Valid 212000Z – 221200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
MINNESOTA AND WISCONSIN…AND OVER PORTIONS OF THE CENTRAL PLAINS…
…SUMMARY…
Large hail, damaging winds, and perhaps a couple of tornadoes remain
possible this afternoon and evening from Kansas to Iowa, and across
parts of central Minnesota into northwest Wisconsin.
…20Z Update…
A small adjustment has been made to the Marginal Risk across the
northern Plains into western MN to account for the eastward
progression of a surface front. Convection across central MN
along/ahead of the front has struggled to intensify so far this
afternoon, even with moderate to strong shear and instability
present across the warm sector. This may be related to the presence
of a subtle inversion around 700 mb suggested in various RAP
forecast soundings across this region. Even so, some increase in
convective coverage and intensity still seems probable across
central MN into northwestern WI through the remainder of the
afternoon into the early evening as a shortwave trough moves over
this region. All severe hazards remain possible. Large-scale lift
and low-level convergence along the cold front are weaker with
southward extent into southern MN and northern IA. Still, some
chance exists for a isolated strong to severe storm across this area
where modest surface heating has occurred.
Farther south, no changes have been made to the Slight Risk
extending from KS into southern/central IA. Convective development
will probably occur across western into central KS along the front
within the next few hours. Strong instability and enough effective
bulk shear to support supercells suggests a large hail threat
initially, with strong to damaging winds becoming a concern as
storms grow upscale along the front. A couple of tornadoes also
cannot be ruled out early in the convective event when storms are at
least semi-discrete.
..Gleason.. 09/21/2019
.PREV DISCUSSION… /ISSUED 1114 AM CDT Sat Sep 21 2019/
…MN/WI…
Morning surface analysis shows a weak low over northwest MN, with a
trailing cold front extending southward across the eastern Dakotas.
This front will move eastward today into central MN by
mid-afternoon, where a combination of daytime heating and forcing
associated with an upper trough tracking across the northern Plains
will lead to the development of widely scattered thunderstorms. 12z
model solutions are inconsistent on coverage of storms in this area,
but forecast soundings suggest a conditional risk of rotating storms
and the potential for hail and damaging wind in the strongest cells
that form. An isolated tornado or two is also possible, given the
presence of remnant outflow boundaries and ample low-level moisture.
Storms will move into central WI and weaken this evening.
…Central Plains…
The aforementioned surface cold front extends southward into central
NE/KS. Strong heating is expected this afternoon ahead of the front
from central/eastern KS into southeast NE, with MLCAPE values
approaching 3000 J/kg. Convective initiation is expected during the
late afternoon over central KS. Storms will build northeastward
across the SLGT risk area during the evening, with the potential for
large hail and damaging winds. Shear profiles will promote initial
supercell structures with a risk of very large hail. Persistent
clouds limit confidence in severe threat extending into
central/eastern IA.
$$