SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0756 AM CDT Sun Aug 25 2024
Valid 251300Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
FOUR CORNERS TO SOUTHWESTERN WYOMING...AND OVER PORTIONS OF WESTERN
MINNESOTA AND THE EASTERN DAKOTAS...
...SUMMARY...
Severe gusts and hail are most probable today over parts of the Four
Corners to southwestern Wyoming, and this evening over portions of
western Minnesota and the eastern Dakotas.
...Synopsis...
A seasonally well-amplified mid/upper-level pattern will persist
over the CONUS, but with some important changes due to the
progression of embedded synoptic- to subsynoptic-scale features. An
intermittently closed cyclone now over north-central NV will eject
slowly northeastward to southeastern ID through the period, while
devolving to an open-wave trough. By 12Z tomorrow, the full trough
should extend from the MT Rockies southward through the remnant
low's vorticity max, then southwestward to southern NV.
Meanwhile, a broad anticyclone -- initially centered over northern
OK/southern KS -- will move northeastward, with the 500-mb high over
central MO by 12Z, and ridging north-northeastward across Lake
Superior. The northward component of this shift should occur partly
in response to a well-defined low/trough in the subtropical
easterlies, now evident over the north-central to southwestern Gulf.
This feature should move ashore on the lower/middle TX Coast around
the end of the period. Northeast of the high, a strong shortwave
trough now over northwestern QC will dig south-southeastward through
tonight, reaching the St. Lawrence Valley and approaching northern
NY by 12Z.
At the surface, 11Z analysis showed a persistent, very slowly
frontolytic, quasistationary boundary from just offshore of the
Carolinas to the north-central Gulf Coast, becoming a diffuse warm
front over astern parts of TX/OK. Rich low-level moisture has
returned northward near this boundary in OK/AR and farther north
across parts of the central/northern Plains. A low was drawn north
of MIB, with cold front southwestward across the northwestern corner
area of SD, into north-central WY. By 00Z, the front should reach
from near the MN/ND/MB border confluence across the eastern Dakotas,
parts of north-central/southwestern NE and near CYS. The front
should proceed overnight to a 12Z position from northeastern MN to
the FSD/SUX vicinity, central NE and eastern CO.
...Northern Plains/Upper Midwest...
Scattered thunderstorms are expected to develop along/ahead of the
cold front -- predominantly after 00Z, though enough moistening/
heating may occur for isolated late-afternoon development.
Supercells and bowing lines/clusters will be possible, offering
large hail and severe gusts.
Activity should occur as the front encounters what is left of a
diurnally well-heated and still-unstable boundary-layer airmass,
with only minimal diabatic cooling, due to the presence of rich
moisture near the surface, Surface dewpoints already are commonly
in the 70s F in a swath of the warm sector with 11Z axis from
north-central ND south-southeastward across eastern portions of
SD/NE/KS and the Arklatex region. Forecast soundings indicate the
moist layer should deepen through the day beneath strong capping,
and steep midlevel lapse rates. With a deep troposphere still
present over this area, atop the favorably unstable boundary layer,
MLCAPE in the 3000-4000 J/kg range will be possible. This will be
collocated with slowly strengthening flow aloft, such that 30-45-kt
effective-shear magnitudes will be possible from late afternoon
through the evening.
...Central/southern Rockies and vicinity, south-central Plains...
Scattered showers and thunderstorms, with at least isolated
strong-severe gust potential, should become common by this afternoon
in a vast, roughly triangular area in and near the outlook, from
western NM and eastern AZ to parts of southeastern MT and southern
KS. Within that area, the greatest potential for severe should be
on the western fringes, with supercells and bowing clusters possible
in a corridor spanning portions of the Four Corners area to
southwestern WY. In that region, the greatest diurnal
destabilization will overlap the most favorable deep-layer lapse
rates, and strengthening vertical shear ahead of the progressive
western trough.
Mid/upper-level moisture-channel imagery, available GPS PW data, and
surface analysis show a persistent plume of deep-layer monsoonal
moisture, maintaining some continuity from the ITCZ well south of
mainland MX, across parts of west-central MX and Chihuahua, to NM,
CO, WY and western NE. Enough low-level moisture (PW above 1 inch
in some areas) to support organized convective potential extends
farther westward over the Four Corners area, especially into
northeastern AZ, eastern UT and western CO. Forecast soundings in
that corridor show favorable deep shear for supercells, with
effective-shear magnitudes in the 40-55-kt range, and elongated,
rather straight low/middle-level hodographs favoring a large-hail
threat, in addition to the broader severe-gust potential.
Farther east, much of the higher terrain across the "marginal" area
will reach convective temperature relatively early in the
diurnal-heating cycle, with little CINH, resulting in abundant
development, slowing the pace of additional destabilization. Still,
isolated strong/briefly severe gusts may occur, especially where
north-northeastward moving clusters of convection encounter pockets
of well-mixed boundary layer suitable for intense downdrafts.
Farther east onto the High Plains, more-favorable heating/mixing
will occur, but with weak shear nearer to the mid/upper high.
..Edwards/Bentley.. 08/25/2024
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