Day 1 Severe Weather Outlook

Valid 241200Z – 251200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
FAR EASTERN NEBRASKA…IOWA…SOUTHEAST MINNESOTA AND FAR WESTERN
WISCONSIN…

…SUMMARY…
Scattered severe thunderstorms are expected to develop this
afternoon and evening from the central Plains northeastward into the
upper Mississippi Valley. Tornadoes, very large hail and wind damage
will be possible, especially from eastern Nebraska to western
Wisconsin. A few marginally severe wind gusts and hail will also be
possible in southern Arizona.

…Central Plains/Mid Missouri Valley/Upper Mississippi Valley…
An upper-level trough will move eastward across the north-central
U.S. today as the exit region of a mid-level jet overspreads the
northern Plains. At the surface, a cold front will advance
southeastward into the upper Mississippi Valley and central Plains
as a low develops across southern Minnesota. Moisture advection will
take place ahead of the front with surface dewpoints rapidly
increasing into the upper 60s and lower 70s F. This will contribute
to moderate to strong destabilization by afternoon. Convection is
forecast to first initiate along the front during the late afternoon
from southern Minnesota southwestward into eastern Nebraska. This
convection should grow upscale quickly into several clusters of
thunderstorms with MCS formation possible.

Concerning the severe potential, late afternoon RAP forecast
soundings from Omaha, Nebraska northeastward to near La Crosse,
Wisconsin have MLCAPE values peaking in the 2500 to 3500 J/kg range.
Wind profiles have moderate deep-layer shear and substantial
directional shear in the in the lowest 2 km AGL. Hodographs will
lengthen during the early evening as a 40 to 60 kt low-level jet
consolidates and moves northeastward across the region. Supercells
with tornadoes will be possible to the northwest of the center of
the low-level jet with the greatest potential across north-central
Iowa and southeast Minnesota. A strong tornado or two may develop in
this area during the late afternoon or early evening. In addition to
tornadoes, large hail will be likely with supercells. The more
dominant supercells may produce hailstones of greater than 2 inches
in diameter, mainly from eastern Nebraska to western Wisconsin where
the combination of instability and deep-layer shear is forecast to
be maximized. Wind damage will also be possible with supercells and
bowing line segments.

…Southern Arizona/Far Southwest New Mexico…
An upper-level low will move southward from southwestern Arizona
into northwest Mexico today. At the surface, a most airmass will be
in place across southern Arizona where dewpoints should be in the
lower 60s F. In response to surface heating, a pocket of instability
should be in place in southern Arizona and far southwest New Mexico
by midday. Thunderstorms that develop across the zone of stronger
instability during the late morning and early afternoon should have
an isolated severe threat. Marginally severe wind gusts and hail
will be possible with the stronger multicells. The threat should
diminish during the late afternoon and early evening as the
upper-level low moves southward away from the region.

..Broyles/Gleason.. 09/24/2019

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