SPC Mar 18, 2025 1730 UTC Day 2 Convective Outlook

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1225 PM CDT Tue Mar 18 2025

Valid 191200Z - 201200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
EASTERN ILLINOIS INTO WESTERN INDIANA...

...SUMMARY...
Scattered severe thunderstorms are possible on Wednesday across
parts of eastern Illinois and western Indiana from mid-afternoon to
early evening. Severe gusts and hail will be the primary threats,
though a couple of tornadoes are also possible.

...Synopsis...
A potent mid-level trough will traverse the Plains and overspread
the MS Valley, encouraging an intense surface cyclone to track from
the Mid-MO Valley to the Great Lakes on Wednesday. Despite the
intensity of the surface low and accompanying upper support from the
mid-level trough, buoyancy is expected to remain scant within the
warm sector given very limited low-level moisture. Nonetheless,
strong isallobaric surface flow and the presence of a low-level jet
ahead of the surface cyclone will allow for the northward advection
of some moisture (however marginal) to support low-topped storms
amid a highly sheared airmass. Some of these storms may become
strong to occasionally severe, particularly in the Midwest toward
the OH Valley.

...Midwest to OH Valley...
Modest clearing and associated insolation ahead of the surface low
will promote modest boundary-layer mixing and destabilization amid
marginal moisture. Surface dewpoints of at least 50 F with the
aforementioned heating, beneath 7-8 C/km mid-level lapse rates, will
support 250-750 J/kg SBCAPE by mid to late afternoon. During this
time frame, surface winds are expected to be backed from the
southeast, with a 50+ kt southwesterly low-level jet contributing to
substantial veering/strengthening of the low-level wind profile.
Somewhat curved and elongated hodographs will contribute to over 300
m2/s2 of sfc-3km SRH. Given marginal buoyancy, an arcing band of
low-topped supercells will precede the surface low, accompanied by a
damaging gust/hail threat. Furthermore, the strong low-level shear,
and increasing low-level vertical vorticity (as the surface low
approaches) will also foster a risk for at least an isolated
tornado.

There are some discrepancies among guidance members regarding the
degree of low-level destabilization across portions of the OH Valley
ahead of the surface low. RAP forecast soundings show mid 50s F
surface dewpoints beneath 8+ C/km mid-level rates, supporting a
relatively higher tornado threat compared to NAM, which shows low
50s F dewpoints overspread by 6.5-7.5 C/km lapse rates. Higher
tornado probabilities may be needed in future outlooks if guidance
consensus depicts more boundary-layer instability.  

...TN Valley into the Southeast...
At least isolated thunderstorms are expected to develop by late
afternoon into evening along the surface cold front, which will
sweep across the TN Valley/Southeast areas. These storms are
expected to be low-topped in nature, but highly sheared (given a 40+
kt southwesterly low-level jet, beneath 80+ kts of southwesterly 500
mb flow, contributing to 50-70 kts of effective bulk shear). In
addition to marginal low-level moisture, modest warming in the
850-700 mb layer will also limit thunderstorm intensity and
coverage. Given strong low-level and deep-layer directional and
speed shear, any storms that can become established and sustained
may become supercellular, posing mainly a risk for a few instances
of damaging gusts/hail.

..Squitieri.. 03/18/2025

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