ACUS01 KWNS 241426
SWODY1
SPC AC 241424
Day 1 Convective Outlook RESENT 1
NWS Storm Prediction Center Norman OK
0924 AM CDT Sat Jun 24 2023
Valid 241300Z - 251200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF IOWA
AND MINNESOTA INTO THE MID MISSISSIPPI VALLEY...
...SUMMARY...
Severe thunderstorms, associated with wind damage, large hail and
perhaps a couple of tornadoes, will be possible from parts of Iowa
and Minnesota into the mid Mississippi Valley this afternoon and
evening.
...Synopsis...
Although the mean/large-scale ridge position aloft still extends
from the northern MX high across the central Plains to ON, it will
be penetrated today by a compact cyclone now evident in moisture-
channel imagery over the northern High Plains. The 500-mb low is
forecast to move erratically eastward to near the MN Twin Cities
region by the end of the period, with shortwave trough to
northeastern KS. Today, strongly difluent mid/upper flow will
spread across the mid/upper Mississippi Valley to its east and
southeast. A longstanding, gradually weakening upper trough over
the eastern CONUS will shift slowly eastward toward the East Coast.
At the surface, 11Z analysis showed a low over east-central SD near
HON, with cold front across south-central NE to northwestern KS and southeastern CO. The low will closely follow the mid/upper vortex
center track, while occluding. The cold front should reach western
IA, northeastern KS and southeastern CO by 00Z. By 12Z tomorrow,
the cold/occluded front will reach eastern parts of IA/MO as well as southwestern MO, southwestern OK and eastern NM. A warm front was
drawn over the lower Missouri Valley, and should move/redevelop
northeastward across the mid/upper Mississippi Valley through the
period ahead of the low.
...Mid/upper Mississippi Valley region...
Scattered thunderstorms may develop this afternoon between the
northern Red River Valley near the deep-layer low, and the mid
Mississippi Valley region. The greatest severe threat still appears
to be over parts of IA, MO and IL, though a good deal of it remains conditional. Tornadoes, along with large to very large hail and
locally severe gusts, are possible.
An ongoing, slowly weakening, mostly non-severe thunderstorm complex
across southwestern MN to northern/western IA will exert substantial
influence on later severe-weather potential across the same region
and nearby mesoscale areas. The northern part is expected to
continue moving into a lower-theta-e environment over MN, WI and
northern IA. Meanwhile, the southern part should shift eastward/
southeastward across IA, with some afternoon rejuvenation possible
along the associated pressure perturbation and UVV plume. The more
probable scenario is for a new round of severe thunderstorms to form
in its wake as warm/moist advection occur from the south and
southwest, and diurnal heating destabilizes the boundary layer
following departure of substantial associated cloud cover. Surface
dewpoints in the 60s and perhaps near 70 F, and a few hours of
heating, should support peak, prefrontal/preconvective MLCAPE in the
2000-3000 J/kg range.
This area also will reside in a regime of deep large-scale ascent --
overlying the warm advection with midlevel DCVA and lift beneath the
left-exit region of 250-300-mb jet. Sufficient flow aloft will
exist for 35-45-kt effective-shear magnitudes, with a pocket of
large low-level hodographs and SRH potentially developing over
central/northern IA or southern MN. With the bulk of the morning
MCS still over or west of the possible area of interest, there still
is considerable uncertainty on where the optimal mesoscale pseudo-
warm-frontal transition zone may set up in the wake of the complex.
A focused area of enhanced supercell/tornado and large-hail
potential may develop on any such boundary, with lower buoyancy,
backed flow and high SRH to its northeast, and greater heating/CAPE
but still adequate low-level shear to its southwest. If a dominant
supercell or two can occur in this environment prior to upscale
growth (not yet a given), greater tornado probabilities and perhaps significant-tornado area may be needed.
Farther north across portions of central/northern MN, boundary-layer instability will be weaker this afternoon. However, a narrow plume
of favorable, effectively surface-based parcels may develop east and
southeast of the deep-layer low, supporting an arc of convection
that may include low-topped supercells. Given uncertainties
regarding airmass recovery, with considerable cloud cover likely
over the area for most of the day, unconditional tornado potential
remains marginal, but the most-aggressive destabilization scenarios
support the potential for a few in that area.
...Southwest to north TX...
Isolated to widely scattered, high-based thunderstorms are expected
to develop along/ahead of a dryline drawn this morning over eastern
NM and far west TX, but expected to mix rapidly eastward across west
TX today, amidst seasonally intense diurnal heating. Activity will
originate atop a very deep/well-mixed boundary layer with dry-
adiabatic lapse rates (superadiabatic very near surface), with
moisture content increasing and surface temperatures decreasing
northeastward. Strong/isolated severe downbursts will be possible
over the entire outlook area, and enough inflow-layer moisture to
support isolated large hail is possible over northwest TX and
perhaps parts of the Big Country.
..Edwards/Mosier.. 06/24/2023
$$
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