SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0300 PM CDT Tue Jul 11 2023
Valid 112000Z - 121200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN/CENTRAL PLAINS INTO THE MIDWEST...
...SUMMARY...
Supercells capable of large to very large hail are possible this
afternoon and evening from eastern Montana and western South Dakota
into Nebraska. One or more convective lines may evolve late this
evening and overnight from central Nebraska into Iowa. Severe wind
gusts may occur with this convection.
...20Z Update...
The Slight Risk area has been expanded to include more of
northeastern MT, western ND, and northwestern SD. Multiple
supercells capable of producing large to very large hail should
spread east-southeastward across this area through the rest of the
afternoon and evening. See Mesoscale Discussion 1498 for more
details on the short-term severe threat across this region.
Otherwise, minimal changes have been made to the ongoing forecast.
Additional convection should eventually develop across parts of NE
along/near a front as low-level warm advection gradually increases
later this evening. This activity may initially pose a threat for
large hail. But, quick upscale growth into an MCS should occur, with
a greater threat for severe/damaging winds through the overnight
hours across eastern NE into IA.
Reference Mesoscale Discussion 1499 for additional information
regarding thunderstorms which have recently developed across parts
of northern IL and far northwestern IN along a weak front. Isolated
strong to severe wind gusts should be the main threat with this
activity.
..Gleason.. 07/11/2023
.PREV DISCUSSION... /ISSUED 1130 AM CDT Tue Jul 11 2023/
...Eastern MT into SD/NE...
Early morning surface analysis places a lee trough across eastern MT
to weak low over the northwest NE. Airmass in the vicinity of the
lee trough is currently characterized by mid 50s to low 60s
dewpoints. Modest mid-level warm advection is fostering widespread
cloud cover to the east of the lee troughing across far eastern MT
and the Dakotas. This area of cloud cover is forecast to shift
eastward over time, allowing for daytime heating to destabilize the
air mass. Low-level convergence along the lee trough will be
augmented by increasing large-scale forcing as the shortwave trough
currently over western MT moves eastward into the region.
This combination of air mass destabilization and forcing for ascent
is expected to foster thunderstorm development across western MT
during the afternoon. Low 60s afternoon dewpoints and temperatures
in the 80s beneath steep mid-level lapse rates will support moderate
to strong buoyancy, with MLCAPE from 1500 to 2500 J/kg. Enhanced
mid-level flow will extend across the region as well, supporting
long hodographs and 50+ kt of 0-6 km vertical shear. These
environmental conditions will result in an initially supercellular
mode, with large to very large hail possible for both left and right
splits. High cloud bases suggest strong downbursts are possible as
well. Evolution into one or more bowing segments is anticipated
after a few hours, with these segments progressing eastward into
southwest ND and western SD during the evening, and eventually into
parts of NE. Severe threat will continue with these bowing segments,
with damaging gusts as the primary risk.
...Central SD/NE into IA...
Early morning surface analysis places a cold front from southern IA
westward across central NE, linking up with lee troughing across
eastern MT via a weak low over the northwest NE Panhandle. Elevated
showers and thunderstorm are currently ongoing north of the front
across central SD and central NE, with outflow from these storms now
emerging across north-central NE. A trend towards more surface-based
activity is possible as this outflow continues
southward/southeastward as well as with the more southerly storm
across central NE. There is also the chance for more in-situ
development along the front later this afternoon if skies clear.
Main uncertainty is whether or not the air mass sufficiently
destabilizes given the current cloudiness and relatively warm
mid-levels. As such, a conditional threat for severe thunderstorm is
anticipated throughout the afternoon and into the evening, with
strong shear likely supporting supercells capable of large to very
large hail and damaging gusts.
Later this evening, whatever evolves out of the upstream activity
across eastern MT and NE Panhandle will likely progress into the
region, supporting the potential for damaging gusts and isolated
hail throughout the night. Some significant gusts are possible,
depending on how organized the system is as it moves into central
NE. Even at this shorter range, guidance remains varied on the
preferred corridor for this potential MCS, limiting the confidence
needed to introduce higher wind probabilities with this outlook.
...North TX into southern MS/AL...
MCS that moved across OK last night has weakened considerably, but a
well-developed MCV in it wake. This MCV is forecast to drift
southeastward throughout the day, with thunderstorms likely in its
vicinity as well as along the leading edge of the
southeastward-progressing outflow. Winds aloft are weak, which
should keep the overall severity limited, with a risk of locally
damaging wind gusts in the stronger cells.
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