Subj : HVYRAIN: Excessive Rainfa
To   : All
From : Mike Powell
Date : Mon Apr 14 2025 08:56:00

FOUS30 KWBC 140804
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Mon Apr 14 2025

Day 1
Valid 12Z Mon Apr 14 2025 - 12Z Tue Apr 15 2025

....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST VIRGINIA...

A Marginal Risk area for much of the central and southern regions
of West Virginia was introduced with this update. The latest CAMs
guidance continues to hint at a line of training showers and
thunderstorms that will press southeastward this afternoon and
evening, while the storms move more eastward, suggesting at least
some potential for training. The biggest question will be how 
strong the storms with the heaviest rain cores will get. A few of
the CAMs, such as the NAMnest, 00Z HRRR and, GEM Regional all
suggest there will be a line of training storms, albeit in
different areas of the state. Thus, as is typical with Marginals,
the area is likely far bigger than the portions of the state that
end up actually seeing flash flooding. Given the aforementioned
uncertainty, the area is bigger to account for that. MUCAPE values
will be around 500 J/kg this afternoon, and PWATs will be between 1
and 1.25 inches, which are values that should support the isolated
flash flooding threat consistent with a Marginal Risk, given the
general agreement for limited areas of training. 

Wegman


Day 2
Valid 12Z Tue Apr 15 2025 - 12Z Wed Apr 16 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Wed Apr 16 2025 - 12Z Thu Apr 17 2025

....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND WESTERN NEVADA...

An upper level low centered off the coast of central California
Wednesday morning will drift eastward towards the coast through the
day. This upper low will spread increasingly cold air in the upper
levels over California and Nevada. This will increase the
instability over the region. Meanwhile, a plume of Pacific moisture
will also move into the region, raising PWATs above 0.75 inches in
some areas, which is over 2 sigma above normal and above the 95th
percentile compared to climatology for the region. Finally,
plentiful warm air in the lower levels of the atmosphere will both
contribute to that instability, as well as raise snow levels to
9-10k ft, so the vast majority of expected precipitation in and
around the Sierras will be in the form of rain. Since expected
MUCAPE values will increase to around 500 J/kg over the area, the
storms that form will be capable of producing cores of heavier
rain, which combined with snowmelt in the Sierras will support an
isolated flash flooding risk, especially in the most prone valleys
and low lying areas. Thus, in coordination with REV/Reno, NV and
STO/Sacramento, CA forecast offices, a Marginal Risk was
introduced with this update. Expect with changing model guidance
that the Marginal area will shift between the morning model run
cycle and Wednesday's event, particularly across western Nevada, as
the amount of convection that may cross the Sierras becomes more
clear.

Wegman

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