208
FOUS30 KWBC 190744
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
344 AM EDT Thu Jun 19 2025

Day 1
Valid 12Z Thu Jun 19 2025 - 12Z Fri Jun 20 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL APPALACHIANS...

...Central Appalachians...

Cold front progression from the Ohio Valley through the Eastern
CONUS will lead to scattered instances of heavy convection along
and just ahead of the front during its evolution. As of now, the
highlight will be a steady forward motion of cells given the mean
field and general speed shear located within the warm sector of the
synoptically driven ascent field. Thunderstorms from eastern OH
down through the Tennessee Valley and points east will be capable
of 1-2"/hr rates within any convective development thanks to
relevant buoyancy out ahead of the surface front and deep moist
plume still running between +1 to +2 deviations above normal from
the Southeast CONUS up through New England. The greatest area of
concern remains the interior Mid Atlantic positioned between WV up
through far eastern OH and Western PA where remnant soils have been
significantly compromised over a succession of 5 straight
convective impact days. Area FFG`s for 1hr indices are down as low
as 0.5/hr for exceedance with some spots even lower within the
confines of the Central Appalachians in Northern WV and the
northern Laurels in PA. The prospects for considerable rates and
training potential are lower than previous days due to the more
progressive nature of precip, but the environment is very capable
of producing rates sufficient for localized flooding in those areas
that have been hardest hit. In coordination with the local WFO`s
across the interior Mid Atlantic, a targeted SLGT risk was added
for the areas with the lowest FFG`s and more susceptible flash
flood capabilities.

...Mid Atlantic and Northeast...

Across the Mid Atlantic and Northeast, the front will produce some
heavy thunderstorms capable of flash flooding within the urban
corridor extending from Richmond up through NYC, however the
progressive nature of the storms should curb the worst impacts and
relegate to a more isolated threat with the urban centers the most
at risk within the nature of the setup. A quick 1-2" with locally
higher is forecast over the above I-95 megalopolis corridor, but
the threat is not anticipated to be widespread with areal average
QPF closer to ~0.5" in most areas with stripes of heavier QPF noted
in the 00z HREF blended mean output. This entire area up through
parts of New England remain firmly in a MRGL with a small chance of
upgrade over parts of the Mid Atlantic if morning CAMs indicate
more aggressive signals.

...Southeast...

Another heavy rain scenario is depicted over parts of the Lower
Mississippi into the Southeast U.S. where convection developing
overnight into a MCS will lead to heavy rain pushing south through
parts of AR/MS/AL tomorrow morning with an flanking line
potentially inducing some backbuilding/training scenarios over
Southern AR. The expected magnitude of the precip should thwart
significant flash flood concerns within the primary axis of the
complex, but a few areas could certainly be impacted enough to
produce localized flash flooding. Over the flanking side of the
complex back west, that area will need to be monitored closely due
to the backbuilding potential and slower storm motions as mean flow
is much weaker and more susceptible to this potential. A MRGL risk
is in place over the Southeast to highlight the threat.

Kleebauer


Day 2
Valid 12Z Fri Jun 20 2025 - 12Z Sat Jun 21 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE UPPER
MIDWEST AND PORTIONS OF THE NORTHERN ROCKIES...

...Upper Midwest...

Amplified mid and upper pattern across the Western CONUS into the
Northern Plains will lead to a stout area of low pressure
developing across the Western High Plains, moving east through the
Upper Midwest as we move through Friday into Saturday morning.
South of the area will see a strong ridge pattern unfold creating
an enhanced thermal gradient pattern with a budding warm sector and
ample surface based instability with most guidance inferring
3000-5000 J/kg of SBCAPE across the area between the eastern
Dakotas over into the Upper Midwest by Friday evening. Split jet
max ejecting northeast out of the Plains will lead to increasing
ascent within the diffluent area of the jet and along the eastern
flank of the intensifying surface low. Thunderstorms will blossom
across the Eastern Dakotas and quickly advance eastward with
upscale growth thanks to the speed shear presence and strong
thermal gradient to maintain general MCS structure. Inflow around
the eastern flank of the low will aid in the overall maintenance
of the expected complex with smaller convective cells forming out
ahead and being integrated into the complex as it maneuvers through
MN. Areal QPF within the latest blend and ensemble bias corrected
forecast indicates a general 0.5-1" coverage with maxima likely to
locally exceed 2" over a short period of time within a zone
extending through Central MN into the Arrowhead. Will monitor for
backbuilding within the flanking portion of any complex which could
lead to local enhancement of precip and enhance flash flood
prospects in pockets where the MCS crosses. For now, the
progression of the MCS would favor limited training potential and
more impacts based on enhanced rates given the favorable upper
level dynamics, instability, and rich moisture pull within the
eastern side of the surface low. The previous MRGL risk was
maintained with only some minor adjustments based on QPF
distribution this forecast.

...Northern Rockies...

Guidance remains pretty locked in for the anticipated evolution of
the next disturbance entering the Pacific Northwest with convective
impacts forecast for the Northern Rockies and adjacent areas to the
east. Extremely anomalous mid- level low will dig into the Pacific
Northwest Friday, pushing 500mb heights to below the 00Z minimum
of the climatological database from NAEFS. Where intense height
falls/PVA combine with the increasing diffluence along the LFQ of a
Pacific jet streak, surface pressure falls will result, leading to
a deepening low pressure moving from ID into MT. The increasingly
backed low-level flow ahead of this system will cause 850mb winds
to surge out of the E at 15-25 kts, lifting favorably to upslope
into the terrain while additionally transporting a ribbon of PWs
above 1 inch into the High Plains and banking into the eastern side
of the Rockies. This will cause expanding precipitation, with
embedded rainfall rates above 0.5"/hr where a narrow corridor of
instability exists over northern MT. Locally heavy rainfall of
1-2", with locally higher amounts, will result, leading to the
potential for isolated flash flooding atop FFG that is only
1-1.5"/3hrs. The previous MRGL risk was maintained given the
expected evolution with general run to run continuity among
deterministic and global ensemble means.

Weiss/Kleebauer


Day 3
Valid 12Z Sat Jun 21 2025 - 12Z Sun Jun 22 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN GREAT LAKES, INTERIOR NORTHEAST, AND NORTHERN MONTANA...

...Northern Great Lakes into the Northeast...

Shortwave progression out of the Upper Midwest will ride along the
northern periphery of our expansive ridge as we move through
Saturday. Ensemble consensus pins the energy over the Northern
Great Lakes with remnant thunderstorms propagating through the
Michigan U.P. into adjacent Ontario with a break in convective
activity in the CONUS before the disturbance re-emerges over
Upstate NY and Northern New England. Some discrepancy on the
northern extent of the shortwave progression as the ECMWF bias is
further north with sights on Western ME and far northern VT/NH as
the impact points. Other guidance is suggesting a bit further south
into Upstate NY through Central New England with heavier precip
threats more likely into the central and northern Hudson Valley.
This is a period to monitor closely as flash flood impacts will be
tied to the progression of a fairly robust complex as it exits out
of Canada. QPF distribution is skewed towards heavier precip
located within North Country of NY state with some secondary maxima
over Northern New England down through the Adirondacks. Heavy rain
over the U.P. may be progressive enough to curb significant
impacts, but the eastern portion of the U.P. is the area of concern
with the vacating complex as it moves quickly through the region.
The MRGL risk inherited was adjusted slightly to account for the
uncertainty in the exact progression of the complex.

...Northern Montana...

Our potent mid-upper trough will meander over the Pacific Northwest
with a strong shortwave migrating out of the base of the trough,
eventually closing off right along the International Border to the
northeast of Glacier National. Surface low development in-of the
closed upper reflection should provide a narrow axis of convergence
just outside the eastern slopes of the Rockies generating a period
of heavy rainfall in the adjacent valleys to the east of the
mountains. Height falls across the region will be significant
creating favorable mid-level lapse rates within any convective
scheme along with ample moisture advection east of the terrain. The
threat is very much tied to the northern tier of MT where the
convergence pattern is greatest. The threat is less pronounced
further south and east comparatively, however some inference of a
few mid-level perturbations ejecting downstream over Northern WY
into Southern MT could provide a secondary area of focus for heavy
rainfall. For now, the main threat within the QPF distribution is
for locally heavy rainfall in that small zone of convergence under
the surface low formation near the border. Local 1-2" is forecast
in that area with some ensemble QPF output closer to 3" not too far
away into Canada. Targeted MRGL risk for that area east of the
terrain remains with little change from previous forecast.

Kleebauer


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