Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1202 PM EDT Fri May 16 2025

Day 1
Valid 12Z Fri May 16 2025 - 12Z Sat May 17 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
OHIO AND TENNESSEE VALLEYS AS WELL AS WESTERN MAINE...


...Arklatex into the OH/TN Valleys...
An active morning of convection will continue across portions of
the TN and OH Valleys (generally western TN through KY and into
WV), with a secondary round of convection likely this evening,
maybe just a bit displaced south from the current activity. This
convection is blossoming within an impressive warm sector behind a
warm front and ahead of an approaching cold front. Forcing for
ascent is maximizing in this region due to an overlap of weak
height falls downstream of an impressive upper low positioned in
the Northern Plains, into which progressive shortwave impulses are
rotating E/NE, and the LFQ of a generally zonal jet streak is
impinging. The southerly flow ahead of the cold front will
gradually veer more to the SW with time, but still maintain
efficient moisture transport, surging PWs to above 1.75 inches on
850mb inflow that may reach 40-50 kts. This will lead to intense
moisture confluence into the approaching front, and as 0-6km mean
winds veer to become more boundary-parallel, this will likely setup
a situation to encourage WSW to ENE training of heavy rain rates.

The inherited MRGL and SLGT risks across this region have been
modified generally just for cosmetic purposes, but some subtle S/E
trends in the probabilities and axis of greatest instability drove
most of the changes. With MUCAPE likely reaching 2000-3000 J/kg
thanks to the presence of an EML, the resultant enhanced
thermodynamics will support rainfall rates of 1-2"/hr as progged by
the HREF, and short-term rates of 3"/hr are also possible as
suggested by the 15-min HRRR precipitation accumulations. Despite
storm motions that will remain generally quick at 30-40 kts (using
0-6km mean winds as proxy), shorter Corfidi vectors aligned to the
mean flow additionally supports the risk for short-term training or
multiple rounds of convection. Where rainfall repeats most
significantly, both the HREF and REFS probabilities exceed 60% for
3"/24 hrs, although FFG exceedance probabilities are far more
modest. This suggests that the inherited SLGT risk is warranted,
with subtle modifications to account for the newer guidance, but no
additional upgrade is needed. The MRGL risk was also extended a bit
SW into far NE TX to account for some potential modest
backbuilding/regenerative activity this evening.


...Mid-Atlantic/Northeast...
The impressive closed low moving from the Northern Plains into the
Upper Midwest will force downstream WAA and mid-level divergence
across the Mid-Atlantic and into the Northeast. This will
additionally result in scattered to widespread thunderstorms today,
with clusters of organized thunderstorms likely thanks to 20-40 kts
of bulk shear. Ascent will likely maximize late this aftn and
through the evening, especially across northern New England as a
modest mid-level impulse/vorticity maxima lifts northeast and
interacts with robust thermodynamics characterized by PWs of around
1.25 inches (above the 90th percentile according to the SPC
sounding climatology) overlapping MUCAPE as high as 2000 J/kg.
Locally backed 850mb flow angling more from the SE will provide
some upslope ascent as well, becoming an impetus for additional
convective development into the terrain of Maine, some of which
could become tied to features to slow overall motion until the
shortwave swings through this evening.

Convection that develops ahead of the shortwave will be slow moving
as reflected by Corfidi vectors that are just around 5-10 kts, and
both HREF and REFS probabilities indicate rain rates have a high
chance (>70%) of exceeding 1"/hr. Where storms move slowly, and
then are swept out by more organized thunderstorm activity
developing beneath the shortwave this evening, rainfall exceeding
3" is likely in some areas, especially western Maine. Although
soils are generally dry here as reflected by NASA SPoRT due to a
lack of recent rainfall, this still poses an increased flash flood
risk and the SLGT risk inherited was modified only cosmetically. To
the SW of this slight risk, and as far south as the Mid-Atlantic,
periodic clusters of convection, potentially as small mesoscale
convective systems (MCS) may generate and track rapidly eastward
across portions of the region. While these will likely contain
heavy rain rates exceeding 1"/hr, fast forward motions should limit
the flash flood risk even atop some areas that are more sensitive
due to recent rainfall, so the MRGL risk remains.


Weiss


Day 2
Valid 12Z Sat May 17 2025 - 12Z Sun May 18 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL TEXAS
THROUGH CENTRAL ALABAMA AS WELL AS FOR PORTIONS OF THE NORTHEAST...

...Texas to Alabama...
Convection will likely develop across portions of central TX
Saturday afternoon as increasing forcing overruns the dryline.
Enough mid level vort energy, upper jet support and robust
instability to support likely upscale growth of convection as it
moves into northeast TX. The extent of the flash flood risk will
likely come down to convective longevity at any one location as
cells should be pretty fast moving. This fast movement may limit
the areal coverage of flash flooding, however certainly an
opportunity for at least some training as convection grows upscale.
The 00z CAMS and AIFS suggest northeast TX towards the AR/LA
border has the best chance of seeing convective training
potentially leading to a flash flood threat. Overall still not
enough model support to suggest Slight risk coverage, but localized
flash flooding appears probable with this setup, especially over
more sensitive urban areas. Areal averaged rainfall may only
average around an inch, but would expect to see localized totals
around 3", much of which would fall in just a couple hour period.

Convection will likely be ongoing across the eastern portion of the
Marginal risk (MS/AL) at 12z Saturday, but likely in a decaying
state. However we could see at least isolated to scattered
redevelopment along the leftover outflow during the day Saturday,
and anything that does develop would likely be slow moving and
capable of resulting in a localized flash flood threat.

...Northeast...
A couple rounds of convection appear likely across the Northeast
on Saturday...one in the morning and another by afternoon/evening.
Both rounds are expected to feature quick moving cells with deep
layer mean flow over 30 kts. This will likely limit the extent of
any flash flood risk, however localized 1" in an hour amounts are
still probable given the moisture and instability forecast. These
higher rates combined with the potential for multiple convective
rounds supports an isolated flash flood threat and a continued
Marginal risk area.

Chenard

Day 3
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 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 across
portions of eastern KS into MO and/or AR near the aforementioned
stationary/warm front. Convection should first develop over central
KS with upscale growth into an MCS likely. Mean flow is off to the
northeast, however with a strong southwesterly low level jet,
Corfidi vectors are pointed more towards the southeast. Thus would
expect convection to turn easterly and then southeasterly as it
organizes Sunday evening. 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 uncertain, but think the 00z GFS is likely
too far to the north and east...with the aforementioned expected MCS
propagation taking it on a farther southern track. Thus tend to
think something closer to the 00z ECMWF and UKMET is more likely.
The experimental 00z RRFS (the first CAM to go out into day 3) also
seems to show a plausible evolution and placement over eastern KS
into western MO. The setup does have the potential for a swath of
over 5" of rainfall where training/backbuilding ends up being
maximized. Thus this is trending towards a higher end Slight risk,
with at least scattered flash flooding probable.

Another area of interest will be farther northwest into western NE
and southwest SD, 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 was not ready to go with a
large Slight risk area yet. But we will continue to monitor trends
and an expansion of the Slight risk may eventually be necessary.

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 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