SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0250 PM CDT Wed Jun 12 2024
Valid 122000Z - 131200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS IN
NORTH-CENTRAL/NORTHEAST MN TO FAR NORTHWEST WI...
...SUMMARY...
Scattered severe thunderstorms capable of very large hail, damaging
wind gusts, and a few tornadoes are expected across parts of the
Upper Midwest during the late afternoon and early evening.
...20Z Update...
No appreciable changes were made to the categorical outlook across
the Upper Midwest, beyond a minor adjustment over the Red River
Valley, with the previous forecast appearing to be largely on track.
Given destabilization trends and impact of early-day/ongoing
convection, the southern extent of the severe threat in the IA
vicinity might be a bit overdone. Still, some severe wind/hail risk
may spread back into northern portions of the state later this
evening. For additional short-term discussion regarding the severe
threat across the Upper Midwest, see MCDs 1238 and 1239.
Over south FL, low-level winds have both slightly weakened and
veered relative to late morning when transient supercell structures
occurred. Still, with deep convection persisting across the southern
peninsula into early evening, will maintain the level 1-MRGL risk
for another outlook cycle.
..Grams.. 06/12/2024
.PREV DISCUSSION... /ISSUED 1131 AM CDT Wed Jun 12 2024/
...Upper Mississippi Valley/Upper Midwest...
Bands/clusters of convection are ongoing at this time, from the
Minnesota Arrowhead southward across northern Wisconsin, and
southwestward across southern Minnesota into the Mid Missouri Valley
region. This convection continues moving eastward, toward a less
unstable airmass, which should temper severe risk over the next few
hours. Greatest potential for a couple of marginally severe storms
appears to exist over southwestern Minnesota and adjacent portions
of southeastern South Dakota and northwestern Iowa, where the
greatest instability resides.
Greater severe risk is forecast to evolve later this afternoon,
north of the ongoing storms. Here, daytime heating ahead of an
advancing cold front (associated with a low-amplitude short-wave
trough embedded in fast westerly flow aloft, and moving eastward
along the international border) will result in moderate to strong
instability (2500 to 3500 J/kg mixed-layer CAPE) within the
pre-frontal warm sector.
Weakly veering but strengthening flow with height across the region
will provide favorable shear for organized/rotating storms. Initial
convection -- expected to initiate in the 12/20Z to 12/21Z time
frame -- should quickly organize/become supercellular, with very
large hail likely to be the initial/primary severe risk. A couple
of tornadoes will also be possible, though veered low-level flow
should temper overall tornado risk. With time, storms are expected
to cluster/grow upscale, with small-scale bowing expected, which
should yield a gradual increase in damaging-wind potential. Storms
will spread rather quickly eastward/southeastward with time, with
some severe risk likely to linger through the evening before
gradually diminishing overnight.
...South Florida...
Widespread convection is ongoing across southern Florida, within a
zone of low-level warm advection south of a weak cold front lying
across central portions of the Peninsula. Modest lapse rates aloft,
but a very moist/tropical boundary layer is resulting in deep CAPE,
which will continue to support the widespread convection.
While mid-level flow remains rather weak, flow veers/increases with
height through the lower troposphere. This is providing ample shear
for generally weak/transient updraft rotation. Given the associated
potential for a brief tornado or two, low (2%) tornado
probability/MRGL risk is being introduced across portions of South
Florida.
For additional short-term information, please refer to SWOMCD #1237.
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