Subj : DAY48SVR: Nws Storm Prediction Center Norman Ok
To   : wx-storm@lists.illinois.edu
From : COD Weather Processor
Date : Sat Oct 21 2023 08:47:47

ACUS48 KWNS 210847
SWOD48
SPC AC 210846

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0346 AM CDT Sat Oct 21 2023

Valid 241200Z - 291200Z

....DISCUSSION...
The southern branch of an upper trough over the western U.S. will
become increasingly cut off from northern stream flow early in the
Day 4-8 period. An upper ridge over the Southeast will block
eastward progression of the upper low over the Southwest until late
in the week. Meanwhile, the northern stream shortwave trough over
the northern Rockies early Day 4/Tue will develop east across the
northern Plains/Great Lakes to the Northeast through Day 6/Thu. Of
note, the GFS appears to be an outlier among the EC/UK/CMC suite of
guidance, indicating more ridging across the north-central portions
of the U.S. compared to other forecast guidance. Given consensus
among other data, the GFS solution has largely been thrown out.

Some severe potential is possible on Day 4/Tue across parts of the
Mid/Upper MS Valley vicinity. Strong west/southwesterly deep-layer
flow, and a 45-65 kt 500 mb jet streak associated with the upper
shortwave trough, will overspread the Upper Midwest. Meanwhile,
south/southwesterly low-level flow will transport mid-60s F
dewpoints across MO and IA into parts of southeast MN, WI and
northern IL. Thunderstorms are likely to develop along/ahead of a
southeastward-advancing cold front Tuesday. Sufficient instability
and effective shear magnitudes around 30-40 kt should support a risk
of severe thunderstorms ahead of the eastward progressing surface
low and cold front across parts of IA into southeast MN and WI.

While moderate to strong mid/upper westerlies will persist on Day
5/Wed across the northern Plains to the Great Lakes, large-scale
ascent will be focused north of the international border. Showers
and thunderstorms are likely across parts of the southern/central
Plains and Midwest as the surface cold front shifts east, but severe
potential appears low.

Heading into Day 6/Thu and beyond, forecast guidance and ensembles
handle the evolution of the cut-off/southern stream upper trough/low
over the Southwest very differently, resulting in low
predictability.

...Leitman.. 10/21/2023

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