392
FOUS30 KWBC 171548
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1148 AM 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 2
Valid 12Z Sun May 18 2025 - 12Z Mon May 19 2025

...THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE
CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY...

...Plains, MS Valley and Southeast...
A favorable pattern for widespread convection and heavy rainfall
on Sunday across the middle portion of the country. A longwave
trough and embedded shortwaves pushing eastward across the Rockies
and into the Plains will provide ascent over a broad region. The
frontal pattern will likely feature a developing low moving into
the High Plains of CO/NE, a warm or stationary front extending east
of this low across portions of OK/KS into MO/AR, and a dryline
across OK/TX. A large pool of instability is expected east of the
dryline and south of the stationary front, with values upwards of
4000-5000 j/kg. Some convection may be ongoing near the
warm/stationary front Sunday morning, but more robust development
is expected by Sunday afternoon into the overnight hours as
stronger forcing ejects into the Plains.

The main area of focus for flash flooding appears to be near the
aforementioned warm/stationary front where some training of convection
is probable. The overall expectation is that robust convective
development occurs along/ahead of the dryline over central OK/KS
during the afternoon hours. Strong deep layer southwesterly flow
will likely quickly push this activity northeastward towards the
warm/stationary front. Activity will likely grow upscale Sunday
evening into the overnight, potentially into an MCS, at which point
it should feel the impact of the strong southwesterly low level jet
and begin turning easterly and then southeasterly aligned with the
upwind Corfidi Vectors. As this process occurs some
training/backbuilding on the south/southwest extent is probable
resulting in a scattered flash flood threat.

The exact MCS location/track remains a bit uncertain. All but
certain that the 00z NAM is too far north, as its QPF evolution is
an outlier and does not align with pattern recognition. The GFS
and GEFS mean are also probably too far to the northeast, with the
aforementioned expected MCS propagation taking it on a farther
south track. The 00z Gem Reg and RRFS are very far south, tracking
across eastern OK into central/northern AR. This may be too far
south, but can not be ruled out as easily as the farther north NAM
can. The AIFS has a track record of pinpointing organized
convection location quite well, and the 00z AIFS favors northeast
OK into southeast KS and southwest MO. This also happens to
generally be the middle ground of the other available models, and
so does appear to represent the most likely outcome at this time.
Thus a higher end Slight risk will remain in place across this
corridor, where scattered flash flooding is expected. The Slight
risk area was cut back a bit on its northern extent and expanded
to the southwest into more of OK and AR to account for the southern
trend. Can not rule out additional southward shifts/expansion..but
for now think the Slight risk area represents the general region
of most concern well.

Another area of interest will be farther northwest into central and
western NE, closer to the low track. A multi-model signal exists
for a second QPF max over this area...which makes sense given what
should be impressive synoptic driven ascent and instability along
the stationary/occluded front. Considered expanding the Slight risk
into this area...but opted against that for now. This area remains
in severe to extreme drought and possible the rainfall max ends up
over the high FFG sand hills. But we will continue to monitor
trends and an additional focused Slight risk may eventually be
needed.

A localized flash flood risk also exists both along the dryline from
central TX into OK...and over the Southeast where a lingering
boundary and substantial instability will support heavy rainfall
with any storms that area able to develop.

...Northeast...
A Marginal risk continues over portions of northern New England on
Sunday as a deep layered low remains overhead. This will be the
3rd straight day of localized heavy rainfall, and thus some areas
may be more sensitive by this time. Generally thinking both
instability and moisture will be lower by Sunday, however cold air
aloft under the low will likely still support some heavier
convective cells and an isolated flash flood risk.

Chenard


Day 3
Valid 12Z Mon May 19 2025 - 12Z Tue May 20 2025

...THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE
CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY...

Another day of widespread heavy rainfall expected across the middle
portion of the country on Monday as the longwave trough and embedded
shortwave energy slowly moves eastward. It will likely be an active
day/night with a broad region where flash flooding will be possible.
A Slight risk stretches from northeast TX northward into southern IA.
The northern extent of the Slight risk will be close to or just
north of the stationary front extending east of the central Plains
low pressure. Strong forcing overrunning this boundary will likely
result in an expansive area of convection, with some
training/backbuilding possible. The southern portion of the Slight
risk is driven by initial dryline convective development, likely
enhanced by the eventual eastward acceleration of the cold front
south of the low. The convergence from the dryline, cold front and
lifting warm front will all aid in convective development. The
general stagnant nature of the frontal feature through much of the
day (both the warm/stationary front and dryline) support a
training/backbuilding convective threat.

Exactly where the focus of highest QPF ends up remains a bit of a
question, but generally anywhere within the Slight risk is in play.
There is some signal that the axis from northeast TX into southern
MO could actually see the most intense rates and greatest flash
flood potential, with both the 00z CMC and AIFS highlighting this
corridor. However ingredients will also be quite favorable farther
north into MO and southern IA as well (pending exact frontal
positioning) and thus both of these areas could end up seeing
training/backbuilding at some point Monday and/or Monday night.
Some of these areas will also likely see heavy rain on day 2 as
well, and so hydrologic conditions could be more sensitive by this
day 3 time frame. Thus do consider just about all of the Slight
risk as a higher end Slight at this time, and think areas of flash
flooding are likely. It seems possible (maybe even probable) that
an embedded MDT risk will eventually be needed somewhere within
the broad Slight risk area. However uncertainty in both the
convective details and hydrologic conditions (pending what rainfall
happens on day 1 and 2) leads to low confidence in any MDT risk
location at this time. Thus we will hold with a higher end Slight
and continue to monitor trends.

Chenard


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt