Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Thu Apr 24 2025

Day 1
Valid 01Z Fri Apr 25 2025 - 12Z Fri Apr 25 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN
LOUISIANA and  NORTHEAST TEXAS, AND NORTHEASTERN KANSAS THROUGH
CENTRAL IOWA...


...01Z Update...
Removed one of the remaining two Slight risks given the history of
convection through the afternoon and fairly limited additional
amounts of rainfall expected during the overnight. Combined with
dry antecedent conditions across Iowa...removed the northern most
Slight risk area but maintained a low-confidence Marginal given the
strength of on-going convection and its potential for isolated
heavy rainfall rates before convection tapers off. Farther
south...maintained the broad Marginal risk area across the western
High Plains eastward across the Central and Southern Plains with
an embedded Slight Risk over part of eastern Texas based on short
term radar trends which supported the earlier high-res guidance.
Removed the Marginal risk area farther east as convection weakens
and dissipates with the loss of daytime heating.

Bann


...16Z Update...

The inherited Slight Risk across much of the Plains was divided
into two much smaller risk areas with this update, with the middle
carved out back down to a Marginal. The CAMs guidance has been in
poor agreement regarding how these MCSs, as well as any new
convection later this afternoon, will behave through tonight. Where
there is at least some consensus among the guidance is for greater
amounts of convection in the new, smaller Slight Risk areas and
less in the middle, which was the catalyst behind dividing the
Slight.

...Eastern Louisiana...

The newly issued Slight in eastern Louisiana is in place for an
ongoing MCS that has a history of producing rainfall rates of 2-4
inches per hour, along with training and backbuilding that has
allowed this heavy rainfall to occur over a small area for an
extended period of time. On the whole the MCS has been weakening,
but local cores with rates that high continue this morning. CAMs
guidance is in reasonable agreement that some level of heavy rain
from convection will continue in this area, likely north of Lake
Pontchartrain, through 18Z. It appears likely that this Slight
will be able to be downgraded this afternoon with higher confidence
in the convective trends.

...Northeast Texas...

The southern end of the previous Slight has been expanded south and
east in response to a much more robust MCS (compared to the one in
Louisiana) across northern Texas this morning. Individual cores of
heavy rain have been far more persistent into the late morning, and
have developed into an east-west oriented line that is now
generally south of the I-20 corridor, and fortunately well south
and west of the Dallas-Fort Worth Metroplex. While urban concerns
remain heightened in the Metroplex for the possibility of
additional storm development this afternoon, the greatest near term
concerns will be between I-20 and I-35 in central Texas. With peak
heating, expect additional storms to develop south of the current
storms in the area of greatest inflow of moisture and instability.

...Kansas to Iowa...

Multiple small clusters of storms from northern Kansas through
central Iowa are largely weakening this morning. However, there is
some CAMs consensus that additional strong convection will break
out in southeast Nebraska this afternoon. This convection will
advect northeast into Iowa, where previous rounds of heavy rain
have depressed FFGs. Thus, despite a lower likelihood of training
and faster moving cells, the low-confidence Slight remains in place
with a small expansion northeast further into central Iowa based on
the trends in the guidance. There remains some possibility tonight
that additional convection will fire along remnant outflow
boundaries in northern Kansas where the current line of showers has
already cleared, which is why for now the Slight remains for that
area.

...Marginal Risk...

Elsewhere, as mentioned above expect a relative minimum of
convection over most of Oklahoma and southern Kansas today and
tonight, though isolated cells remain very possible. For the
Southeast, no changes were made, with expected individual cells or
small clusters of storms developing this afternoon with peak
heating. The coverage of storms in this area is unlikely to be very
robust, precluded the need for a Slight Risk upgrade for now.

Wegman


Day 2
Valid 12Z Fri Apr 25 2025 - 12Z Sat Apr 26 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH TEXAS AND SOUTHWEST OKLAHOMA...

...2030Z Update...

In coordination with LUB/Lubbock, TX and SJT/San Angelo, TX
forecast offices, a Slight Risk was hoisted for portions of north
Texas and southwest Oklahoma this afternoon. Surprisingly good
agreement among the 12Z suite of CAMs guidance suggests persistent
and slow-moving thunderstorms will develop across this region
Friday afternoon. The storms will likely initiate along the
dryline, and congeal Friday evening into an MCS. The forecast for
the individual cells contained within the MCS is uncertain for
Friday night, but there is some concern that stronger
storms/supercells may be stalled or very slow moving, causing
localized flash flooding impacts, while the MCS as a whole remains
over an area hard hit with heavy rain in recent days.

The Marginal Risk remains in place over much of the Midwest and
Tennessee Valleys for more stratiform, but longer lived rainfall
across the region, with embedded convection that may cause isolated
instances of flash flooding, especially in the lower FFG areas hit
with heavy rain recently, resulting in locally saturated soils. The
Marginal Risk into Arkansas and Missouri was removed with this
update as it appears more likely that the MCS out west and the
rainfall across the east remain separate.

Wegman

...Previous Discussion...

A strengthening surface low developing ahead of a strengthening
upper level shortwave trough will drive the storms from a forcing
standpoint through the period. Starting in the Plains it will track
northeast into the Great Lakes Friday evening, with showers and
storms forming along the cold front to the south and east of the
low center. This will bring periodically heavy rainfall to
portions of the Northeast, which has been in a drier period the
past week. Gulf moisture continuing to advect in from the south
will lead to shower and thunderstorm activity starting Friday
afternoon across the Tennessee and Ohio Valleys, and continuing
into Friday night as the front lags behind (south and west) of the
surface low. Meanwhile, the trailing (western) end of the cold
front may be the trigger for additional thunderstorms into southern
Oklahoma, north Texas, and the southern Texas Panhandle.

In coordination will the offices in the Northeast the Marginal Risk
was removed from New Hampshire, Vermont, much of New York and
northeast Pennsylvania. This area has been drier of late and with
green up already in progress, much of the rain will be absorbed
with minimal if any flooding possible. Elsewhere, the Marginal had
minor reshaping to reflect the latest WPC QPF and model trends.

Campbell/Wegman


Day 3
Valid 12Z Sat Apr 26 2025 - 12Z Sun Apr 27 2025

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

...2030Z Update...

...Southern Plains...

An MCS that developed over the Texas Panhandle Friday and Friday
night will persist into Saturday across the Marginal Risk area,
with less areal coverage of rain on Saturday as compared with
Friday. Uncertainty regarding MCS behavior on Saturday remains very
high, and further updates and the move of this time frame into CAMs
range may require a later upgrade to a Slight, particularly if
heavy rain remains further west towards the Panhandle for longer.
Regardless, additional convection is likely to form again Saturday
afternoon along the dryline near the New Mexico border, which may
further worsen flooding concerns in the areas where the storms
develop.

...New England...

Does an inch of rain in Maine fall swiftly enough to cause pain?
The answer is no, so the Marginal has been removed.

Largely stratiform rain and the fast movement of the rainfall
plume over soils that are at or drier than normal should preclude
flash flooding in this area.

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