FOUS30 KWBC 130748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
348 AM EDT Tue May 13 2025

Day 1
Valid 12Z Tue May 13 2025 - 12Z Wed May 14 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC...

A closed upper low will gradually lift north over the Tennessee
and Ohio Valleys, accompanied by a very slow-moving surface low
pressure system. A stream of deep Gulf/Atlantic moisture will move
into the Mid-Atlantic ahead of the system, increasing PWAT values
to 1.5-2 inches. There will be enough instability (MUCAPE > 1500
J/kg) to support thunderstorm development, some of which may be
strong/deep. The best chances for deep convection with high rain
rates (1-2 inches) will likely be over portions of Virginia and
eastern North Carolina where higher instability will reside, but
persistent upslope enhancement will also contribute to elevated
rain rates over portions of the Appalachians from central and
southern Pennsylvania to Virginia. These areas will likely see 1-3
inches of rainfall today, with locally higher amounts in terrain
and heavier convection. Heavy rain will have the potential to
create isolated to scattered flooding concerns across these areas,
warranting a Slight Risk of excessive rainfall.

Elsewhere, a Marginal Risk of excessive rainfall is in effect for
the Ohio Valley and portions of the Tennessee Valley and interior
Southeast where scattered diurnal convection is expected underneath
the upper low. Slow storm motion will contribute to elevated rain
rates (1+ inch per hour) that could reach or exceed 1 and 3 hour
FFGs (1-2 inches). Given that soils in these areas are already
saturated from recent heavy rain, these rain rates could create
isolated instances of flash flooding.

Dolan


Day 2
Valid 12Z Wed May 14 2025 - 12Z Thu May 15 2025

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

...Northern Plains...

A deep upper level trough will push across the northern Rockies
into the northern Plains late Tuesday into Wednesday, resulting in
a strengthening low pressure system tracking from Wyoming through
North Dakota. Stratiform rain with some embedded convection is
expected on the northern and western side of the low, which should
result in a swath of 0.5-1+ inches of rainfall across central and
eastern Montana on Wednesday. Storm total rainfall will be higher
across Montana due to additional rain falling at the end of the Day
1 period (Tuesday night). The main concern with this activity will
be the duration, with rainfall rates potentially reaching 6 hour
FFGs. This could support isolated flash and river flooding concerns.

Over the Dakotas, convection is expected to develop along a
strengthening cold front that will push east across the region.
With favorable upper level and surface support, some convection may
be deep enough to produce 1+ inch per hour rain rates that could
reach or exceed the 1 and 3 hr FFGs (around 1.5-2.5 inches).
Conditions will be supportive of localized heavy rain in stronger
convection/thunderstorms, with plentiful moisture (PWATs 1-1.5
inches) and instability (CAPE > 1500 J/kg) in place. The Marginal
Risk of excessive rainfall in the northern Plains includes portions
of Montana and the Dakotas where isolated to potentially scattered
instances of flash flooding will be possible. It's possible an
upgrade may be needed for the Dakotas if precip rates and QPF trend
higher, but we should have a better idea of potential impacts once
the period is fully within range of the CAMs.

...Mid-Atlantic...

Soils are expected to be thoroughly saturated across the Mid-
Atlantic by Wednesday due to heavy rain earlier in the week, which
will likely result in a lingering flash flood threat on Wednesday.
The slow-moving surface low is forecast to become nearly stationary
over the region while gradually weakening, resulting in scattered
precipitation through Wednesday night. Enough moisture and
instability are expected in the vicinity of the surface low to
support convection with relatively higher rain rates that could
lead to flooding. The 00Z HREF shows a high probability (60-80%) of
1 inch per hour rain rates from the Maryland and West Virginia
Panhandles through Virginia to eastern North Carolina and even a
low/isolated probability (~20%) of 2 inch per hour rain rates. A
Marginal Risk is in place from southern Pennsylvania through North
Carolina to northeast South Carolina to account for this flood
potential.

Dolan


Day 3
Valid 12Z Thu May 15 2025 - 12Z Fri May 16 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST AND NORTHERN PLAINS...

An occluded low pressure system will lift northeast across the
Upper Midwest, with showers and storms ahead of the occluded and
cold fronts and some precipitation wrapping around the backside of
the system into the northern Plains as well. Slow storm motion and
repeat activity look like they will be the main threat, which may
result in rain rates that may reach or exceed the 3 hour FFGs.
There will be plenty of moisture in place to support elevated rain
rates with PWAT values reaching 1-1.5 inches. However, instability
may be a limiting factor. The best instability is expected to be
further south along the cold front, displaced from the higher QPF
values. Based on available guidance, a Marginal Risk area should
suffice to account for the low end flash flood potential.
Precipitation is forecast to extend across the Great Lakes region
as well, but rain rates will likely be moderate at best, not
supporting a flash flood threat at this time.

Dolan

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