394
FOUS30 KWBC 171558
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1158 AM EDT Tue Jun 17 2025

Day 1
Valid 16Z Tue Jun 17 2025 - 12Z Wed Jun 18 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL APPALACHIANS AND THE SOUTH-CENTRAL GREAT PLAINS...

...Central Appalachians to the Central Gulf Coast...
Extreme thermodynamics expanding across the eastern CONUS will
produce an environment favorable for widespread convection with
heavy rainfall today and tonight.

A broad Bermuda-type ridge centered off the southeast Atlantic
coast will expand northwestward, while a shortwave trough persists
over the Mississippi/Ohio Valleys. This will accelerate the SWly
850-500mb flow which contains multiple mid-level impulses that
provide waves of enhanced ascent from the Central Gulf Coast
through the Mid-Atlantic states.

12Z CAMs remain in good agreement on diurnal expansion of
thunderstorms in a swath from Louisiana into Upstate NY, though
poorly organized with bulk shear 20-25 kts and progressive to the
northeast on 20-30 kts of mean layer wind. However, any storm will
likely contain impressive rain rates in deep Gulf-sourced moisture
with PWs up around 2" through this swath. Daily record high PW is
possible at both OKX and PIT. This high PW and deep warm cloud
depths of 13000-14000 ft will support high hourly rainfall,
particularly around south-central PA that remains a favored area
for QPF max today where a warm front is lifting.

12Z HREF neighborhood probs have 20-30% risk of of exceeding
2"/hr in south-central PA where a Moderate Risk persists. Sub-
hourly rain rates of 3-4"/hr are possible in this environment.

FFG is extremely compromised in parts of the central Appalachians
with a Slight Risk remaining for eastern KY into northern PA. The
Marginal Risk still extending all the way to the central Gulf
Coast where isolated flash flooding is ongoing.


...South-Central Plains into the Midwest...
16Z Update...
Shrank north side of Slight Risk to focus on organized convective
activity over KS (where a Moderate Risk persists) with expansion
downstream in OK and western MO.

A potent shortwave shifts across the central Rockies this
afternoon and amplifies sharply over the Plains this evening. This
should enable vigorous terrain induced convection off the Front
Range into WY with some organization as it shift east this evening.

This will be after the ongoing activity centered on KS which is
after a powerful MCS blew through last night. 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. The 12Z CAM consensus is for southern KS to be
the MCS focus this evening with repeating heavy rain where earlier
activity fell, raising the threat level.


Weiss/Jackson

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