ACUS01 KWNS 042000
SWODY1
SPC AC 041959
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0259 PM CDT Thu Sep 04 2025
Valid 042000Z - 051200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
NEW YORK...TO THE MID ATLANTIC...SOUTHERN APPALACHIANS AND MID
MISSOURI VALLEY...
...SUMMARY...
Isolated damaging gusts remain possible across parts of New York to
the Mid-Atlantic and Southern Appalachians through early this
evening. Isolated severe gusts are also possible across parts of the
mid Missouri Valley late this afternoon and early evening.
...20z Update Mid Atlantic and southern Appalachians...
Several clusters of strong to severe thunderstorms have matured and
are expected to continue through early this evening along and ahead
of the slow-moving cold front from the Mid Atlantic southward.
Despite poor mid-level lapse rates, heating of the modestly moist
air mass is supporting around 500-1000 J/kg of MLCAPE, sufficient
for a few stronger updrafts. As convection gradually increases in
coverage through the remainder of this afternoon, clustering and
30-40 kt of mid-level flow will continue to support isolated
damaging gust potential into early this evening. Some hail remains
possible with the more cellular elements, especially father
southwest over the eastern TN Valley where storm coverage should
remain more isolated. The primary change was to align the western
edge of the MRGL and severe probabilities with current frontal
position, and shift them slightly east for ongoing storms on the
border of NC and southern VA.
...New York...
As the line of showers and weak thunderstorms continues eastward
along the front, occasional damaging gusts remain possible owing to
mixing of stronger mid-level flow downward this afternoon. Some
heating, but marginal surface moisture should keep buoyancy and the
resulting updrafts weak. With MLCAPE generally less than 500 J/kg,
showers and storms should steadily weaken with the loss of diurnal
heating. The MRGL risk area has been trimmed behind the ongoing
line. Otherwise, the prior outlook remains unchanged, see the
previous discussion.
..Lyons.. 09/04/2025
.PREV DISCUSSION... /ISSUED 1124 AM CDT Thu Sep 04 2025/
...New York into the Mid-Atlantic and Tennessee Valley...
A closed mid/upper-level low will remain centered over Ontario
today. Multiple mid-level perturbations will rotate around this
feature through the period, including a shortwave trough forecast to
advance generally east-northeastward across parts of the central Appalachians/Mid-Atlantic into the Northeast. Showers and
thunderstorms are ongoing mainly ahead of a surface cold front from
parts of western NY southward to the TN Valley in a low/mid-level
moisture plume. Generally clear skies ahead of this activity should
allow for ample daytime heating of a modestly moistened low-level
airmass. Even so, 12Z observed soundings from BUF, IAD, and RNK/GSO
show rather poor mid-level lapse rates, which will likely hinder
updraft strength even as weak instability develops this afternoon
with continued diurnal heating.
Modestly enhanced southwesterly low/mid-level flow should still aid
in some thunderstorm organization even with the apparent
thermodynamic limitations, with multicells the dominant convective
mode. Wherever low-level lapse rates can become steepened, a risk
for at least isolated damaging winds remains apparent. Some hail may
also occur with the strongest cores across parts of the TN
Valley/southern Appalachians where locally greater instability
should be present this afternoon. The Marginal Risk has been
adjusted for ongoing trends, but the potential for a more focused
corridor of scattered severe/damaging winds remains unclear.
...Mid Missouri Valley...
A mid-level shortwave trough and attendant strong speed maximum will
move quickly southeastward today from central Canada and the
northern Plains towards the mid MO Valley and vicinity. Pronounced
ascent associated with this jet should encourage convection to
develop by late this afternoon across parts of eastern SD and
vicinity. While low-level moisture and MLCAPE are both expected to
remain very limited, enhanced low/mid-level northwesterly flow may
still support some risk for severe gusts with any low-topped
convection that can develop and spread quickly southeastward before
weakening this evening with the loss of daytime heating.
...Kansas...
A sharp cold front will sweep southward across the central Plains
tonight. Elevated convection should form to the cool side of this
boundary late tonight through early Friday morning given sufficient
MUCAPE. Locally strong surface gusts and small hail may accompany
this activity amid robust low-level northerlies, but the overall
severe potential should remain fairly low given the tendency for
convection to be mostly elevated.
...Arizona/Lower Colorado River Valley...
Thunderstorms should develop again today across parts of
central/southern AZ and the lower CO River Valley, beneath weak
mid-level flow and on the periphery of increasing mid-level moisture
associated with Tropical Storm Lorena. Weak shear should keep
thunderstorms mostly disorganized, but occasional strong/gusty winds
may occur where low-level lapse rates can become steepened.
$$
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