- 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
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
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
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
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.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 43808454 43448303 42368281 40828392 39508518 38638671
38668908 39448927 40418837 41418666 43198600 43808454