Day 1 Severe Weather Outlook

Valid 121300Z – 131200Z


Severe thunderstorms are likely today over the middle Mississippi
Valley to the Ohio Valley. Damaging gusts are the primary risk but
a few tornadoes are possible.

…IA/IL this afternoon/evening…
A well-defined MCV is moving eastward over western IA this morning,
per reflectivity mosaics and VWP observations of 50-60 kt midlevel
flow at OAX/DMX. The associated MCS and low-level cold pool have
weakened since 09z, though convective outflow appears to extend
farther south into KS/MO than most model guidance. Thus, recovery
of the boundary layer into IA is a primary concern for this
afternoon. Assuming surface heating in cloud breaks by early-mid
afternoon and erosion of the morning cold pool, boundary-layer
dewpoints in the low-mid 70s will contribute to MLCAPE in excess of
2000 J/kg east of the surface low and south of the warm front in IA.
Renewed thunderstorm development is probable by mid afternoon near
the surface low, with storms expected to subsequently move/develop
east-southeastward along the warm front into IL by this evening.

The initial storms will likely be supercells capable of producing
tornadoes and damaging winds, given the enhanced low-level
shear/hodograph curvature near the cyclone/warm front, along the
north edge of the unstable warm sector. Upscale growth into a
cluster is expected by this evening from southeast IA/northeast MO
into northwest and central IL, where damaging winds will become the
more prominent threat into tonight. The MCV/cyclone will begin to
move east of the surface warm sector late tonight, which should
result in gradual weakening of the storms across IN. The northward
extent and magnitude of the severe/tornado threat this afternoon
will depend heavily on the degree of recovery from the
ongoing/weakening MCS, which is still a bit uncertain.

…TX Panhandle this afternoon to southeast KS tonight…
A remnant midlevel trough will drift slowly east-northeastward over
the TX Panhandle today, as a weak cold front moves slowly
southeastward from KS/CO. Surface heating and deep mixing will
result in inverted-v profiles along and south of the front, and will
support the potential for isolated strong downburst winds.
Convection will tend to develop east-northeastward tonight along the
front, with strong downdrafts still possible in an environment of
lingering large CAPE/DCAPE.

…Maine this afternoon…
A midlevel trough over James Bay will pivot east-southeastward to
Maine overnight. In advance of this trough, a weak pre-frontal
trough at the surface and band of weak ascent will provide a focus
for thunderstorm development this afternoon. Though moisture and
lapse rates will be modest, increasing deep-layer westerly shear and
surface heating on the south edge of the thicker clouds could
support storms capable of producing isolated damaging gusts.

…Dakotas this afternoon/evening…
Widespread clouds and rain are spreading slowly eastward across the
eastern Dakotas in association with a couple of diffuse MCVs from
overnight convection. A little clearing of clouds and surface
heating will be possible from west to east this afternoon, in
advance of a larger-scale trough moving east-southeastward from MT
to ND. Isolated thunderstorm development and a low-end severe
threat will be possible this afternoon if sufficient near-surface
destabilization occurs, given enough vertical shear for organized

..Thompson/Kerr.. 08/12/2019


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