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SPC Sep 24, 2020 0100 UTC Day 1 Convective Outlook

SPC 0100Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0751 PM CDT Wed Sep 23 2020

Valid 240100Z - 241200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ISOLATED STRONG
STORMS MAY POSE SOME RISK FOR SEVERE WEATHER TONIGHT ACROSS PARTS OF
CENTRAL AND SOUTHERN MISSISSIPPI...PORTIONS OF THE UPPER
MIDWEST...AND WEST OF THE WASHINGTON CASCADES...

...SUMMARY...
Thunderstorm activity which could pose at least some risk for a
tornado may persist tonight across parts of central and southern
Mississippi.

...01Z Outlook Update...

...Lower Mississippi Valley...
Thunderstorm activity has generally been confined to a short narrow
band, just in advance of the center of Beta, apparently aided by a
weak mid-level thermal trough (as suggested by the Rapid Refresh
near/just above 700 mb).  As the surface low center of Beta migrates
across the Mississippi River into southwest/west central Mississippi
overnight, it is possible that this may continue.  Supported by
inflow of boundary-layer moisture characterized by lower/mid 70s F
dew points, in the presence of modest clockwise curved low-level
hodographs, it is possible that there could be occasional low-level
mesocyclone strengthening which could pose at least some risk for a
tornado.


...Elsewhere...

...Thunderstorm activity may continue to increase this evening, into
the overnight hours, within a narrow corridor along a frontal zone
across parts of north central Minnesota into northern Wisconsin and
adjacent Upper Michigan.  This may be aided by forcing associated
with a short wave trough digging into the region, and its possible
that thermodynamic profiles and cloud-bearing layer shear may be
marginally conducive from some hail and gusty winds in stronger
storms.

...Isolated thunderstorm development has also occur ahead of the
digging mid-level trough across parts of the western Dakotas, where
boundary-layer instability largely driven by daytime heating became
locally maximized.  This activity is now waning north/northeast of
the Black Hills vicinity, but could perhaps continue into the 02-04Z
time frame.

...As the base of a 500 mb thermal trough with temperatures near or
below -20C migrates inland, scattered thunderstorm development
appears increasingly possible west of the Washington Cascades by
04-06Z.  It is possible that boundary-layer instability may become
marginally sufficient, in the presence of moderate to strong
deep-layer mean flow and shear, to support some risk for hail and
gusty winds in stronger activity.

..Kerr.. 09/24/2020

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SPC Sep 24, 2020 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1217 AM CDT Thu Sep 24 2020

Valid 241200Z - 251200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY ACROSS
PARTS OF SOUTHEASTERN MISSISSIPPI...CENTRAL AND SOUTHERN
ALABAMA...SOUTHWESTERN GEORGIA AND THE FLORIDA PANHANDLE...

...SUMMARY...
A few strong thunderstorms may impact parts of the eastern Gulf
Coast states today, accompanied by at least some severe weather
potential.  This may include the risk for a couple of tornadoes,
mainly this afternoon across parts of south central Alabama.

...Synopsis...
Strong zonal flow emanating from the northern mid-latitude Pacific
appears likely to rapidly overspread the Pacific Northwest through
the northern Great Plains during this period.  A vigorous short wave
trough on the leading edge of this regime is now progressing inland
of the British Columbia/Pacific Northwest coast, and is forecast to
progress east-northeastward through the Canadian Prairies today
through tonight.  This probably will be accompanied by significant
surface cyclogenesis from the lee of the Canadian Rockies through
northern Saskatchewan by late tonight.  A cold front may also surge
southeastward to the lee of the northern U.S. Rockies by late this
afternoon.  This probably will be preceded by strengthening
southwesterly low-level flow across the northern Great Plains,
near/east of pre-frontal surface troughing, but initially modest
moisture content beneath the eastward advecting nose of a plume of
warm elevated mixed-layer air is expected to result in low
convective potential.

Models indicate that seasonably high precipitable water content will
largely remain confined to a couple of northeastward advecting
plumes, east of mid-level troughing, within a remnant branch of weak
westerlies, gradually shifting east of the middle/lower Mississippi
Valley toward the middle/southern Atlantic Coast.  This includes
mid-level remnants of Beta, which may become increasingly
sheared/deformed while progressing across the central Gulf States
through the lee of the southern Appalachians, slightly in advance of
the associated weakening surface low.

...Southeast...
Despite the lower/mid tropospheric moistening, relatively warm
mid-level air may limit destabilization, and perhaps tend to
suppress thunderstorm development across much of the region. 
Initially dry/potentially cool boundary-layer air emanating from
surface ridging across the lower Ohio Valley into the southwestern
Atlantic may also slow boundary-layer destabilization.  However, an
influx of lower/mid 70s F surface dew points, within a narrow plume
off the Gulf of Mexico into the vicinity of the surface low center,
may continue.  And models suggest that this may overspread southern
into central Alabama and the western Florida Panhandle by early this
afternoon, contributing to mixed-layer CAPE on the order of 1000+
J/kg. 

Although stronger lower/mid tropospheric wind fields may tend to
develop northeastward into the residual drier/more stable air, 30+
kt flow in the 850-700 mb layer may persist as far south/southwest
as south central Alabama, as the boundary layer destabilizes this
afternoon.  This may contribute to sizable clockwise-curved
low-level hodographs supportive of low-level mesocyclone
development, as large-scale forcing for ascent and daytime heating
aid widely scattered to scattered intensifying thunderstorm
activity.  Some of this could pose a risk for a tornado, before
convection wanes late this afternoon or evening.

..Kerr/Wendt.. 09/24/2020

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