FOUS30 KWBC 172032
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 PM 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF MICHIGAN...

...Central States/Midwest... 21Z Update...
Slight Risk introduced for the L.P. of Michigan.

A shortwave trough lifts north from northern Missouri through the 
L.P. of Michigan Wednesday. This is in advance of the longer wave 
trough axis shifting east from the central Plains Wednesday and in 
the right entrance region of a SWly upper jet streak lingering over
the northern Great Lakes through this time. These dynamics will 
allow the surface low pressure coincident with the shortwave to 
deepen through its track as it crosses northern Illinois and the 
L.P. of MI. This low will move along a baroclinic zone to the 
northeast, dragging its accompanying cold front east into the 
Midwest Wednesday afternoon and tracking over the Mid-South and
Ohio Valleys Wednesday night, leading to two main swaths of heavy rain.

The environment along the low track will support prolonged
convergence/ascent as intensifying 850mb inflow from the Gulf will
draw higher PWs (1.75 to 2" and with sufficient instability 
(MUCAPE around 1000 J/kg) northward. 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 within a modest TROWAL that will pivot northeast. The 
confidence on higher rain rates have increased with 12Z HREF 
neighborhood probabilities for 1"/hr rates now 40-60% over the 
central/lower L.P. through the afternoon (with 2-4" likely in the 
swath which warrants an introduction of a Slight Risk despite the 
typically higher FFGs in MI. Rates do ramp up over northern IL and 
especially over Chicago, so a Slight Risk could become warranted 
for the Chicago metro should trends continue to increase.

Farther south along the front, instability is more robust with 
MUCAPE progged to be 1000-2000 J/kg, promoting scattered convection
with embedded heavy rates particularly in the late
afternoon/evening over southern Missouri and Arkansas east into
Kentucky/Indiana. This area should also have ongoing activity from
tonight which will further precondition soils. Given the
progressive nature of the activity, the Marginal is maintained 
with expansion over OK/northeast TX/southern AR and central KY/TN.

...Mid-Atlantic into New England... 
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 
reach 2" Wednesday afternoon from the DC metro up through southern 
New England as a weak surface trough swings east over the Midwest.
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 
Marginal Risk is maintained/expanded a bit into central PA where 
morning activity should be occurring from overnight tonight. 

Weiss/Jackson


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

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR AN AREA FROM
THE MID-SOUTH THROUGH INTERIOR NEW ENGLAND...

A mid-level longwave trough axis will progress eastward from the
lower/mid-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 reach the Eastern Seaboard by 12Z 
Friday. The overlap of the front with its parent trough will drive 
another strong corridor of lift through height falls/convergence, 
into an environment supportive of convection with heavy rain. 
Despite the rapid progression of this front, the Marginal Risk is
expanded from TN through southern AR 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 12Z 
consensus including the experimental RRFS which is the first CAM to
go out through Day 3. While in general this pattern suggests only 
an isolated flash flood risk, there are rather vulnerable soils in 
the central Appalachians, so a targeted Slight could be needed in 
future updates, particularly as it gets more into the typical CAM range.

Weiss/Jackson

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