Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Fri Mar 21 2025

Day 1
Valid 12Z Fri Mar 21 2025 - 12Z Sat Mar 22 2025

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

Wegman

Day 2
Valid 12Z Sat Mar 22 2025 - 12Z Sun Mar 23 2025

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

Wegman

Day 3
Valid 12Z Sun Mar 23 2025 - 12Z Mon Mar 24 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI AND TENNESSEE VALLEYS AS WELL AS FOR THE
WESTERN OLYMPIC PENINSULA OF WASHINGTON STATE...

...Lower Mississippi and Tennessee Valleys...

A strong cold front trailing behind a deep low tracking across the
Upper Midwest will be the primary forcing supporting widespread
shower and thunderstorm activity across portions of the Mississippi
and Tennessee Valleys on Sunday. The front will be driven with a
robust longwave trough moving across the Midwest. Much of the
Marginal Risk area will be in the favorable RER of the jet. As is
typical with thunderstorm outbreaks, there will be an LLJ out of
the Gulf that will be in place ahead of the front. The
southwesterly LLJ will be parallel to the orientation of the front
(southwest to northeast), which will support training of the
leading convection. 5-10 kt southwesterly Corfidi Vectors further
support the potential for training. Thus, the forward speed of the
front, which should be slowing...but should be fast enough to limit
training potential...will be the deciding factor as to how
widespread the instances of flash flooding will be.

FFGs decrease markedly towards the north into the Ohio Valley, but
the lack of atmospheric instability in that region will keep most
of the rainfall in this area to stratiform rain. Further, when the
front is moving southeastward across the Ohio Valley, the forcing
and moisture availability will not be as optimized as a bit later
when the front is further south. Thus, the inherited Marginal Risk
over much of Kentucky and southern Indiana was trimmed with this
update. The greater instability and Gulf moisture will be closer to
the Gulf, where the Marginal Risk remains with this update. The
flooding threat will be partially offset by the higher FFGs in the
lower Mississippi Valley, so between that and the forward speed of
the front, instances of flash flooding should remain isolated,
favoring a continuation of the Marginal Risk. As always, a slowing
of the front or additional moisture/instability with future updates
could require a Slight Risk upgrade. Should the front become more
progressive then it's possible the Marginal Risk could be
downgraded or shrunk further.

...Olympic Peninsula of Washington...

A strong trailing cold front will move into the coast of the
Pacific Northwest on Sunday. IVT values will peak around 600-650
kg/ms during this time. The western Olympic Peninsula's soils are
well saturated from repeated rounds of rain over the past few
weeks, so this round could result in isolated flash flooding on the
smaller streams and creeks that drain the Olympic Mountains. There
were no changes to the inherited Marginal Risk. Expect 2 to 4
inches of new rainfall in the area through the day on Sunday. 

Wegman


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