LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0732 AM CDT Wed Sep 06 2023
Valid 061300Z - 071200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHERN
LOUISIANA TO PARTS OF THE TENNESSEE VALLEY REGION...
...SUMMARY...
Severe thunderstorms capable of damaging gusts and isolated hail are
possible from northern Louisiana to parts of the Tennessee Valley
region.
...Synopsis...
A mid/upper-level anticyclone is building over the NM/far west TX
region, with ridging northward across WY to the northern Plains. A
progressive shortwave train will be maintained on the anticyclone's
northern and northeastern periphery, from the Pacific Northwest to
the Appalachians. The currently strongest of the shortwave troughs
-- apparent in moisture-channel imagery over the mid/upper
Mississippi Valley -- will shift eastward to an axis from western
Lake Superior to western IN through 00Z, before weakening and slowly
moving eastward across the remainder of the Upper Great Lakes.
Meanwhile, an upstream perturbation -- initially over WY -- will
pivot eastward then southeastward away from the mean-ridge position
and strengthen as it traverses downstream northwest flow. By 00Z,
this feature should reach western IA, southeastern NE and northern
KS. By 12Z tomorrow, the trough should extend over the Wabash River
area and southwestward across parts of the Mid-South region, with a
500-mb low developing near EVV.
At the surface, 11Z analysis showed frontal-wave lows over the
Keweenaw Peninsula and western WI, collected by a cold front that
extended across northern/southwestern IL, southeastern MO,
southeastern OK, to a weak low over northwest TX, then
quasistationary across the Llano Estacado into central NM. By 00Z,
the front should reach eastern Upper/northern Lower MI, southern IN,
the DYR area, south-central AR, and the Red River Valley. The
western segment of the front should stall over north TX today into
tonight, and return northward diffusely across the TX Panhandle and
northeastern NM tonight. Meanwhile, by the end of the period, the
rest of the front should reach eastern portions of OH/KY/TN,
northern AL, central MS, and southern AR.
...Lower Mississippi Valley to Great Lakes...
Scattered thunderstorms are expected to develop this afternoon
along/ahead of the cold front, and near any remaining prefrontal
outflow/ and differential-heating boundaries from prior/morning
convection. The most-favorable shear/instability parameter space,
and best-organized severe potential, should be in and near the
15%/"slight" area, where damaging to severe gusts will be the main
threat, with large hail also possible. Isolated, marginal severe
potential extends northward from there past the Ohio Valley into
portions of the Great Lakes.
The strongest winds aloft and associated deep shear will remain
largely behind the leading mid/upper trough -- with one important
exception: a channel of enhanced 500-250-mb layer flow extending
from the central Plains across the Ozarks toward the Mid-South by
00Z. The leading edge of intensifying gradient winds aloft will
extend over parts of the warm sector from northern LA into the
Mid-South today, even as the strongest mid/upper flow lags the
surface front/outflow. This will increase deep shear enough to aid
in multicellular organization from northern LA to the Tennessee
Valley, with effective-shear magnitudes relatively maximized at
around 25-35 kt, compared to 15-25 kt farther north across the Great
Lakes States.
The southern corridor also will correspond to the greatest
preconvective buoyancy, with a combination of steep low-level lapse
rates (from diurnal heating) and rich moisture (70s F surface
dewpoints) -- each of which will be weaker northward into the Great
Lakes. This should result in a plume of 2000-3000 J/kg MLCAPE
(locally higher) over the lower Mississippi Valley region,
diminishing to around 1000-1500 J/kg north of the Ohio Valley. A
secondary, weaker relative max in buoyancy (contributing to MUCAPE
near 2000 J/kg) may be noted over parts of western/northern Lower
MI, under stronger cooling aloft, though low/middle-level flow and
vertical shear will be weak there.
..Edwards/Leitman.. 09/06/2023
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