146
FOUS30 KWBC 162026
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Mon Jun 16 2025

Day 1
Valid 16Z Mon Jun 16 2025 - 12Z Tue Jun 17 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO VALLEY AND MID-ATLANTIC STATES...

16Z Update...

...Far South Texas...
Separated Marginal Risk on TX coast to cut out the Middle Texas
Coast and include more of the lower Rio Grande Valley. Ongoing
activity south of Corpus Christi (detailed in MPD 457 which will be
updated if the threat is notable for the lower RGV) is pivoting on
itself as it shifts SSW. A downward trend is noted in the past
hour. However, inflow and extreme moisture with PW of 2.3" over
the Lower TX Coast would allow rainfall rates to locally exceed
2"/hr.

...Ohio Valley through the Mid-Atlantic...
16Z Update...
Slight Risks are on track. Was able to trim the northern section
of the Slight Risk near Pittsburgh given 12Z HREF consensus.

09Z Discussion...
Broad warm sector south of a wavering front draped from the
Central Plains through the Ohio Valley and into the Carolinas will
fuel widespread thunderstorm activity today. As this front wavers
across the region through today and tonight, shortwave impulses
embedded within broad SW flow around a Bermuda- type ridge will
lift across the area. These impulses, especially where they move
atop the residual baroclinic zone accompanying the front, will
serve as a focus for convective development leading to rounds of
thunderstorms. Although, in general, shear will be modest
suggesting primarily disorganized pulse-type thunderstorms which
are also suggested by simulated reflectivity within CAMs, there may
be two areas of focused thunderstorm development leading to a
higher potential of excessive rainfall.

The first will be in a stripe from the Bootheel of Missouri east
into southern Ohio. Here, the front will linger through today while
a potent vorticity maxima rotates overhead. The interaction of these
two may cause low-pressure development to locally enhance ascent.
Thermodynamics across this region will be impressive, characterized
by MUCAPE of 1000-2000 J/kg coincident with a plume of PWs exceeding
2 inches, more than 2 sigma above the climatological mean.
Additionally, some modestly enhanced shear approaching 35 kts will
lift northward to help organize thunderstorms, enhancing both the
intensity and longevity of heavy rainfall. With rates progged by
both the HREF and REFS likely exceeding 1"/hr (50-70% chance), and
an increasing likelihood of at least short-term training as Corfidi
vectors weaken to just 5 kts, this has the potential to produce
locally 3-5 inches of rain which has a 30-50% chance of exceeding
the 3-hr FFG.

The other locally enhanced threat for excessive rainfall will again
be along this wavering front, but farther east from SW Pennsylvania
through North Carolina. Here, a stalled backdoor front will
provide the impetus for thunderstorm development, especially during
the aftn/eve when thermodynamics are most impressive. The high-res
CAMs suggest the best threat for any organization will be across
VA/NC this evening, likely in response to a convectively enhanced
shortwave moving eastward and interacting with the front,
potentially leading to an MCS or multiple clusters of thunderstorms
moving across this area. With rainfall rates likely exceeding
2"/hr and just 5 kt Corfidi vectors aligned anti-parallel to the
mean westerly flow, training of cells is expected which results in
HREF and REFS probabilities for 3" of rain exceeding 70%. This
could produce excessive rainfall despite relatively higher FFG,
especially in NC where MRMS 24-hr rainfall has been locally as much
as 6 inches, creating more vulnerable soils. There was some
coordination tonight with WFO MHX/RAH to discuss a possible
upgrade to a MDT risk, but modest HREF/REFS ensembles and
uncertainty in placement of the afternoon convection (which could
be displaced south of ongoing activity due to convective adjustment
of the front itself) prompted a delay in any upgrade at this time.
However, an adjustment to the SLGT was made to focus the highest
risk over the HREF 3" EAS probabilities and low-percentage 100-yr
RI exceedances.

Farther upstream across VA and back as far as PA, recent heavy
rain has lowered FFG to just 1-2"/3hrs, so even less organized
thunderstorms over these more sensitive soils will promote a flash
flood risk.

Displaced to the south from these more focused excessive rainfall
risk regions, rounds of thunderstorms developing within robust
thermodynamics, especially during peak aftn/eve heating, will pose
at least an isolated flash flood risk. This will be due to rainfall
rates which have a 30-50% chance of exceeding 2"/hr, and could be
slow moving to produce locally more than 3 inches of rain. Both REFS
and HREF probabilities of exceedance are scattered across the
region, suggesting the inherited MRGL risk is sufficient, and this
was just cosmetically adjusted for new guidance.


...Dakotas to the western Great Lakes...
16Z Update...
Adjustments to Marginal Risks with expansion in central WI given
ongoing and upstream activity, and expansion over northeast WY and
south-central KS given 12Z CAM consensus. Activity should be
progressive enough in the stronger flow east of the trough axis
over the northern Rockies to preclude targeted Slight Risks.

09Z Discussion...
A messy mid-level pattern persists over the northern tier of the
CONUS as dual troughs, one over the Pacific Coast and another
lifting over the Great Lakes leaves pinched but zonal flow from the
Rockies into the Upper Midwest. However, this pattern will evolve
today as the primary trough digs into the Great Basin and sheds
lobes of vorticity eastward within the progressive flow. Any of
these impulses could become the impetus for thunderstorms,
especially this evening and tonight as a cold front digs from the
Northern Rockies into the Upper Midwest. The CAMs feature a variety
of different MCS solutions tonight as the LLJ ramps up to 30-40
kts over KS/OK, surging elevated PWs above 1.75 inches and
increasing MUCAPE northward. Where this most efficiently impinges
into the boundary, resultant convection will likely blossom through
the convergence/isentropic ascent, and where this occurs beneath a
vorticity maxima, MCS development will likely be the result.

With broad SW flow aloft helping to increasing column moisture from
MT all the way into the Great Lakes, any of these clusters of
thunderstorms which develop could produce an excessive rain risk.
However, there is limited spatial agreement among the deterministic
models as to which area will have the greatest focus. Additionally,
mean 0-6km winds of 20-30 kts suggest storms will be fast moving,
somewhat limiting the flash flood risk despite rain rates eclipsing
1"/hr. For this reason, no upgrade from the MRGL risk is warranted
with this update. However, a subtle locally higher risk may exist
across NE/KS/IA tonight where the environment appears to support at
least a subtly higher training potential due to Corfidi vectors
angled more right of the mean wind and possibly regeneration
occurring along the western tail of the front into more impressive
thermodynamics. However, exceedance probabilities remain modest, so
for now the MRGL risk remains but a targeted upgrade may be needed
with later issuances.

Weiss/Jackson


Day 2
Valid 12Z Tue Jun 17 2025 - 12Z Wed Jun 18 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL APPALACHIANS AND THE CENTRAL PLAINS/MIDWEST...

...Central Appalachians to the Central Gulf Coast...
Broad SW mid-level flow downstream of an amplifying trough will
produce another day of active convection across the east-central
CONUS. As the SW flow amplifies, convectively charged shortwaves
embedded within the amplifying flow will surge northeast,
interacting with a residual front to produce a wave of low pressure
lifting into the Great Lakes Tuesday evening. Downstream of this
low, the front is likely to lift north as a warm front, aided by
return low-level flow around a ridge off the Southeast coast.
Together, these features will produce favorable thermodynamics and
kinematics for heavy rain producing convection.

Deep SW flow through much of the column from the Gulf Coast into
the northern Mid-Atlantic states will produce an environment
favorable to heavy rain producing thunderstorms from Louisiana to
Pennsylvania. Moisture will be high PW anomalies that reach above
+2 sigma across this swath. Widespread convective development is
supported in this environment 1"/hr rainfall likely. 12Z guidance
(and now the 18Z HRRR) has keyed in on central Pennsylvania as
having the greatest flash flood potential with a continued heavy
rain signal down to eastern KY. This is where the warm front will
focus more organized convection thanks to 35-45 kts of bulk shear,
which when combined with Corfidi vectors collapsing to 5-10 kts,
will result in a subtly enhanced repeating/training risk. This is
also where FFG is most compromised due to recent heavy rain events
(and more expected into this evening) which led to further
expansion of the Slight Risk, particularly in central Pennsylvania
where this can be considered a higher end Slight Risk. There is
potential for a targeted Moderate Risk to be drawn when the
confidence is there.

...Central Plains and Midwest...
12Z consensus continues SW trend in the 24hr QPF max with central
Kansas favored in the HREF mean. The HRRR, including now the 18Z
HRRR is more favoring south-central Kansas. This is from both
overnight activity tonight and the main event Tuesday night ahead
of the wave pushing onto the central Plains. The Slight Risk is
expanded across central Kansas to the Oklahoma border, as well as
farther north in Nebraska where the comma head will set up ahead of
the developing low. The threat of repeating heavy rain over
portions of Kansas warrants the Slight Risk to be considered higher
end with the potential for a moderate risk in subsequent issuances.


Weiss/Jackson


Day 3
Valid 12Z Wed Jun 18 2025 - 12Z Thu Jun 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST, MID-SOUTH, GREAT LAKES, AND NORTHEAST URBAN CORRIDOR...

...Central States/Midwest...
A potent shortwave trough shifts northeast from Kansas to Michigan
on Wednesday. This feature will remain amplified through its
lifespan, interacting with a cold front across the Plains to drive
surface low pressure from northern Missouri Wednesday morning to
Lake Huron by the evening and enhance ascent through the track.
This lift will work with favorable thermodynamics for heavy
rainfall, with 1.75 to 2" PW (+2 standard deviations above normal)
and a ribbon of MUCAPE of 2000-3000 J/kg. This will support
widespread showers and thunderstorms, with hourly rainfall
exceeding 1 inch at times as storms move along the front. Storm
motions are likely to remain progressive much of Wednesday as 0-6km
mean winds are forecast to reach 30 kts, but repeating rounds are
possible as these winds track parallel to the front. This will
support stripes of heavy rainfall both with the comma head of the
upper low over Iowa/Wisconsin/Michigan, and in the warm sector over
Arkansas, southern Illinois, Indiana into Ohio. The Marginal Risk
was adjusted to depict a dry slot between these features.

...Northeast...
Broad southwesterly flow east of a trough axis over the Midwest
will pump Gulf-sourced moisture up the Eastern Seaboard. Global
guidance continues to depict a 2" contour of PW from northern
Virginia through southern New England which is around 2.5 standard
deviations above normal. Strong flow should maintain movement to
activity, but isolated heavy thunderstorms are forecast, so a
Marginal Risk is raised for the Bos-Wash megalopolis.


Weiss/Jackson


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