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FOUS30 KWBC 180819
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Sat May 18 2024

Day 1
Valid 12Z Sat May 18 2024 - 12Z Sun May 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/CENTRAL GULF COAST...

...Southern Alabama/Georgia into the Florida Panhandle...

Mid-level trough/weak closed low over the Lower MS Valley/Mid-
South this morning will continue eastward across southern TN today
and into the southern Appalachians by early Sunday. On its
southeast side, a stream of mid- level impulses will move across
the FL Panhandle and southern AL/GA today ahead of a cold front
that will finally clear through the area by tomorrow. SW flow out
of the Gulf will maintain relatively high precipitable water values
(~1.75" or +1.5-2 sigma) into the region ahead of the trough and
surface front, with more impressive 850-700mb moisture flux
anomalies into the region (+3 to +4 sigma). With ongoing convection
over southern LA moving eastward, expect this to continue past 12Z
along the I-10 corridor from southern MS eastward through early
afternoon. Thereafter, models suggest a loss of mid-level vorticity
dynamics to sustain heavier rainfall but this may be underdone
given the approach of the cold front with still sufficient moisture
available. Slight Risk outline was generally constrained to the
higher probabilities of >2" QPF in the 12-18Z period this morning.
Broader Marginal Risk covers much of the rest of the Southeast into
the Carolinas where isolated heavier rain rates are possible.
Trimmed away the northern/northeastern side of the Marginal Risk
where instability is low (or non-existent) on easterly flow.

Fracasso


Day 2
Valid 12Z Mon May 20 2024 - 12Z Tue May 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS/CORN BELT...

Broad SW flow over the central Plains across the Corn Belt will
bring in increasing moisture to the region on Sunday with a surface
frontal boundary hung up over Iowa. Mid-level vorticity will exit
the central Rockies and moves across the central Plains as
precipitable water values rise to around 1.50" (+1.5-2 sigma) with
some instability in the afternoon/early evening (500-2000J/kg) that
could yield some 1"/hr rates (per 00Z HREF thru 00Z Mon) which are
near FFG values over Iowa and nearby areas. The region has been
fairly dry the past week (most of the region has had about 50% of
the normal rainfall) so much of the rain will be beneficial, but
given 1-hr FFG values near 1"/hr (between MCI-OMA- DSM), maintained
the Marginal Risk outline for the region. Higher probabilities
exist farther southwest into central Kansas (to where the Marginal
was extended), but FFG values are higher as well (2"/hr). 12Z CAM
guidance through 12Z Mon may shed a little more light on the higher
rainfall potential over the entire 24 hours.


Fracasso


Day 3
Valid 12Z Mon May 20 2024 - 12Z Tue May 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS/CORN BELT...

Upper pattern remains fairly similar to D2 across the Plains, with
broad SW flow over the region. One wave will exit through the
Upper Midwest as troughing over the West deepens via northern and
incoming southern stream vorticity. Upper diffluence east of the
Rockies will support widespread showers/storms over the central
Plains Monday into Monday night along the stalled surface boundary,
with sufficient moisture in place and sfc CAPE 500-1500J/kg.
Heaviest rainfall per the ensembles lie over western Nebraska (Sand
Hills) where FFG values are highest, so focused the area to the
southeast and east into Iowa where rainfall on D2 may lower FFG
values a bit (from already somewhat lower numbers).

Fracasso


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt