FOUS30 KWBC 170832
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Sat May 17 2025

Day 1
Valid 12Z 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...
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... 
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


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

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