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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
333 AM EDT Wed May 14 2025

Day 1
Valid 12Z Wed May 14 2025 - 12Z Thu May 15 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHERN PLAINS AND MID-ATLANTIC...

...Northern Plains...

A deep upper level trough centered over the Intermountain West this
morning will become negatively tilted as an impressive shortwave
draws plentiful upper level energy into the northern Plains.
Surface cyclogenesis is expected ahead of this trough over Kansas,
which will track NNE into south-central North Dakota by Thursday
morning. A line of convection associated with the leading/eastern
edge of the trough is moving into far western South Dakota from
Wyoming, and this line of storms will track NNE over the western
Dakotas through the morning. The primary flooding threat will be
from a new line of storms which will develop over western South
Dakota with peak heating this afternoon, then track very slowly
eastward, as embedded cells move north, parallel to the line. This
will result in some training of convection, leading to an isolated
flash flooding threat.

In coordination with UNR/Rapid City, SD; FSD/Sioux Falls, SD; and
ABR/Aberdeen, SD forecast offices, the inherited Slight Risk was
removed with this update, and the surrounding Marginal shifted well
to the west of inherited. The westward shift was due largely to a
shift in the guidance slowing the forward motion of the trough.
While this increased forecast rainfall amounts some, the area of
the Dakotas now highlighted is in Moderate to in some localized
cases Severe drought. Nasa Sport soil moisture imagery shows the
area has next to no moisture. Thus, despite the potential for heavy
rainfall at times in some areas, the flooding threat was determined
to be isolated.

...Mid-Atlantic...

Lingering moisture from a slow moving storm system associated with
a weak upper level negatively tilted trough along with MUCAPE
values in the area between 1,000 and 1,500 J/kg this afternoon will
lead to a renewed round of showers and thunderstorms across much of
the Mid-Atlantic. Due to the movement of the trough however, the
storms should be rather progressive, which will limit the amount of
rainfall any one area sees. While the entire area has picked up 1-2
inches of rain yesterday, thoroughly saturating the near-surface
soils, expect most areas to see less than an inch of new rain.
Those localized areas under the stronger storms may perhaps 
approach an inch of new rainfall. This should keep flash flooding
isolated and localized, albeit with some uncertainty. For now the
area remains in a higher end Marginal with few changes from
inherited. 00Z HREF guidance shows a high probability of exceeding
3 and 6 hour FFGs from far western Maryland southeast to the
Tidewater of Virginia with these storms and low FFGs, but it
remains unclear as to the impacts from the resultant flooding given
the significantly lower amounts of rain expected today as compared
with yesterday.

Wegman


Day 2
Valid 12Z Thu May 15 2025 - 12Z Fri May 16 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHERN PLAINS...

The low over the northern Plains will become vertically stacked as
the upper trough catches up with the surface low. With the new
vertically stacked low moving only very slowly towards the
northeast, this will make for a prolonged period of rain on the
cold/northwest side of the center, as it becomes the comma head
region of the low. This area of the low is expected to be largely
devoid of convection given the lack of instability making it quite
that far north and west. Thus, expect the 2-3 inches of additional
rainfall from today to have only a minimal impact given the still
largely dry soils over much of North Dakota. This Marginal was
shifted well to the west following the latest guidance trends, so
the Marginal was removed from much of Minnesota due to lack of
expected rainfall.

Wegman


Day 3
Valid 12Z Fri May 16 2025 - 12Z Sat May 17 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST, TENNESSEE VALLEY, AND OZARKS...

A potent shortwave trough will round the periphery of the
vertically stacked low over the northern Plains and upper Great
Lakes on Friday. This shortwave will tap into a low level jet of
Gulf moisture riding the prior low's occluded front. This will
reinvigorate the front, resulting in rather fast northeastward
moving storms across the Marginal Risk area. Much of this area
remains saturated for soil moisture from recent prior storms, so
any training thunderstorms will be capable of producing isolated
flash flooding. There is poor agreement on the character of the
storms, which better agreement may have warranted a Slight Risk
upgrade in part due to the wet soils in this area. For now, will
continue to monitor the trends in the rainfall guidance for better
indication on if a Slight Risk upgrade is needed.

Wegman

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