Valid 301630Z – 011200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING FOR
SOUTHEAST MN AND NORTHWEST WI…
…SUMMARY…
Isolated large hail and damaging gusts will be possible this evening
across parts of the upper Mississippi Valley.
…Upper MS Valley this afternoon/evening…
Little change is expected with the larger-scale pattern, as a closed
high persists over the lower MS Valley, and a deep trough remains
over the northern Rockies and Great Basin. An embedded speed max
over WY as of late morning will eject to ND this evening and western
ON overnight, around the northwest periphery of the MS Valley high.
An associated surface low now in northeast SD will move
northeastward across MN/northwest WI by this evening, and Lake
Superior overnight, while weakening gradually.
A moist/unstable warm sector will persist today south and east of
the surface low, where boundary-layer dewpoints near 70 F and
afternoon temperatures in the 80s will contribute to MLCAPE of
2000-3000 J/kg. The majority of the warm sector will remain capped
beneath a relatively warm elevated mixed layer, and large-scale
forcing for ascent will be minimal in proximity to the surface warm
sector. Thus, storm development will depend on weak low-level
ascent near the surface low and along the trailing cold front this
evening. If surface-based storms do form along the cold front, the
environment with effective bulk shear greater than 40 kt will favor
supercells capable of producing large hail, damaging winds, and
perhaps a tornado or two (primarily near the warm front where
low-level shear/hodograph curvature will be greater).
…Central Plains this evening into early tonight…
The trailing cold front will move slowly southeastward across NE
through tonight. Surface-based storm development along the front is
in question given a largely capped warm sector. The more probable
scenario is for elevated storms to form on the cool side of the
boundary, as warm advection increases late evening atop the frontal
surface across southern NE. Elevated supercells will be possible
for a few hours early tonight, with an associated threat for large
hail/damaging winds. Storm coverage should increase through the
overnight hours, though storm intensity will likely diminish.
…Southern High Plains this afternoon/evening…
Low-level moisture from the Gulf of Mexico and mid/high-level
moisture from tropical cyclone Narda near the southern Gulf of
California will stream across the southern High Plains through
tonight. There will be some potential for gusty outflow winds with
the initial convection this afternoon/evening across eastern NM and
vicinity with marginal supercell structures, though moistening
profiles by tonight suggest that locally heavy rainfall will become
the main concern.
..Thompson/Leitman.. 09/30/2019
$$