Valid 301300Z – 011200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
UPPER MISSISSIPPI VALLEY…
…SUMMARY…
The greatest severe-storm threat today will be over parts of the
upper Mississippi Valley this afternoon and evening.
…Synopsis…
In mid/upper levels, a positively tilted synoptic pattern will
prevail over the CONUS in the form of:
1. Western troughing, anchored by a cyclone now centered over the
interior Pacific Northwest, and
2. A high over the Mid-South region, with ridging northeastward up
the Ohio Valley and southwestward to northeastern MX.
In between, a series of low-amplitude, convectively augmented/
generated vorticity maxima will traverse a long-fetch southwesterly
flow belt from northwestern MX and NM to the upper Mississippi
Valley and upper Great Lakes.
At the surface, the 11Z analysis showed a strong low between
HON-ABR, with cold front southwestward across western NE and
northeastern CO. A warm front was drawn across southern MN and
western IL, and should move northeastward today across eastern MN
and WI. The low will continue moving northeastward through the
period, reaching north-central/northeastern MN around 00Z, and ON
northeast of Lake Superior by 12Z. The cold front will move
southeastward but decelerate through the period, beneath the
aforementioned stretch of parallel southwesterlies aloft. By 00Z
the front should be located from the low southwestward over the
Siouxland area of SD/IA, to south-central NE east-central CO. By
12Z, the front should extend across eastern Lake Superior, central
WI, southwestern IA, northwestern KS, and southeastern CO.
…Upper Midwest…
Isolated, marginally severe hail may occur from the most intense
cells amongst a broad plume of morning precip/convection now located
across the northern parts of the outlook area, ahead of the low and
warm front. However, the more-substantial severe threat appears to
be this afternoon and early evening.
Widely scattered to scattered thunderstorms should develop close to
the surface front across portions of the Upper Midwest, especially
over the MN segment where low-level mass response to the cyclone
(and related convergence) is the strongest. The foregoing
environment across eastern MN and WI will become very favorable
conditionally, so with an increasing potential that storms will
interact therewith, the probabilities are increased a level for all
severe hazards.
As the western trough assumes more positive tilt, a tightening
midlevel height gradient and related speed max will develop and
shift northeastward across this region, contributing to increasingly
favorable deep shear spreading over the cold front and across the
northern rim of the warm sector. Meanwhile, relatively backed
surface winds are expected amidst favorable isallobaric forcing,
within a zone encompassing the warm-frontal zone, and about 100 nm
to its south. In conjunction with a 40-45-kt southwesterly LLJ,
this will yield favorable hodographs for supercells, with effective
SRH commonly in the 250-400 J/kg range in forecast soundings
(decreasing southwestward from southern MN). Because of the more
meridional frontal alignment here compared to farther southwest, the
potential for discrete to semi-discrete storms to move off the
boundary is greater.
Along/ahead of the cold front, a residual EML, still present
northeast of the tropical moisture plume mentioned below, will
maintain steep midlevel lapse rates. Low-level moisture south of
the warm front will remain rich, with surface dew points commonly in
the upper 60s and low 70s F. Diabatic heating in the warm sector
thus will contribute to the development of peak preconvective MLCAPE
in the 2500-3500 J/kg range, decreasing eastward and with time
across WI toward Lake Michigan. This CAPE/shear parameter space
supports potential for damaging gusts, large hail and tornadoes,
especially with any sustained supercells that can develop from the
cold-frontal convection.
…Central Plains…
Scattered thunderstorms are expected to develop by late afternoon
and early evening across this region, offering the potential for
isolated severe hail/gusts, and perhaps damaging wind closer to the
front.
Given the slowly progressive nature of the front beneath largely
parallel flow aloft, the convective scenario appears anafrontal,
with activity struggling to maintain access to the near-surface warm
sector. Still, convection should increase in coverage across the
region, both on and north of the surface boundary, as a favorably
moist air mass is advected over the front, and isentropically/
frontally lifted to LFC. The northeast edge of the weaker midlevel
lapse rates related to the tropical/subtropical moisture plume aloft
will spread across the area, but with enough low-level moisture to
still support 1000-1500 J/kg peak MLCAPE this afternoon along and
south of the front, and about that magnitude of MUCAPE to its north.
35-45 kt effective-shear magnitudes (locally higher) suggest a mix
of multicell and supercell modes possible. For now, the potential
for fairly messy convective structures to develop quickly, and the
presence of weaker lapse rates and shear than farther northeast near
the surface cyclone, preclude more than marginal unconditional
probabilities at this time.
…Southern High Plains…
Widely scattered to scattered thunderstorms are expected to develop
across this corridor, primarily this afternoon and evening, moving
northeastward while offering isolated damaging to locally severe
gusts.
A plume of rich midlevel moisture and related cloudiness, emanating
from the vicinity of T.D. Narda near the Mexican Pacific Coast, will
overlap the zone of deep low-level convergence responsible for the
convection. It also will limit lapse rates aloft and diabatic
surface heating compared to previous days, across much of the
outlook domain. Still, in the absence of a substantial EML, the air
mass will be weakly capped for surface-based inflow parcels. Enough
heating still should occur to support a field of preconvective
MLCAPE in the 1000-2000 J/kg range. Deep shear will be modest, with
effective-shear magnitudes generally in the 25-35-kt range,
indicating predominantly multicellular mode.
..Edwards/Broyles.. 09/30/2019
$$