980
FOUS30 KWBC 161950
QPFERD

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

Day 1
Valid 16Z 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NEW
ENGLAND...

Impressive mid-level closed low (NAEFS 500-700mb heights below the
1st percentile to start the forecast period) will lift northeast
from the Great Lakes to New England D2 while slowly filling.
Southeast of this feature, confluent flow will result in increased
westerly into the Southern Plains and Mid-Mississippi Valley, with
height falls and downstream divergence driving plentiful ascent into
New England. This broad synoptic ascent related to this large closed
low will produce periods of thunderstorms with heavy rain,
potentially resulting in areas of flash flooding on Saturday.

...Northeast...
Downstream to the east of the upper low, height falls and
increasing divergence will drive intense ascent on Saturday into
New England and other parts of the Northeast. Within this deep
layer ascent, mid-level vorticity maxima will periodically shed
from the low and rotate cyclonically over the Northeast. Where
these interact with the most impressive thermodynamics (PWs to 1.25
inches and MUCAPE ~500 J/kg), clusters of thunderstorms are
expected to develop with rain rates of 1-2"/hr. Generally fast
storm motions should limit the duration of any distinct heavy rain
producing cell, but still the HREF and REFS indicate a low end risk
(10-30%) for up to 3" of rain on Saturday. Considered a small SLGT
risk area from northern NH into western ME due to this rainfall on
Saturday falling atop soils that will likely be sensitive from
heavy rain on D1. There continues to be some uncertainty into how
effectively the D2 rainfall will overlap the footprint of heaviest
rain on D1, but after coordination with WFO GYX, have included a
targeted slight risk to account for the greatest potential of
high rainfall probabilities D2 with the greater rainfall potential
D1.

...Texas to Alabama...
Showers and thunderstorms may be ongoing Saturday morning across
MS/AL, but should wane quickly by morning. Thereafter, convection
is likely to rapidly blossom within a de-facto warm sector between
a dry line wavering across west Texas and a stationary front
draped west-to-east from OK to TN. As instability climbs during the
late morning (MUCAPE surging as high as 3000 J/kg) on return flow
out of the Gulf, the overlap of this CAPE combined with PWs
reaching towards 1.75 inches will result in an extremely favorable
thermodynamic environment for heavy rainfall producing convection.
The high-res CAMs are in good agreement that convection will become
widespread during the aftn/eve, and both HREF and REFS
probabilities are high (>70%) for 1"/hr rates, with modest
probabilities (20-30%) for 2"/hr rates. With the pronounced
thermodynamics in place, where any additional mesoscale forcing can
occur through convergence along the front or across any
differential heating boundaries that may develop, brief rain rates
of 3"/hr or more are possible.

This will result in scattered to widespread heavy rain producing
thunderstorms on Saturday, especially in an arc from the Big Bend of
Texas through the Red River Valley of the South, eastward into
central Alabama. Recent rainfall across the Southern Plains and into
the Lower MS VLY has been below normal, although points farther east
have seen more than 300% of normal rainfall the past 7 days. This is
reflected by wetter soils across MS/AL than points east which could
suggest a greater risk across the eastern portions of this arc.
However, thermodynamics are more impressive to the west, indicating
a higher risk for excessive rain rates, and 0-6km mean winds, used
as a proxy for storm speed, remains quite fast Saturday (20-30 kts).
This will somewhat limit the excessive rain risk, but bulk shear as
high as 50 kts will help organize storms into clusters, and where
any short-term training can occur isolated flash flooding could
occur which is reflected by the MRGL risk.


Weiss


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 MIDDLE MISSISSIPPI VALLEY...

...Southern Plains through Mid-Mississippi Valley...
A longwave trough will begin to amplify across the Rockies Sunday
morning, and then amplify into a large closed low over the West as
a secondary shortwave digging out of Canada merges with a shortwave
ejecting from the Four Corners. Downstream of this closed low,
modest height falls will spread into the Central Plains, while
divergence intensifies downstream into the Southern Plains and Mid-
Mississippi Valley. Through this evolution, an upper jet streak
will intensify and rotate cyclonically around the base of trough,
and where the favorably diffluent LFQ overlaps with the best height
falls, surface cyclogenesis is expected across the Central Plains.
To the east of this low, a warm/stationary front is likely to
extend to the east into the MS VLY and portions of the Southeast,
with a dry line likely pushing east across Texas during the aftn.

The primary focus for flash flooding potential should be from
eastern Kansas through northern AR along the stationary front. This
will be where convection is most likely to grow upscale from
Oklahoma on 40-50 kts of bulk shear into an MCS. Corfidi vectors
becoming increasingly angled to the right of the mean wind (ESE vs
NNW) suggesting an increased risk for at least short term training
as the entire cluster (or clusters) attempt to lift northeast the
latter half of D3. This is still quite a ways out into the future,
and as expected, SREF, GEFS, and ECENS probabilities still feature
considerable spread in both timing and orientation, leading to
lower than typical confidence. However, there is better agreement
among the ECMWF, AIFS, UKMET, and RRFS for this evolution, with the
GFS Graphcast AI also aligned SW of the GEFS. For this reason the
inherited SLGT risk was pulled just a bit far SW and extended west
as well to account for possible upstream training along OFBs into
more impressive thermodynamics over KS. Rainfall amounts of 3-5"
are probable within the heaviest convection D3, and at least
scattered flash flood instances are anticipated.

Additionally, 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.

...Central Plains through Northern High Plains...
A bit Northwest of the surface low and displaced from the greater
flash flood risk over the Mid-Mississippi Valley, a secondary
maxima of rainfall is forecast from near the Nebraska Panhandle
into western South Dakota and even eastern Montana. As the surface
low deepens, increasingly impressive theta-e advection will wrap
cyclonically into the system from the east, forming at least a
modest TROWAL into the region. This forces more intense moist
isentropic ascent, and into the TROWAL and its accompanying
elevated instability, streaks of heavy rainfall are likely.
Forecast soundings across this region suggested minimal CAPE with
temperatures only in the 40s/50s, so rain rates may struggle to
even reach 0.5"/hr. This should limit the excessive rainfall risk,
at least modestly, so while a SLGT risk was considered, at this
time opted to maintain the MRGL risk and see how instability can
evolve as the event gets closer.


...Northeast...
A Marginal risk continues over portions of northern New England on
Sunday in response to a third consecutive day of potential heavy
rain over New England beneath a slowly meandering upper low. While
the intensity and coverage on Sunday may be decreased from Friday
and Saturday, any rainfall that occurs on top of what will become
increasingly vulnerable soils could result in instances of flash
flooding and the inherited MRGL risk was modestly tailored
cosmetically.

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