ACUS01 KWNS 051251
SWODY1
SPC AC 051250

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0650 AM CST Tue Nov 05 2024

Valid 051300Z - 061200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS A PORTION
OF THE SABINE TO LOWER MISSISSIPPI VALLEYS...AND CENTRAL TO SOUTHERN
WISCONSIN...

...SUMMARY...
A brief tornado or two and locally damaging winds may occur across a
portion of the Sabine to Lower Mississippi Valleys, and central to
southern Wisconsin.

...Synopsis...
Mid/upper-level longwave troughing has become well-established over
the western CONUS.  That will persist through this and more outlook
periods, as a series of synoptic to large shortwave troughs
traverses the associated cyclonic-flow field.  The leading such
perturbation -- now located from the central Plains south-
southwestward over the southern High Plains to far west TX -- will
eject northeastward and weaken.  By 00Z, the residual, strongly
positively tilted trough should extend from a vorticity max over
western WI, across central/southwestern MO to central TX.  The
perturbation will deamplify greatly overnight and accelerate to a
position near the western QC/ON border, Lake Huron and IN by 12Z.
Meanwhile, an upstream shortwave trough -- evident in moisture-
channel imagery over ID -- will dig southeastward and strengthen
greatly through the period.  By 12Z, it should be oriented
northeast/southwest through a developing 500-mb low over the Four
Corners.

The 11Z surface analysis showed an elongated area of low pressure
from southern WI to northwestern MO, with cold front through central
OK and north-central/central TX.  The front was preceded by an
extensive  convective band and associated outflow boundary from
central IL to central AR, east TX, and deep south TX.  By 00Z, the
front should catch up (or nearly so) to what remains of the
convective boundary, with the combined baroclinic zone extending
from a consolidated low over northern WI across the Mid-South region
to the northwestern Gulf shelf waters off TX.  By 12Z, the front
should extend from Lower MI across western portions of KY/TN/MS to
southwestern LA and the northwestern Gulf.

...Sabine to Lower Mississippi Valleys...
The ongoing convective band, near the western edge of the "Marginal
Risk" area, should weaken overall through mid-morning.  However, an
isolated damaging gust or brief tornado cannot be ruled out in the
meantime with any associated or preceding cell that may take
advantage of lingering favorable deep shear and high theta-e, in a
northward-narrowing corridor.

After a relative lull of a few hours, widely scattered to scattered
thunderstorms are forecast along the boundary this afternoon into
early evening, with isolated damaging gusts and/or a tornado
possible.  Afternoon boundary-layer destabilization is anticipated
ahead of the convective boundary, from both fragmented/cloud-
modulated surface heating and low-level theta-e advection.  A
corridor of surface dewpoints in the upper 60s to low 70s F will
persist an expand slightly northward ahead of the boundary.  With no
antecedent EML and minimal MLCINH, this should offset modest
midlevel lapse rates enough to result in effectively uninhibited
buoyancy, supporting an increase in convective coverage and local
intensity this afternoon.  Peak MLCAPE should range from around
500-800 J/kg over a narrow corridor of the Mid-South,
widening/increasing to around 1000-1500 J/kg over southern LA.  By
contrast, lift along the boundary, and in the form of deep-layer
ascent preceding the weakening mid/upper trough, will be greater
over the Mid-South, until some nebulous point where too little
instability exists to support organized convection.  The outlook has
been elongated northward somewhat to cover the marginal-buoyancy
plume near the Mississippi River.

...WI...
Isolated to widely scattered, shallow thunderstorms are possible
this afternoon, in an arc near the front and the organizing surface
low.  A few cells may produce damaging gusts.  A brief tornado also
may occur, especially with any cells that can linger within a narrow
zone of overlap between favorably, isallobarically backed near-
surface winds and weak boundary-layer instability.  A low-confidence
and quite conditional scenario exists here, with abundant antecedent
clouds and precip likely to limit destabilization for most of the
day.  Forecast soundings suggest enough large-scale ascent aloft to
cool midlevels a degree or two C, though lapse rates still will be
meager.  Areas that can experience a couple hours of relatively
unimpeded heating may attain 250-500 J/kg MLCAPE.

..Edwards/Mosier.. 11/05/2024

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