Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Thu Apr 10 2025

Day 1
Valid 16Z Thu Apr 10 2025 - 12Z Fri Apr 11 2025

...16Z Update...

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

Latest hi-res guidance continues to suggest the potential of
thunderstorms with up to 1 to 1.5"/hour rainfall rates broadly
from the Ohio Valley into the Southeast, particularly for portions
of the Tennessee Valley into the southern Appalachians. However,
storm coverage and progression will likely limit overall totals to
these isolated quick bursts which will keep any flooding concerns
related to some isolated ponding on roadways in urban areas.

Putnam

...Previous Discussion...

...OH Valley and Mid-South...
The latest GOES-E WV suite shows a mid-level trough gradually
amplifying across the Midwest as shortwave energy digs
southeastward from the northern Plains. The model guidance shows a
further amplification of mid-level height falls down across the OH
Valley and Mid-South by later today and tonight which will allow
for a cold front and an attendant area of low pressure to cross
portions of the region. Locally heavy showers and thunderstorms are
expected to develop in the warm sector this afternoon and evening
as a moderately buoyant airmass characterized by MLCAPE values of
1000 to 1500 J/kg sets up and couples with proximity of left-exit
region upper jet dynamics associated with the arrival of a jet
streak. Adding to the convective footprint will be steep mid-level
lapse rates given the amplifying trough overhead, and a sufficient
amount of shear will be in place for some supercell convection.
While moisture is not going to be particularly supportive of heavy
rainfall totals, the convection will be locally well-organized and
capable of producing some hourly rainfall totals of as much as 1 to
1.5 inches. Areas from KY/TN down through northern MS/AL will
generally see scattered pockets of this convection with this
rainfall potential. Given the wet antecedent conditions from the
most recent high-end rainfall/flooding event, it is possible that
these additional rains may result in an isolated concern for some
flash flooding. For now, a Marginal Risk will not be depicted given
some question marks on the overall coverage of convection and
modest rainfall totals, but a non-zero threat of additional
flooding will exist at least on a small-scale and highly localized
basis.

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

Orrison


Day 2
Valid 12Z Fri Apr 11 2025 - 12Z Sat Apr 12 2025

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
MID-ATLANTIC STATES...

...2030Z Update...

Previous forecast remains on track overall, including continued
support from the hi-res guidance for a more southerly QPF footprint
from the central/southern Appalachians east-northeast through
North Carolina and into the southern Mid-Atlantic, which led to
minimal spatial changes to the inherited Marginal Risk. A
combination of showers and some thunderstorms will support areal
average rainfall totals of 1-2", with amounts locally as high as
3". However, generally expect rain rates to remain moderate and on
the steady side, especially on the western/colder side of the
system, with isolated heavier rain rates of around 1" per hour
likely limited to areas along and east of I-95 where some
instability will be present. As noted in the prior discussion, this
steady rainfall will likely be more beneficial for most areas, but
some isolated urban flooding/ponding will be possible particularly
along the I-95 corridor, and possibly for more terrain sensitive
areas into the Appalachians of southwestern Virginia and southern
West Virginia.

Putnam

...Prior Discussion...

...Mid-Atlantic States...
The 00Z model guidance continues to strongly support the arrival of
an amplifying deep layer trough across the central Appalachians and
the Mid-Atlantic states by Friday as an upper-level jet streak rounds
the base of the trough and lifts northeastward up across the coastal
plain. The model consensus suggests a 500mb low center developing
over the VA Piedmont area, however there is some latitudinal spread
with this as some guidance like the 00Z NAM is farther south, and
other guidance like the 00Z GFS is farther north and faster.
Regardless of exact placement, large areas of the Mid-Atlantic
region will come under left-exit region jet dynamics for ascent
which coupled with strengthening frontogenetical forcing poleward
of a deepening area of low pressure, and relatively strong moisture
convergence, should promote the development and expansion of
moderate to locally heavy rainfall. Instability will be lacking
over much of the region with exception of southern VA and
especially NC where closer proximity to the warm sector will be
noted. However, there will be steepening mid-level lapse rates
associated with the closed low development, and there will be areas
of slow-moving, and rather shallow convection that will be capable
of producing heavy rainfall rates. A few model solutions such as
the GEM regional, HRW-ARW2, and NAM-Conest support locally heavy
rainfall totals (2 to 4 inches) over parts of southwest VA and NC
in particular due to slow cell-motions. Farther north into the
cooler side of the storm, the I-95 urban corridor from central VA
up into southeast PA/NJ may see locally heavy rains impact this
region with totals as high as 1 to 2 inches. Generally, these rains
should be beneficial aside from some localized urban runoff
concerns. However, there may be a more defined concern for some
localized flash flooding down into areas of southwest to south-
central VA and NC, along with some potential for southern WV to see
some of this threat. Given the southward trends in the guidance
for the overall heavy rainfall footprint, the Marginal Risk area
has been accordingly shifted farther south compared to continuity.

Orrison


Day 3
Valid 12Z Sat Apr 12 2025 - 12Z Sun Apr 13 2025

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

Putnam


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