed.


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SPC Sep 27, 2026 1630 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1630Z Day 1 Outlook


Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
1137 AM CDT Sun Sep 27 2026

Valid 271630Z - 281200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

CORRECTED TO REMOVE DUPLICATE TSTM AREA IN NEW ENGLAND

...SUMMARY...
Organized severe storms are not expected through early Monday.

...Synopsis...
An occluded cyclone will slowly weaken near the northern
Mid-Atlantic coast, with lingering potential for elevated convection
and isolated lightning flashes to its northeast across southeast New
England. Farther west, weak ridging aloft will extend from the
southern High Plains toward MN. A few thunderstorms will be
possible late this afternoon/evening along a diffuse surface
boundary from north TX into southern OK. The strongest storms may
produce gusty outflow winds given steep low-level lapse rates with
afternoon mixing, but the severe threat appears too low to justify
adding severe-wind probabilities.

Slightly elevated convection is ongoing across central SD in a zone
of weak low-level frontogenesis, coincident with midlevel lapse
rates near 8 C/km and MUCAPE close to 1000 J/kg. Deep-layer
vertical shear is on the lower margins for storm organization, but
it appears this convection has already peaked. Additional elevated
thunderstorms are expected overnight from western SD into ND as
low-level warm advection increases in association with embedded
speed maxima downstream from a midlevel trough moving across CA/NV.
While an isolated hail report could occur overnight as far northeast
as southwest ND, the overall threat is low and when combined with
the questionable threat for diurnal convection across the central
High Plains, the MRGL area has been removed.

..Thompson/Chalmers.. 09/27/2026


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