SPC MD 163

SPC MD 163


Mesoscale Discussion 0163
NWS Storm Prediction Center Norman OK
0250 PM CST Tue Feb 27 2024

Areas affected…Northern IL and far northwest IN

Concerning…Severe potential…Tornado Watch likely

Valid 272050Z – 272245Z

Probability of Watch Issuance…80 percent

SUMMARY…A few supercells are expected to develop over parts of
northwest Illinois and then spread east-northeast across northern
Illinois into far northwest Indiana through early evening. Very
large hail to around baseball size will be the primary initial
threat. A conditional strong tornado scenario may develop later over
northeast Illinois into far northwest Indiana.

DISCUSSION…20Z subjective surface analysis placed a 990-mb cyclone
near the Quad Cities along a pronounced cold front sweeping
southeast across the Upper MS to Lower MO Valleys. A bent-back plume
of relatively richer surface dew points from the mid to upper 50s
lies immediately ahead of this wave, along the northwest periphery
of the broader warm-moist sector across the OH Valley and lower
Midwest. Within the exit region of an intense mid-level jet shifting
east from the southern Great Plains to the Lower OH Valley,
continued ascent should yield sufficient weakening of MLCIN during
the 22-23Z time frame. Initial storm development is most likely in
the immediate vicinity of the surface low.

The environment will be favorable for discrete supercells forming
despite initially modest low-level shear. Amid very steep mid-level
lapse rates, large to very large hail production is expected, with
peak intensity around 2 to 3.5 inches possible. The primary
uncertainty is with how convection evolves downstream given the
relatively narrow warm/moist sector where storms develop. There
should be a tendency for convection to spread into a drier boundary
layer towards the WI border. It is plausible, within a scenario
highlighted by various 12Z MPAS members and occasionally by later
WoFS/RRFS/HRRR runs, that a longer-tracked supercell or two may
develop into the broader warm-moist sector across northeast IL into
far northwest IN. Low-level shear within this region will be
increasing towards and after dusk, which will conditionally support
potential for a strong tornado into the EF2-EF3 range (peak
estimated gusts around 120 to 150 mph).

..Grams/Hart.. 02/27/2024

…Please see www.spc.noaa.gov for graphic product…


LAT…LON 41339107 41889019 42428914 42648804 42478749 41728666
41188637 40588664 40008714 40078770 40758927 40549065

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