Day 1 Severe Weather Outlook

Valid 242000Z – 251200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NORTHERN
IOWA INTO SOUTHEAST MINNESOTA AND WESTERN WISCONSIN…

…SUMMARY…
Severe thunderstorms will threaten parts of the central Plains and
Midwest today. Large hail, damaging gusts and a few tornadoes are
possible.

…Discussion…

Primary change to previous outlook has been to reintroduce a small
10% tornado probability for southeast MN into western WI. Moisture
return has been robust along a 30 kt southwesterly low-level jet
with corridor of 70 F dewpoints now approaching the southern MN
border. The low-level jet will strengthen in pre-frontal warm sector
this evening downstream from an amplifying northern-stream trough.
Vertical wind profiles will become increasingly favorable for
organized storms with effective bulk shear up to 50 kt and 0-1 km
storm relative helicity from 200-300 m2/s2 in presence of a
moistening boundary layer and moderate to strong instability
(2000-3000 J/kg MLCAPE). Storms are expected to develop along and
ahead of the cold front from southeast MN, western WI into IA by
early evening and spread east. Discrete supercells will probably be
the initial storm mode before upscale growth into line segments
occurs. While the parameter space will be favorable for supercells
capable of producing a few tornadoes, overall tornado threat will be
dependent in part on duration of discrete storm modes. Otherwise,
damaging wind and large to very large hail will be likely with some
of the storms into this evening.

..Dial.. 09/24/2019

.PREV DISCUSSION… /ISSUED 1105 AM CDT Tue Sep 24 2019/

…IA to WI…
A strong upper trough and associated midlevel jet will move into the
northern Plains today, with height falls and large-scale forcing
overspreading parts of MN/IA/WI/Upper MI. A surface cold front
extends from western MN into eastern SD, and will track eastward
into IA later today. This boundary will be the focus for afternoon
thunderstorm development. Visible satellite imagery shows strong
heating is occurring in this region. Dewpoints are only in the 50s
to lower 60s, but are forecast to climb into the mid/upper 60s by
mid-late afternoon as advection occurs from the southwest. This
will result in a corridor of strong instability across much of IA
into southeast MN and western WI. Initial storm mode will likely be
supercellular, given strong deep-layer vertical shear, relatively
steep midlevel lapse rates, and high CAPE values. Very large hail
will be main threat with these storms, although damaging winds and a
few tornadoes are possible. The main tornado threat appears to be
over parts of western/central WI, but is contingent on robust
low-level moisture return depicted in model guidance. Storms will
likely congeal into linear segments by early evening and spread
eastward into WI with a continued risk of damaging winds and a few
tornadoes.

…KS…
The aforementioned cold front extends southward into KS, where ample
low-level moisture and strong heating will be present today.
Large-scale forcing is much weaker this far south, but weak frontal
convergence and a weak cap will likely result in widely scattered
storms over central KS this afternoon. Sufficient vertical shear
suggests rotating storms are possible with a risk of large hail and
gusty winds. The primary severe risk with this activity should
diminish by mid-evening.

$$

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