U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 251940 
Storm Prediction Center ac 251938 

Day 1 convective outlook 
National Weather Service 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 
middle-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 mesoscale convective system 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 
more detail. 

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 
near/after 00z. 

.Cook.. 09/25/2018 

Previous discussion... /issued 1125 am CDT Tue Sep 25 2018/ 

A weak surface cyclone in south central Iowa 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/Iowa to central ks) will move 
southeastward to North Texas 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 Ohio this afternoon/evening... 
The ongoing storms across in will spread northeastward today as weak 
embedded mcvs interact with the moist environment across the Ohio 
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 Ohio with weak surface-based buoyancy, and a 
couple of tornadoes will be possible. 

..eastern Iowa/northern Illinois/southern WI this afternoon/evening... 
The threat for isolated strong/severe storms should evolve from the 
ongoing convection approaching central Iowa. These storms have been 
elevated through the morning, but will gradually phase with the 
surface low/cold front and warm sector from eastern Iowa to northern 
Illinois/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 Ohio 
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 Michigan 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/Illinois could reach 
western lower Michigan by early tonight, with additional storm development 
possible from in into southeastern lower Michigan. A modest increase in 
low-level flow/shear is possible this evening across lower mi, 
though forecast hodographs do not look particularly favorable for 
tornadic supercells. 


Mesoscale Discussion

acus11 kwns 252221 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 252221 

Mesoscale discussion 1501 
National Weather Service Storm Prediction Center Norman OK 
0521 PM CDT Tue Sep 25 2018 

Areas affected...lower Michigan...Northwest Ohio...Indiana...east 
central Illinois 

Concerning...severe potential...watch possible 

Valid 252221z - 260015z 

Probability of watch issuance...40 percent 

Summary...thunderstorm activity will spread into, and continue to 
develop across, much of the region into the 8-11 PM EDT time frame, 
accompanied by at least some risk for severe wind gusts. It is not 
yet clear that this will require an additional severe weather watch, 
but trends are being monitored for this possibility. 

Discussion...thunderstorms are beginning to initiate in the warm 
sector, across parts of lower Michigan southwestward across Indiana 
into east central Illinois. This is ahead of the forced line of 
storms now spreading into/through the Interstate 55 corridor of 
northeast and central Illinois. 

The northwestern periphery of Richer boundary layer moisture return, 
characterized by dew points around 70, currently extends along the 
Interstate 70 corridor of central Illinois into Indiana, and 
northward, roughly near and east of the Interstate 69 corridor 
across central Indiana into northwestern Ohio/southeastern lower 
Michigan. This moisture, coupled with daytime heating, appears to 
be contributing to cape on the order of 1000-2000 j/kg. This is in 
the presence of generally strong deep layer shear beneath 40-50 
southwesterly mid-level flow. 

Pre-frontal low-level wind fields are somewhat weak and veered 
resulting in generally modest low-level hodographs across much of 
the region. This may be at least one mitigating factor to a more 
substantive severe weather risk into the 01-03z time frame. 
However, in the lingering moderate boundary layer instability, the 
environment may remain conducive to convection capable of at least 
producing localized potentially damaging wind gusts and perhaps some 
hail into/through the evening hours. 

.Kerr/Hart.. 09/25/2018 

..please see www.Spc.NOAA.Gov for graphic product... 


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