Areas affected…Portions of western/central IA…far southeastern
SD…far southwestern MN…and far eastern NE
Concerning…Severe potential…Severe Thunderstorm Watch likely
Valid 200405Z – 200600Z
Probability of Watch Issuance…80 percent
SUMMARY…A threat for mainly large hail should increase over the
next couple of hours as storms increase in coverage and intensity.
Severe Thunderstorm Watch issuance is likely.
DISCUSSION…A well-defined surface warm front extends eastward from
a low over western SD across southern SD, roughly along the NE/IA
border, and into southern IA. Warm air advection is occurring atop
this front as a southwesterly low-level jet gradually strengthens
late this evening. The VWP from KOAX has shown an increase in the
0-2 km layer over the past couple of hours, generally to around
30-35 kt above 1 km. Ascent associated with this low-level jet has
encouraged elevated showers to develop across parts of northwestern
IA, with a few recent lightning strikes noted with this activity.
The 00Z sounding from OAX showed very steep mid-level lapse rates
(around 8.6 C/km in the 700-500 mb layer) associated with an EML
were present across this region. These lapse rates aloft coupled
with ample low-level moisture are supporting moderate to very strong
instability along and to the cool side of the front (MUCAPE of
2000-4500 ) per latest mesoanalysis. Westerly winds strengthen with
height through mid levels, and a subtle vorticity maximum along the
NE/SD border maximum noted on water vapor may provide an additional
source of lift to encourage at least scattered storm development.
Around 40-50 kt of effective bulk shear will likely remain across
this region through the overnight hours, which will support
supercells initially. Given the large degree of elevated instability
and strong shear present, robust updrafts appear likely. Large hail
should be the primary hazard with initial storm development, with
these storms likely remaining elevated above a near-surface stable
layer to the cool side of the surface front. However, there is some
potential for upscale growth into a MCS later tonight across parts
of central IA, which could result in an eventual damaging wind
threat materializing. Watch issuance is likely in the next couple of
hours to address the initial large hail risk.
…Please see www.spc.noaa.gov for graphic product…
LAT…LON 42999379 42499378 41829411 41399485 41359556 41539614
42029655 42799688 43569680 43709587 43659501 43559445