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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0804Z May 19, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Sun May 19 2024

Day 1
Valid 12Z Sun May 19 2024 - 12Z Mon May 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS, UPPER MIDWEST, AND COASTAL SOUTH
CAROLINA/GEORGIA...

...Central Plains through the Corn Belt...

Southwesterly flow east of the Rockies in the mid/upper levels
will carry waves of vorticity across the central Plains into the
Corn Belt today/tonight, culminating in a more coherent/robust vort
max out of NE into Iowa overnight. Surface frontal boundary through
CO/KS will lift back northward today as a warm front with
increasingmoisture in the boundary layer (precipitable water
values ~1.50" or +1.5 sigma), leading to a severe weather threat
over KS late this afternoon. Additionally, warm front lifting
through Iowa will help spur some rainfall (though generally
beneficial) atop relatively lower FFG values (compared to KS). 00Z
HREF probs for exceeding FFG values run about 10-50% from KS
northeastward to southern MN, roughly in line with the Marginal
Risk outline and weighted toward late afternoon into the overnight
hours as the stronger vort max moves through. Question will be the
details of the mode of convection and how things play out
downstream.


...Coastal South Carolina and Georgia...

Vort max over northern GA this morning will slip southeastward
across the coastal areas this afternoon, yielding another chance
for showers/storms as it passes overhead. Maintained the Marginal
Risk for this area of coastal GA/SC beneath its path where surface
convergence is greater. 00Z HREF probs of exceeding FFG values
(~3"/3h) climb as high as 40% in a very small area where CAPE will
be sufficient. Urban areas will be most susceptible.

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...

Similar upper pattern over the central US on Monday but with
changes upstream over the West. A southern stream shortwave will
race through the southern Rockies just ahead of a digging northern
stream trough into the northern Great Basin. Likely convection late
Sunday/early Monday may leave the area rather stable for the
daytime Monday, though the lingering frontal boundary will act as a
focus for some rainfall into the afternoon. Any heavier rainfall
may hold until overnight (00-12Z Tue) as the shortwave exits CO and
the LLJ increases. Heavier rainfall may lie over the NE Sand Hills
where FFG values are much higher, so the Marginal Risk outline is
on the smaller side to the south and east of this region for any
overnight isolated heavier rainfall as surface cyclogenesis starts
in earnest.


Fracasso



Day 3
Valid 12Z Tue May 21 2024 - 12Z Wed May 22 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER MIDWEST...

Robust southern stream shortwave over eastern CO Tuesday morning
will head northeastward into southwestern MN by early evening as
surface low pressure deepens below 990mb over northeastern MN at
the end of the period. Strong southerly flow in the BL (850mb winds
to 50kts) will help surge moisture northward into and around the
low, with precipitable water values climbing to over 1.50" which is
about +2 sigma and 850-700mb moisture flux anomalies > +5 sigma
from MO northward to MN/WI. QPF mode will be deformation-driven on
the NW side of the wrapped-up/occluded low where rates will be
modest but perhaps exceeding longer-time FFG values vs warm-sector
QPF (MO northward/northeastward) where instability will be present
(CAPE > 2000 J/kg as far north as northern IA). The entire system
will be rather progressive, but expansive. Focused the Slight Risk
on the warm sector rainfall and nosed back toward eastern SD where
FFG values are lower than points farther west (covered by the
Slight Risk). Models show various axes of QPF tied to different
forcings in the evolution of the system, which is covered by the
broad Marginal Risk outline eastward to Lake Michigan. AI guidance
was displaced a bit to the east of the dynamical parent models, but
this was also reflected in the ensemble systems as well
(GEFS/ECMWF/GEPS). QPF spread was largest well into the warm
sector (MO into the Ozarks) where the flash flood threat may be
more isolated depending on the convective evolution.


Fracasso


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