890
ACUS02 KWNS 121721
SWODY2
SPC AC 121720

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1220 PM CDT Wed Jun 12 2024

Valid 131200Z - 141200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS IN SOUTHERN
IA...NORTHWEST IL...AND NORTHERN MO...

...SUMMARY...
Severe thunderstorms are most likely across southern Iowa into
northern portions of Missouri and Illinois during the late afternoon
to early evening on Thursday. Very large hail, destructive wind
gusts, and a couple tornadoes are possible.

...Central Great Plains to central Great Lakes...
Increasing concern exists for a potentially intense severe weather
episode, centered on the late afternoon to early evening across
parts of the Midwest. A highly favorable thermodynamic and kinematic
space should exist for significant severe hail and wind threats with
supercells likely growing upscale into southeast-moving
clusters/MCSs. Spatial confidence is only about average for the D2
time frame, rendering moderate uncertainty on the swaths of greatest
threat. For now, have upgraded to level 3-ENH risk across the most
likely corridor and broadened lower probabilities with added sig
severe highlights surrounding it from eastern KS to northwest IN.

A fast upper-level jet streak will become centered across parts of
the Upper Midwest/Great Lakes by Thursday afternoon, as a series of
embedded shortwave impulses track along the international border
into parts of ON/QC. A surface cyclone attendant to the leading
impulse should move across eastern ON into QC. A cold front will
trail to the southwest, likely centered on southern WI to the KS/NE
border area at 21Z. A secondary thermal low should form over the
central Great Plains portion of the front with hot temperatures from
100-105 F west of the dryline. A plume of large to extreme buoyancy
should develop amid very steep mid-level lapse rates and a corridor
of enhanced boundary-layer moisture ahead of the impinging front.

Guidance does differ somewhat on the spatiotemporal aspects of
convective development near the frontal zone by late Thursday
afternoon. 12Z CAMs such as the HRRR are farther east-northeast with
their centroid of development along the Mid-MS Valley, while the
NSSL-ARW would suggest a centroid farther west over the Mid to Lower
MO Valley. Low-level convergence along the front should be fairly
similar across both regions, suggesting either corridor may sustain
intense storm development.

Strong speed shear above 700 mb amid west-northwest flow will
support an elongated hodograph. This coupled with ample instability
should yield initial splitting supercells capable of very large
hail. Orientation of the shear vector semi to nearly paralleling the
front should yield upscale growth by early evening, especially with
western extent where effective bulk shear is relatively weaker.
Given the big MLCAPE, potential for intense downdrafts will exist
with significant severe wind gusts of 75-95 mph possible. One or two
convective wind swaths may surge southeastward briefly through about
dusk. The eastern lobe of the  pronounced elevated mixed layer
should foster increasing MLCIN after dusk, suggesting that severe
wind gust potential should wane Thursday night.

..Grams.. 06/12/2024

$$