SPC AC 251938
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0238 PM CDT Tue Sep 25 2018
Valid 252000Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE
MID-MISSISSIPPI VALLEY EASTWARD TO LOWER MICHIGAN AND NORTHWESTERN
Scattered damaging gusts and isolated large hail will be possible
this afternoon and evening from eastern Iowa to northern Illinois
and southern Wisconsin, and at least isolated damaging gusts will be
possible into the overnight hours across Lower Michigan. A couple
of tornadoes will be possible this afternoon from northeastern
Indiana into northwestern Ohio.
...20Z Outlook Update...
An ongoing MCS across northwestern Illinois will continue to pose a
risk for damaging wind gusts and perhaps an isolated tornado or two
through this evening. These storms will be sustained by a
moderately unstable downstream airmass along with roughly 40 knots
of deep shear. Ref WW 388 and associated Watch Status Messages for
Downstream of this MCS, airmass recovery was ongoing across Indiana
and Ohio in the wake of morning convection, although this recovery
was tied primarily to advection as only a few breaks in the cloud
deck are present via satellite imagery. Models continue to suggest
that convection evolving in upstream areas (i.e. Illinois and
vicinity) will exhibit mixed modes (linear and cellular) while
migrating toward the region, with low 70s F dewpoints and
strengthening low-level flow supporting near-surface-based
convection along with a modest increase in tornado potential. The
risk for damaging winds will also continue. Thus, 5% tornado/15%
wind probabilities remain in place for this outlook, with the bulk
of the Indiana/Ohio/lower Michigan severe potential occurring
.PREV DISCUSSION... /ISSUED 1125 AM CDT Tue Sep 25 2018/
A weak surface cyclone in south central IA as of late morning will
move northeastward toward southwestern QC and gradually deepen by
the end of the period, in advance of a shortwave trough progressing
from the northern Plains to the Great Lakes. An associated surface
cold front (now from central WI/IA to central KS) will move
southeastward to north TX and the lower Great Lakes by 26/12z. A
broad swath of 68-72 F boundary-layer dewpoints has spread
northeastward in advance of the cold front, though midlevel lapse
rates greater than 7 C/km are confined to a small area of the
central Plains, with lesser lapse rates to the east. Ongoing
clusters of storms from central IN southward are related to a
remnant tropical moisture plume with diffuse embedded speed maxima.
Larger cloud breaks and stronger surface heating is expected west
through northwest of the ongoing IN storms today.
...Eastern IN to northwestern OH this afternoon/evening...
The ongoing storms across IN will spread northeastward today as weak
embedded MCVs interact with the moist environment across the OH
Valley. Storm mode will likely remain somewhat messy clusters, and
surface heating will be slowed by persistent clouds. Still, the
stronger low-level shear this afternoon should be focused from
eastern IN across western OH with weak surface-based buoyancy, and a
couple of tornadoes will be possible.
...Eastern IA/northern IL/southern WI this afternoon/evening...
The threat for isolated strong/severe storms should evolve from the
ongoing convection approaching central IA. These storms have been
elevated through the morning, but will gradually phase with the
surface low/cold front and warm sector from eastern IA to northern
IL/southern WI this afternoon. Surface heating in advance of the
cold front, combined with the frontal circulation and forcing for
ascent preceding the midlevel trough moving eastward from NE/SD,
should support some increase in storm intensity this afternoon.
Though low-level shear is not expected to be particularly strong
(the stronger low-level southwesterly flow is confined to the OH
Valley moisture plume), MLCAPE near 1000 J/kg and effective bulk
shear near 50 kt will favor an organized band of storms (with bowing
segments and embedded supercells) capable of producing damaging
winds and isolated large hail.
...Northern IN to lower MI this evening through tonight...
In the wake of today's ongoing convection across IN/OH, additional
storm development will be possible this evening into tonight. The
remnants of the afternoon/evening storms in WI/IL could reach
western Lower MI by early tonight, with additional storm development
possible from IN into southeastern Lower MI. A modest increase in
low-level flow/shear is possible this evening across Lower MI,
though forecast hodographs do not look particularly favorable for
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