Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 PM EDT Sat May 17 2025
Day 1
Valid 16Z Sat May 17 2025 - 12Z Sun May 18 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHERN NEW ENGLAND...

...Texas to Alabama...

16Z Update: The previous MRGL risk was relatively unchanged with
the only necessary adjustments made were for areas further north
around Northern MS/AR and Southern TN where convection has already
occurred and the threat has diminished. The overall pattern remains
steadfast with the 12z CAMs and subsequent HREF depiction
highlighting the afternoon and evening period of heavy
thunderstorm activity initiating over the Southern Plains and
points east into the Lower Mississippi Valley with emphasis along
the surface front positioning. Regional theta_E assessment gives us
a pretty good inflection marker of where the boundary is situated
and correlates well with the convective pattern anticipated for
points along and north of I-20. The area of great focus will lie
between the DFW metro through the AR/LA border where the tight
theta_E gradient aligns within the expected cold front to quasi-
stationary frontal positioning after 00z. HREF blended mean output
continues to pinpoint the area along the AR/LA line as the area of
greatest interest, likely in part to the stronger low-level
convergence field intersecting with the deepest moisture layer
between the sfc-500mb. That area will also see a dual convective
threat with initiation along the front between 18-00z, then
eventual impact from downstream convective propagation out of the
ArkLaTex thanks to attendant shortwave progression out of the
southern stream jet.

A corridor of elevated neighborhood probs for >3" (40-70%) are
located within a zone extending from Northeast TX across the state
lines inferenced above. The only saving grace of the setup being
more conducive for a higher risk is the inherited FFG 1/3/6 hr
indices are all very high (3-4/5-6/6+ inches) respectively and
that's where the threat falls just below a higher risk threshold.
Will need to monitor the threat closely to see how trends play out
for a possible targeted upgrade, but decided to refrain from the
that with the initial update as rates are geared to be capped at
2-3"/hr at peak intensity.

The potential for elevated convection across Northern OK into
Southeast KS remains with the HRRR the most bullish on the
prospects. The threat remains on the lower end of risk threshold,
but the setup is worthy of maintaining the risk given the potential
for training if the threat ensues.

Kleebauer

..Previous Discussion..

Convection will likely develop across portions of central TX this
afternoon as increasing forcing overruns the dryline. Pretty
impressive thermodynamic environment, with upwards of 5000 j/kg of
CAPE forecast along and east of the dryline, along with substantial
deep layer shear. This will likely result in rather explosive
convective development this afternoon, and while not the strongest
large scale forcing, there is enough mid level vort energy and
upper jet dynamics to support scattered to perhaps numerous
convective coverage. Supercells are likely, with some upscale
growth into convective clusters probable. Given the CAPE/shear
profile, large hail is the main threat, although even with such a
robust CAPE profile the deep layer moisture is actually still
pretty good. Thus these cells will likely be capable of producing
some heavy rainfall rates over 2"/hr as well.

Limiting factor for flash flooding will be quick cell motions
(30-40 kt mean westerly flow) and the high FFG over the area.
However right moving supercell motions are closer to 15-20 kts)
and the various cell movement deviations we see in a supercell
environment will likely lead to some cell mergers as well. Thus do
think we will see isolated flash flooding, with a focus over
central to northeast TX. Not sure we will get the FFG exceedance
coverage needed for a Slight risk given the generally quick cell
motions and high FFG, so we will stick with a Marginal risk. Even
the 00z RRFS, which tends to run hot with QPF rates/totals, only
depicts isolated hourly FFG exceedance.

Convection will likely be ongoing this morning across the eastern
portion of the Marginal risk (MS/AL), but should be in a decaying
state. However we could see at least isolated redevelopment along
the leftover outflow/front this afternoon, and anything that does
develop would likely be slow moving and capable of resulting in a
localized flash flood threat.

The Marginal risk was also expanded to the north into more of OK/AR,
southeast KS and southern MO. There are indications that an
overnight cluster of storms should develop here later tonight into
Sunday morning. This is where the stronger moisture convergence and
instability gradient should be and thus it makes sense that we
would get some elevated convection develop somewhere along this
axis. The 00z HREF is not that aggressive with development here,
but the HREF members can often be slow in catching on to training
elevated convective development. There is enough of a signal in the
00z global guidance to suggest we may see the HREF trend wetter on
future cycles, and there is at least some threat of an isolated
flash flood risk.

...Northeast...

16Z Update: There was little change in the previous forecast as all
the variables below remain on track. 12z CAMs had little to no
deviation from previous forecast with the area across the Central
Greens and White Mountains as the primary area of interest this
afternoon as the ULL moves overhead. Western ME remains the lower
end of the risk threat, however antecedent conditions from the
previous period have primed the region enough to lower the
threshold necessary for flash flood prospects in the the Northern
Appalachian territory. 12z HREF EAS signals of 50-70% for at least
1" are significant within the area outlined in the SLGT, something
that has some historical precedence for at least a few localized
flash flood concerns in the complex terrain of Northern New
England.

Kleebauer

..Previous Discussion..

Multiple rounds of convection are expected across the Northeast
today into this evening. PWs and instability in place support high
rainfall rates, and both the 00z HREF and 18z REFS depict a 30-50%
chance of hourly rainfall over 1" and the 06z HRRR has a swath of
1- 2" per hour rainfall. In general, quick cell motions will help
limit the duration of these heavy rates, but the combination of
multiple convective rounds and high rates indicates at least some
flash flood risk is in place. Recent high res model runs indicate a
stationary/warm front in place over portions of VT/NH this
afternoon, which may act as a focus for some short duration
training of cells. Expect the best chance for isolated to scattered
flash flooding to focus near and south of this boundary, which
ends up encompassing much of central/northern VT into central NH.
This is generally a bit south and west of the inherited Slight
risk area. Thus we will expand the Slight risk into this region,
while maintaining it in northern NH and western ME (less confidence
on higher rates in these latter areas, but heavy rainfall over the
past 24 hours may keep the area a bit more sensitive to additional
rainfall today).

Chenard

...Inter-Mountain West...

A non-zero threat for flash flooding will exist across portions of
the Inter-Mountain West with emphasis over Northern NV, Northwest
UT, and Southern ID where the evolution of a strong upper trough ejecting
into the Great Basin will create a period of convective
enhancement during the late-morning and afternoon period before
subsiding after cold frontal progression stabilizes environment in
wake of fropa. Modest instability with much of the setup rooted
within steep low to mid-level lapse rates and a narrow corridor of
prominent theta_E's advecting into the Snake River Basin during the
front half of the forecast cycle. Scattered showers and storms will
produce locally moderate to heavy rainfall for a time as the
evolution unfolds with the greatest threat over some of the
urbanized zones that carry naturally lower FFG's. The reasoning for
a lack of an ERO risk area is due to the rate expectations being
pretty meager overall. The threat is very isolated with the 3 areas
noted above having the greatest opportunity for possibly seeing
something. Considering very low FFG exceedance probs for 1/3/6 hr.
indices, a nil ERO remains, but wanted to make mention due to the
threat still non-zero but just below the MRGL risk threshold.

Kleebauer


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt