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ACUS11 KWNS 201916
SWOMCD
SPC MCD 201915
WIZ000-MNZ000-IAZ000-SDZ000-202145-

Mesoscale Discussion 1482
NWS Storm Prediction Center Norman OK
0215 PM CDT Thu Sep 20 2018

Areas affected...Southern MN...Northern IA...Far West-Central WI

Concerning...Severe potential...Tornado Watch likely

Valid 201915Z - 202145Z

Probability of Watch Issuance...80 percent

SUMMARY...All severe hazards, including tornadoes, are possible are
the region this afternoon and evening. A Tornado Watch will be
likely be issue over portions of the region by 21Z.

DISCUSSION...Recent surface analysis reveals a fractured frontal
structure with two effective warm fronts draped across the region.
One is more aligned with the temperature gradient and extends from a
low near OFK eastward across northern IA and into northern IL while
the other is better aligned with the moisture gradient, extending
from a low near HON across southern MN and into central WI. Strong
pressure falls across MN suggest a northeasterly motion of the
surface lows, particularly the stronger low across northeast NE,
will continue and perhaps accelerate. Cumulus fields along the front
in NE have become more agitated over the past hour while some
clearing has been noted across northern IA between the two warm
fronts. Overall, trends continue to suggest a quick northward motion
of the southern warm front across northern IA and into southern MN
while the low tracks northeastward and the cold front sweep in from
the east.

Recent mesoanalysis suggests a corridor of strong effective
storm-relative helicity exists along the MN/IA border where surface
winds are still southeasterly. Bulk shear values are also quite
strong as both the low and mid-level flow strengthen ahead of the
approaching shortwave trough. Consequently, kinematic field support
a severe risk, including the potential for tornadoes.

Thermodynamic fields are a bit more questionable, particularly with
northern extent where the cloud cover remains. Even so, the quickly
modifying airmass will likely be free of convective inhibition
within the next hour or so. Once this capping erodes, a narrow
window appears to exist for discrete development within the warm
sector before the approaching cold front leads to a predominately
linear mode. All severe hazards are possible and a Tornado Watch
will likely be issued over portions of the region by 21Z.

..Mosier/Hart.. 09/20/2018

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

ATTN...WFO...ARX...MPX...DMX...FSD...

LAT...LON 43499664 44269605 44999407 45229305 45289214 44889108
43799185 42919318 42369597 43499664



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