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FOUS30 KWBC 170847
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
447 AM EDT Tue Jun 17 2025

Day 1
Valid 12Z Tue Jun 17 2025 - 12Z Wed Jun 18 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL APPALACHIANS AND THE CENTRAL PLAINS...

...Central Appalachians...
Extreme thermodynamics expanding across the eastern CONUS will
produce an environment favorable for widespread convection with
heavy rainfall on Tuesday.

The synoptic pattern responsible for this environment is set up by a
broad Bermuda-type ridge centered off the southeast Atlantic coast.
This ridge will expand northwestward, while a trough digs
concurrently across the Mississippi Valley. This evolution will
result in pinched flow between the two, accelerating the 850-500mb
flow northeast as return flow emerges more impressively from the
Gulf. Within this flow, multiple mid-level impulses/shortwaves
will lift progressively northeast, providing waves of enhanced
ascent from the Central Gulf Coast through the Mid-Atlantic states.
Additionally, the low-level flow surging northeast will reach
20-30 kts at 850mb, lifting isentropically atop a wavering front
anchored NW to SE across the Ohio Valley and Mid-Atlantic. This
will produce additionally enhanced ascent to increase convective
potential in that area.

The high-res CAMs are in good agreement in a ribbon of widespread
thunderstorms during peak aftn/eve heating from Louisiana into
Upstate NY, but organization is expected to be minimal as 0-6km bulk
shear is progged to just be around 20-25 kts, and storms should move
progressively to the northeast on 20-30 kts of mean wind. However,
any storm will likely contain impressive rain rates as PWs, which
are currently analyzed via the CIPS LPW percentiles to be above the
90th percentile from sfc-500mb, may approach 2 inches as far north
as PA and NY, which would be daily records at both OKX and PIT if
reached. This near-record PW and deep warm cloud depths of 13000-
14000 ft will support hourly rainfall that has a chance (20-40%) of
exceeding 2"/hr, with sub-hourly rain rates of 3-4"/hr possible.

Despite a lack of organization, repeating rounds of thunderstorms
with these intense rain rates could produce stripes of rainfall
exceeding 3" (50-60%) and locally 5" (20-30%) with the greatest
potential for the higher accumulations are suggested by both HREF
and REFS guidance occurring over PA invof the front.

With FFG extremely compromised in parts of the Mid-Atlantic, this
has prompted an upgrade to a MDT risk for excessive rainfall.
Elsewhere, a SLGT risk continues for portions of the Central
Appalachians, with a MRGL risk extending all the way to the Gulf
Coast where heavy rain is also expected, but in a less organized
fashion and atop drier antecedent soils with more filtration
capacity.

...Central Plains...
Generally flattened/zonal flow across the western CONUS will
amplify today as a potent shortwave dives out of the Rockies and
amplifies sharply into the Plains this evening. Impressive ascent
downstream of this shortwave will impinge into a cold front draped
across the Plains, resulting in surface low pressure development
this evening as well. This will enhance ascent locally, especially
as the LLJ ramps up to 40-50 kts, and is locally additionally
enhanced downstream of the developing low pressure, especially in
KS/OK. This LLJ will draw impressive thermodynamics northward
reflected by PWs rising above 1.75 inches collocated with MUCAPE
potentially exceeding 4000 J/kg. Where this LLJ converges both
along its nose and into the wavering surface front, convection is
expected to develop this evening, and then grow upscale and
intensify tonight.

In the presence of these impressive thermodynamics, both REFS and
HREF hourly rainfall probabilities have increased to above 40% for
2"/hr rates, and it is likely maximum sub-hourly rainfall rates will
be at least double that. This is in response to increasing shear and
the upscale growth of thunderstorms into clusters or an MCS, to
provide locally enhanced lift through convergence/mergers. The high-
res CAMs are in generally good agreement in this evolution after 00Z
tonight, although some latitudinal spread continues, likely in
response to an MCS presently moving across NE/KS, which will
somewhat impact the evolution 24-hrs from now in response to
residual boundary placement. The greatest potential for the heaviest
rainfall tonight is forecast to be across eastern KS and into
northeast OK where both the REFS and HREF probabilities indicate a
40-70% and 20-40% chance of 3" and 5", respectively. This will occur
atop pre-conditioned soils from antecedent rainfall, leading to FFG
exceedance probabilities above 50%. Despite some uncertainty in
timing and placement, the excessive rainfall risk has been upgraded
to MDT for portions of KS after coordination with the affected
WFOs.

Weiss

Day 2
Valid 12Z Wed Jun 18 2025 - 12Z Thu Jun 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE MID-
SOUTH THROUGH THE GREAT LAKES, AND FOR PORTIONS OF THE COASTAL
MID-ATLANTIC STATES...

...Central States/Midwest...
A shortwave lifting northeast embedded within a larger trough axis
centered over the Mississippi Valley will combine with the RRQ of a
departing upper jet streak to deepen a surface low pressure
emerging out of Missouri early Wednesday. This low will track
steadily northeast, reaching Michigan Wednesday evening and then
departing into Ontario by Thursday morning. This low will move
along a baroclinic zone to the northeast, dragging its accompanying
cold front to the east and into the Midwest by the end of D2,
leading to enhanced convergence/ascent across that region as well.

The associated lift will surge into an environment which will
support scattered to widespread convection with heavy rainfall.
Intensifying 850mb inflow from the Gulf will draw higher PWs
(nearing 2") and instability (MUCAPE above 2000 J/kg) northward to
support heavy rainfall. There may be two areas of heaviest rainfall
across this region on Wednesday, one in a strengthening deformation
axis NW of the surface low track, and a second in a corridor of
convection which develops along the front.

Within the deformation, the 850mb winds will lift a theta-e ridge
northward and cyclonically around the low pressure, producing an
axis of moderate to heavy rainfall which will pivot northeast.
Instability within this deformation will be modest, and elevated
within a modest TROWAL, so rain rates will be somewhat restricted
as reflected by 1"/hr rain rate probabilities from the HREF
(through 00Z Thursday) that peak around 20-40%. However, rainfall
will be persistent as cells track along the axis, leading to a
prolonged period of moderate rain with embedded higher rain rates.
This will produce an axis with GEFS, ECENS, and SREF probabilities
indicating a 5-15% chance for 3" of rain, which may result in
localized excessive rainfall impacts.

Farther south along the front, instability is much more robust and
this is reflected by rainfall rate probabilities that reach 60% for
1"/hr and 20-30% for 2"/hr. However, coverage of precipitation will
be more scattered as thunderstorms develop along the front and
organize into clusters moving rapidly to the northeast along the
front itself. This could result in some training/repeating, but the
progressive nature of individual cells should limit the overall
flash flood risk. Rainfall across this region could again exceed 3"
in a few areas as noted by similarly low probabilities of 3"/24hrs,
but the more intense rates still support a MRGL risk of excessive
rainfall.


...Coastal Mid-Atlantic...
Continued return flow around a Bermuda-type ridge off the Southeast
coast will maintain broad southwesterly flow to pump elevated PWs
northward into the Mid-Atlantic on Wednesday. PWs above 1.75 inches
(above the 97th percentile according to NAEFS) will remain over the
region, as a weak surface trough swings eastward, moving offshore
Wednesday night. This weak convergent axis moving through the area
concurrent with an embedded mid-level vorticity maxima into these
robust thermodynamics will result in thunderstorms with rain
rates of 1-2"/hr. Despite the relatively isolated to scattered
nature of convection expected, any of these torrential rain rates
atop the more urban areas could cause isolated instances of flash
flooding, and the MRGL risk was adjusted accordingly.


Weiss


Day 3
Valid 12Z Thu Jun 19 2025 - 12Z Fri Jun 20 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN
APPALACHIANS THROUGH NEW ENGLAND...

A mid-level longwave trough axis will progress eastward from the
Mississippi Valley to the Atlantic coast on Thursday. This trough
will push an accompanying surface cold front beneath it, and this
front is progged to move off the coast by 12Z Friday /end of D3/.
The overlap of the front with its parent trough will drive a narrow
but strong corridor of lift through height falls/convergence, into
an environment supportive of convection with heavy rain. Despite
the rapid progression of this front, a MRGL risk for excessive
rainfall exists due to potential short-duration training of
thunderstorms due to mean 0-6km winds that are parallel to the
boundary. With rain rates likely eclipsing 1"/hr at times, this
should produce stripes of 1-2" of rainfall, with locally as much as
3" in a few locations as suggested by GEFS and SREF 3"/24hr
probabilities. While in general this suggests only an isolated
flash flood risk, and the updated MRGL risk, should any heavier
rain fall atop the more vulnerable soils of the central
Appalachians, a targeted SLGT could be needed in future updates.

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