RSS Mesoscale Discussions from Storm Prediction Center
SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0743 AM CDT Tue Sep 10 2024
Valid 101300Z - 111200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER COASTAL
AREAS OF LOUISIANA...
...SUMMARY...
Marginal tornado potential may reach coastal areas of Louisiana
during the late overnight/early morning hours.
...Synopsis...
In mid/upper levels, a strong shortwave trough now over the eastern
Gulf of Alaska is forecast to dig southeastward and develop a closed
500-mb cyclone tonight, with the low about 150 nm west of AST by 12Z
tomorrow. Ahead of this progressive and intensifying perturbation,
height falls and strengthening cyclonic flow will spread across the
northwestern CONUS throughout the period, combining with modest low/
middle-level moisture to support thunderstorm potential in parts of
the Northwest, in addition to the diurnal-cycle convection across
the Four Corners region, central/northern Rockies and parts of MT.
Downstream, mid/upper flow should be nearly zonal from the Rockies
across the Great Lakes. An embedded shortwave trough was evident in
moisture-channel imagery over the Dakotas. This feature is forecast
to move eastward to MN by 00Z, then across the Upper Great Lakes to
Lake Huron and adjoining parts of ON by 12Z. Farther south, a weak,
cut-off, mid/upper-level low over north-central TX should drift
erratically near its present location, while increasingly
influencing the path of Tropical Storm Francine.
The 11Z surface analysis showed a wavy, quasistationary frontal zone
fro Atlantic waters offshore SC, across northern FL to near the
Mississippi River mouth, then a warm front arching across Gulf
waters just south of the LA and TX Coasts, and into western parts of
the Francine circulation. This boundary should move little from
about 90W eastward, but remain a warm front over the upper TX and LA
coastal waters, diffusely shifting inland over southern LA through
the period.
...LA Coast...
T.S. Francine -- initially centered southeast of BRO -- is forecast
by NHC to turn northeastward across the northwestern Gulf today and
become a hurricane, then move ashore in Louisiana on day 2.
Suitably rich low-level moisture already is in place over coastal
LA, and scattered to numerous showers and thunderstorms should cover
the outlook area throughout much of the period, even well northeast
of Francine's center. However, supercell potential will await
sufficiently strong wind fields reaching the coastline ahead of the
cyclone to enlarge hodographs favorably -- likely during the last
few hours of the period. Then, tornado potential will be a matter
of having relatively distinct/discrete, outer-band convection that
can survive long enough to mature into supercells before weakening
again in more-stable inflow air -- either inland or in broader
precip areas. Unconditional probabilities are kept at marginal
levels for now, given the late timing of the favorable kinematic
fields, the nocturnal min in overland instability, and convective/
structural uncertainties with Francine.
..Edwards/Kerr.. 09/10/2024
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SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1122 AM CDT Tue Sep 10 2024
Valid 101630Z - 111200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
COASTAL/SOUTHEAST LOUISIANA...
...SUMMARY...
Marginal tornado potential may reach coastal areas of Louisiana
during the late overnight/early morning hours.
...Southern Louisiana Late Tonight/Early Wednesday Morning...
The latest NHC forecast indicates Tropical Cyclone Francine will
move northeastward over the western Gulf of Mexico tonight through
early Wednesday morning. Morning surface observations show a stalled
front remains situated over the northern Gulf, with greater
low-level moisture (mid/upper 70s surface dewpoints) generally
confined near or just off the LA/MS/AL Gulf Coast. As Francine
approaches the LA Coast late tonight, this very moist low-level
airmass over the northern Gulf should gradually spread inland along
parts of the immediate central Gulf Coast, supporting weak
boundary-layer instability. As Francine approaches, a corresponding
increase in low-level flow and related effective SRH should support
some threat for tornadoes with any low-topped supercells in outer
rain bands to the east-northeast of the central circulation. Latest
guidance continues to suggest that the better tornado potential over
land will occur in the Day 2/Wednesday time frame (after 12Z). But,
an isolated/marginal tornado threat still remains apparent along
parts of the LA Coast for the last few hours of the Day 1 period
(08-12Z) early Wednesday morning, where a Marginal Risk has been
maintained.
..Gleason/Bentley.. 09/10/2024
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