508
FOUS30 KWBC 230048
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
848 PM EDT Thu May 22 2025

Day 1
Valid 01Z Fri May 23 2025 - 12Z Fri May 23 2025

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

01Z Update...

Southern Plains...
Expanded the Marginal Risk area in the Southern Plains and shifted
the area southward now that the atmosphere farther north has been
worked over by afternoon convection and storms continued to feed on
instability over Texas. HREF neighborhood probabilities from 18Z
showed non-zero but sub-5 percent probabilities of rainfall
exceeding both 1-hr and 3-hr flash flood guidance with embedded
pockets of 5 to 10 percent risk probabilities into late
evening/early morning hours.

New England...
Strong moisture transport into the eastern part of New England will
persist as low pressure continues to hug the coast on its
northeastward trek. Rates continue to be quite modest but there
could still be isolated problems with excessive rainfall given the
duration of rainfall. No changes were made to the area from the
previous outlook/discussion.

Florida...
Removed the area given the rapidly warming cloud tops and weakening
of echoes on radar. However...some showers may linger along the
southeast Florida coast for a few hours.

Bann

...Previous Excessive Rainfall Discussion...

...Southeastern New England...
Late season Nor`easter will strengthen and continue to move up the
coast just east of Massachusetts today, spreading significant
moisture onshore New England and eastern Long Island. Although
instability will be modest, generally less than 500 J/kg, and
highest right along the immediate coast, tremendous moisture flux
driven via 850mb U-wind below the 1st percentile (so easterly)
according to NAEFS will manifest as continuous moderate to heavy
rainfall today. Rain rates from the HREF are progged to generally
be around 0.5"/hr, although low probabilities (10-20%) for 1"/hr
exist across Southeast MA and the Cape/Islands. 0-6km mean winds of
10-20 kts will become increasingly anti-parallel to the Corfidi
vectors, and this suggests training of echoes will result in
repeating rounds of rainfall. Despite the overall modest rates,
this will cause as much as 3-5" of rain, especially across Rhode
Island, the Cape, and inside 495 around Boston. This could cause
localized flash flooding, especially in urban areas, and the MRGL
risk has been maintained with minimal changes.


...Southeast Florida...
Thunderstorms will blossom along the tail of a cold front that will
settle across the Florida Peninsula this aftn as it gets shunted
southward in response to an amplifying trough over the East. This
will impinge into a pool of anomalous moisture (PWs +1.5 sigma)
with SBCAPE above 2000 J/kg fueling thermodynamics that will
support heavy rain rates within thunderstorms. As convection
develops along the cold front this aftn, storms will track east on
0-6km mean winds of around 5-10 kts, so generally slow moving,
with aligned Corfidi vectors (and aligned to the front itself)
suggesting training/backbuilding cells across portions of South
Florida. The greatest risk for heavy rain will be near the Gold
Coast where the eastward advancing cells will interact with the
westward moving sea breeze to cause at least periods of nearly
stationary motion and redevelopment, enhancing both the rates to
above 3"/hr and duration of these rates. This will result in more
than 3" of rain in some areas as suggested by HREF probabilities
exceeding 90%, and locally as much as 5" of rain is possible. This
portion of Florida has been quite dry so FFG is elevated, but where
any storms stall over urban areas, instances of flash flooding are
possible.


...Southern Oklahoma/North Texas...
A trough amplifying over the Great Lakes/Northeast will yield
lowering heights over the Central Plains, with additional ascent
being forced through modest jet streak development and spokes of
energy via vort maxes rotating SE through the mean flow. At the
surface, this will leave a wavering boundary in place in the
vicinity of the Red River Valley of the South, which will serve as
a focus for convective development this aftn/eve, aided by
increasing return flow from the Gulf pushing PWs up to 1.5" and
forcing pronounced convergence of moisture transport vectors into
the boundary. The CAMs have become more robust with activity later
today, but there still exists considerable temporal and latitudinal
spread with development. What is more certain, however, is that
convection should grow upscale into clusters, especially this
evening, when moisture flux maximizes in the presence of 40-50 kts
of bulk shear. Storms that develop will feature rainfall rates for
which the HREF suggests have a 50% chance of exceeding 1"/hr, which
could produce 2-3" of rainfall where any short-term training can
occur. While this event does not appear to be exceptionally
significant, the highest probabilities for heavy rainfall
effectively overlap relatively wetter soils via NASA SPoRT,
suggesting at least isolated flash flooding instances are possible.
The inherited MRGL risk was expanded just slightly to account for
the continued spread in model guidance.


Weiss


Day 2
Valid 12Z Fri May 23 2025 - 12Z Sat May 24 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN
KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS...

...Central Plains into the Ozarks...
Guidance continues to be confident in that development of a multi-
day heavy rainfall event which will commence Friday night.

The synoptic pattern becomes increasingly supportive of a heavy-
rainfall event as a closed low over the Northeast elongates NW to
SE, leaving NW flow across the Central Plains and into the
Southeast. Within this flow, a jet streak is progged to intensify,
leaving the favorably diffluent RRQ overhead the region, with weak
impulses embedded within the mid-level flow also producing ascent.
Although it is challenging to time/place these impulses even into
D2, which will limit confidence at this range, the overlap of the
diffluent jet portion with these impulses atop a warm front
lifting slowly northeast will create an environment that steadily
becomes more supportive of heavy rainfall. Friday night, as the LLJ
ramps up to 30-50 kts at 850mb and impinges into the front,
increasing isentropic ascent and moisture convergence will drive
the development of convection. Storms that fire will likely
produce impressive rain rates which the HREF suggests has a 70%
chance and 20% chance of exceeding 1"/hr and 2"/hr, respectively.
These rain rates will be supported by thermodynamic advection on
the LLJ, reflected by PWs surging above 1.75 inches and MUCAPE
rising towards 3000 J/kg.

As storms develop later Friday, they will intensify and organize
through pronounced bulk shear of 35-50 kts, indicating the
potential for clusters, supercells, and even an MCS. This
organization could at least briefly enhance rain rates even beyond
2"/hr, and with mean flow aligned to the warm front, with Corfidi
vectors becoming increasingly angled right into the moisture surge,
training of echoes from NW to SE appears likely. This has resulted
in an uptick in QPF probabilities, and the subtly modified SLGT
risk encompasses the area of greatest chance for 3+" of rain from
the HREF, ECENS, and GEFS clusters.


...Southeast Florida...
Another day of aftn showers and thunderstorms is expected to
develop across the Peninsula and track eastward through the
evening. The cold front leftover from D1 should wane, but any
residual low-level convergence will help focus thunderstorms, with
the persistent favorable thermodynamics supporting rainfall rates
of 2-3"/hr. Mean winds will be light on Friday, and as storms
approach the westward advancing sea breeze near the Gold Coast,
another day of convection stalling or moving erratically across the
urban corridor is expected. With heavy rain forecast during D1,
the footprint of this rainfall may overlap the same areas on D2,
and because of this a MRGL risk has been added.


Weiss


Day 3
Valid 12Z Sat May 24 2025 - 12Z Sun May 25 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN
KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS...

Day 2 of what is becoming more certain to be a 3-day rainfall event
occurs Saturday across portions of the Central Plains and into the
Ozarks and Mid-Mississippi River Valley. A warm front wavering
near the OK/KS/MO/AR intersection will continue to be a focus for
convergence and ascent, while broad W/NW persists aloft between an
elongated trough over New England and a secondary trough over the
Great Basin. This fairly static flow will repeatedly shed weak
impulses atop the low-level front, providing additional forcing for
ascent through Saturday.

This forcing will occur within persistently favorable
thermodynamics, as return flow from the Gulf maintains elevated PWs
as high as 1.75 inches, with overlapping MUCAPE of 1000-2000 J/kg.
This will support efficient rainfall rates within convection, which
is likely to blossom each aftn/eve and then grow upscale in
clusters or an MCS training WNW to ESE across the region. While
there still remains some uncertainty into the latitudinal placement
of the heaviest rainfall, there is likely to be at least partial
overlap of the heavy rain footprint from D2 into D3. This is
reflected by NASA SPoRT 40cm soil moisture anomaly forecasts that
skyrocket to above the 98th% on D3, so soils will become much more
vulnerable to runoff/flash flooding D3. Considered an upgrade to a
MDT risk, but there remains just enough spread in the guidance,
including the AI models, to keep excessive rain probabilities below
40%, so a high-end SLGT risk remains at this time. It is possible a
MDT risk will be needed for some areas D3 with future updates,
however, helping to bridge the gap into the new D4 MDT risk issued
for these same areas on Sunday.


Weiss

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