483
FOUS30 KWBC 082009
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Tue Jul 8 2025

Day 1
Valid 202Z Tue Jul 08 2025 - 12Z Wed Jul 09 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
EASTERN MID-ATLANTIC STATES AND SOUTHERN NEW ENGLAND, AS WELL AS
FOR PARTS OF THE ARKANSAS AND EASTERN OKLAHOMA...

2008Z Update: SLGT risk was expanded to include more of the Mid-
Mississippi Valley into Illinois to account for an evolving threat
in the region. More on this threat can be found in the latest MPD
#0622.

Kleebauer

...Mid-Atlantic and Northeast...

16Z Update: The overall synoptic and mesoscale scheme are still
relevant from previous forecast, although one note to point out is
the increasing potential for more locally enhanced flash flood
concerns focused across portions of Southeast VA. 12z CAMs are much
more bullish on the area between I-95 to the mouth of the Potomac
in VA for the highest precip output in the period. A lot of the
setup is driven by the training prospects in the vicinity of
Richmond through Williamsburg up to the areas running adjacent to
the Rappahannock River. 12z HREF neighborhood probs are running
between 40-60% for >3", but between 60-80% for >2" leading to a
strengthening signal compared to previous forecast output as noted
in the previous discussion below. This aligns with the increased
low-level convergence pattern as the mean layer wind becomes
generally unidirectional after 00z across Central and Southeast VA
with the approach of a shortwave expected to eject out of
Southwest VA. With coordination from the local Wakefield, VA WFO,
have expanded the SLGT risk into all of Southeast VA to account for
the threat.

Further north into the Megalopolis of the Mid Atlantic and
Northeast, the best threat for heavy rainfall will likely be
situated over Southern New England down into NYC where a secondary
focus will occur as a shortwave analyzed over Western PA lifts to
the east-northeast and centers over the above zone by later this
evening. Environment remains favorable for locally stronger
convective cores to produce rainfall rates in excess of 1"/hr,
especially with the assessment of 12z KOKX PWATs coming in ~2", a
reading encroaching the 30-day moving max, and eclipsing the daily
max for the 8th. The high urbanization factor always constitutes
close monitoring for locally impactful flash flooding, so the
threat remains well within the threshold for a SLGT risk leading to
general continuity for the forecast.

Kleebauer

..Previous Discussion..

The remnants of post-tropical cyclone Chantal are in the process of
transitioning into an open trough while exiting east of Cape Cod
to start the period, as it continues to get caught up in increasing
southwesterlies downstream of a trough approaching from the Great
Lakes. This trough will continue to translate eastward through the
day, but will be slow to advect due to downstream Bermuda-type
ridging. Between these two features, southwest flow will remain
prevalent over the area, with 850-500mb winds being nearly
unidirectional from the W/SW at 15-20 kts. This will result in a
continued extremely moist environment with PWs likely eclipsing 2"
from eastern VA through southern New England Tuesday afternoon,
coincident with a plume of SBCAPE exceeding 2000 J/kg (per 00z HREF
mean fields).

Into these impressive thermodynamics, a subtle shortwave embedded
within the mean flow will traverse northeast ahead of a cold front
and along a surface trough, providing additional ascent atop the
already impressive convergence on these boundaries. The 00z hi-res
CAM consensus suggests showers and thunderstorms will become
numerous to widespread along and near the front during the mid to
late afternoon, and storm motions are expected to initially be
quite slow (0-1 km mean flow near 10 kts) with the potential for
localized backbuilding and training along the surface trough and
front (as Corfidi vectors collapse to around 5 kts with the
development of deep convection and resulting outflow boundaries).
With warm cloud depths potentially eclipsing 14,000 ft, this will
support efficient warm-rain processes and rain rates above 2"/hr
at times (and resulting localized hourly totals of 1-2"). Where
training occurs, this could result in total rainfall in excess of
3" (per 00z HREF 40-km neighborhood 3" exceedance probabilities
ranging from 10-40% across the SLGT risk area). Scattered
instances of flash flooding are possible across much of the I-95
corridor from Richmond, VA to Boston, MA.

Churchill

...Southern Plains through the Ohio Valley...

16Z Update: A targeted area across Eastern OK into Western AR now
exists as a higher-end SLGT with some locally significant impacts
plausible over the aforementioned area. Current WV satellite and UA
analysis depicts a prevalent mid-level shortwave located within the
confines of Northeast OK and Southeast KS dipping southeast over
the last several hrs. Enhanced sfc-500mb convergence pattern over
Eastern OK to the AR/OK line is already materializing with a
solidified low-level inflow pattern advecting unstable air into the
vicinity of the mid-level circulation. Latest RAP surface analysis
pins a target of 1000-2000 J/kg of SBCAPE located over the axis of
greatest convergence with PWATs noted to be ~2" as 12z KLZK
sounding just to the east depicted a PWAT observation of 1.99",
above the 90th percentile daily output. The combination of a
pronounced area of buoyancy and focused surface convergence on the
southern flank of the shortwave will create an opportunity for
locally enhanced hourly rates between 2-3+"/hr through the next 3-6
hrs before the threat wanes with the progression of the shortwave
migrating eastward. For more information on this threat, please see
MPD #0619 for the latest.

More convection will fire this evening across Southern MO with the
strongest cells likely to induce some 2-3"/hr rates for a short
time, enough to warrant some attention for flash flood prospects
across the Ozarks due to the local topography. The SLGT risk was
expanded to include those areas where 12z CAMs have highlighted the
potential.

No additional changes were made across the Midwest and Mid-
Mississippi Valley, but will monitor the progression and expected
output from the MCV as it migrates into IL this afternoon and
evening. Some spots may see 2-4" of rainfall which could very well
induce flash flooding, especially in any urbanized areas where run
off prospects are highest, generally in-of and between St. Louis to
Springfield, IL.

Kleebauer

..Previous Discussion..

Farther north near the base of the trough (into MO/IL), an MCV has
the potential to locally organize convection with better overall
dynamics (DPVA and shear from right-entrance region of jet streak)
compared to farther south (despite overall tropospheric moisture
likely to remain a bit lower with PWs less than 2.0" (but still at
or above 90th percentile). CAMs are not as aggressive with this
area, but will need to be monitored closely today given the MCV
and added daytime instability (HREF indicating SBCAPE rising to
1500-2500 J/kg).

Churchill

...Upper Midwest...

16Z Update: No changes to the previous forecast as the general
synoptic and mesoscale pattern remains on track with little run to
run deviation.

Kleebauer

..Previous Discussion..

A wave of low pressure and accompanying surface trough will move
progressively eastward from Minnesota into Michigan the first half
of today. While this will be generally fast moving, it will
impinge into a favorable environment to support heavy rain rates
which will likely eclipse 1"/hr, especially where it overlaps a
ribbon of MUCAPE reaching 1000-2000 J/kg. The fast motion will
limit total rainfall, but in some places that receive multiple
rounds of heavy rain, event totals could reach 2-3". This falling
atop sensitive soils (FFG as low as 1-1.5"/3hrs) could produce
localized instances of flash flooding.

Churchill

......Southeastern New Mexico...

16Z Update: The pattern remains favorable for another diurnally
driven convective pattern in-of the terrain of NM, especially in
the confines of the Sacramento Mtns. up to the Sandia`s to the east
of ABQ. No changes were necessary from the previous forecast MRGL
risk issuance.

Kleebauer

..Previous Discussion..

Another daily round of showers and thunderstorms is
expected to develop in the high terrain and then drop southeast
into the High Plains and southeastern portions of New Mexico.
Although storms are expected to be generally progressive as they
come off the terrain (0-6km mean winds from the north around 15
kts), they will move into an axis of more favorable thermodynamics
characterized by PWs of above 1" and MUCAPE approaching 1000 J/kg.
This could support briefly heavy rain rates above 1"/hr, supporting
an isolated risk for flash flood impacts across urban areas or
sensitive terrain and burn scars.

Churchill/Weiss


Day 2
Valid 12Z Wed Jul 09 2025 - 12Z Thu Jul 10 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MID ATLANTIC...

...Mid-Atlantic...

20Z Update: The previous SLGT risk issuance was generally
maintained with some expansion further west into the interior Mid
Atlantic and far Eastern KY, including portions of Southwest VA and
much of WV. The signal within the CAMs output is greatest across
Western NC up through Southwest VA and the southern reaches of WV.
Despite that depiction, the ML is very consistent with a marker of
heavy rainfall a bit further north into WV with ample instability
and elevated PWATs located over the southern half of the state.
When assessing the synoptic setup, the amplified shortwave
progression from the remnant MCV exiting into the Ohio Valley will
allow for a better axis of diffluence downstream of the advancing
disturbance which should increase convective prospects. A tongue of
elevated theta_E`s forecast within the deterministic bisect all of
Southern WV, especially along and east of I-79 within the terrain
leading to MUCAPE signatures between 1500-2500 J/kg during time
frame of interest, Wednesday afternoon. ECMWF AIFS indicates a
convectively driven QPF bullseye to materialize over areas near and
just northeast of Charleston, WV, a bit further north compared to
what is inferred from hi-res at this time. Given the consistency
and general amplification with the approach of the shortwave/MCV,
would not be surprised to see a touch more meridional conjecture
within the convective regime over the area. The threat still looks
pretty solid within the Southwest VA area as there`s pretty good
agreement overall in the prospects for local totals >3" with the
latest HREF running between 60-80% within the neighborhood probs
and even some low-end 15-25% probs for >5" in the same area.
Considering the current outline of higher probs and the consistency
within ML depiction of a bit further north threat of heavier
precip, have expanded the SLGT risk to cover the more sensitive
areas across Western NC up through Southwest VA into WV with the
greatest threat likely over Southwest VA and Southern WV.

Further east to the I-95 corridor, the signal has ramped up for
heavier precip to focus over Northern MD along the M/D from Parrs
Ridge over into Cecil County expanding northeast along I-95 through
Philadelphia and Southern NJ up towards Toms River. This area is
coincident with the frontal positioning and pre-frontal trough
alignment across the Eastern Mid Atlantic leading to focused
convective elements and potential training. HREF neighborhood probs
for >3" are running hot (50-70%) from Richmond up through DC/Balt
and Philadelphia which is very favorable for scattered flash
flooding prospects due to a large footprint of impervious surfaces
within the southern half of the Megalopolis. PWATs between 2-2.2"
will lead to near or exceeding daily PWAT maxima by the time we
reach Wednesday afternoon and evening, an environment suitable for
heavy rain potential with enhanced rates collocated with the
strengthening low-level convergence regime. Hourly rates between
1-2"/hr are likely with intra-hour rates reaching between 2-4"/hr
at times within the heavier cores that initiate over the area.
Areal average rainfall forecast lies between 1-2" with a maxima of
2.5" centered near Richmond with a secondary maxima positioned over
the Delaware River basin near Philadelphia and adjacent South
Jersey urban areas. This threat is suitable for the previous SLGT
risk with an elevated threshold within the risk that could lead to
a targeted upgrade if the signal is amplified further overnight.

Kleebauer

..Previous Discussion..

Quasi-stationary front from Tuesday will linger in a general east-
to-west fashion on Wednesday across the Mid-Atlantic States. Along
this boundary, a much more potent shortwave (likely enhanced in the
latest guidance from the aforementioned MCV currently in the
Plains) will press east within the approaching longwave trough,
leading to weak low pressure development moving east along this
front. The accompanying ascent will act favorably into continued
robust thermodynamics with PWs around 2 inches (above the 90th
percentile according to NAEFS ensemble tables) and 1000-3000 J/kg
SBCAPE during the afternoon. Increasing 0-6km bulk shear and
aligned 0-6km mean winds to the front suggests storms will
repeatedly develop to the SW and then train ENE across the Mid-
Atlantic states. Not only will this result in heavy rainfall which
may exceed 3 inches in some areas, but it will occur across primed
soils from heavy rainfall on prior days, enhancing the flash flood
risk. An inherited SLGT risk was maintained and expanded based on
the new guidance, and high-end (25%+) SLGT probabilities are
indicated from central NC through much of VA and into southern MD.
Upon the full evaluation of the new 12z CAM suite later today, a
targeted upgrade to MOD risk may be necessary for portions of the
SLGT risk (particularly given the more vulnerable antecedent
conditions).

Churchill

...Central Texas through the Southern Ohio Valley...

20Z Update: Spread within the CAMs still maintains a posture of
medium confidence in convective development across Central TX, but
lower confidence in where the heaviest precip could occur, as well
as coverage ranging from generally isolated to numerous pending
hi-res output. Analysis currently indicates an expansive TUTT
migrating into the TX Gulf Coast and Northeast MX south of
Brownsville. This feature will remain a driver in what will occur
upstream as the system moves inland and rotates around the western
flank of the ridge downstream. How much this feature interacts with
the stationary front will decide how much of a potential impact
the area will receive from area convection. Environment will be
conducive for locally heavy rainfall with sufficient deep layer
moisture and relevant instability as referenced below...

HREF EAS probs for at least 1" are actually very low (<15%) but
neighborhood probs for >1" and >2" are generally high (60+%)
meaning the models agree on the potential, but vary with the
spatial recognition of where exactly this will occur. It`s also an
inference for more isolated prospects in those higher totals, so it
will be something to note as we move through the next few forecast
cycles. A better signal and greater overlap from guidance will
likely lead to a targeted upgrade, especially for those highly
compromised areas over Hill Country to the west of the I-35
corridor as they are still recovering from the significant impact
over the weekend. For now, maintain a MRGL risk, but certainly an
area to watch closely.

Downstream over the Southern Ohio Valley, the signal is relatively
similar to the threat across TX with isolated to widely scattered
flash flood instances plausible from the Lower Mississippi Valley
up through TN/KY down into northern AL/MS as they align with the
stationary front. Look for the highest risks to occur within the
fronts confines with emphasis on more urbanized zones. The MRGL
remains for the region mentioned above with local totals between
2-4" possible in the highest impact areas.

Kleebauer

..Previous Discussion..

Stalled frontal boundary will gradually begin to lift northeast on
Wednesday over the OH Valley, but persistent ascent along this
front as weak shortwaves cross it will maintain a convective risk
through the day. Some DPVA and subsequent forcing for ascent from
storms in the Plains may be deflected southward into the warm,
moist air mass of the Southern Plains (possibly as far south as
more vulnerable areas of central TX). Storms will fire in response
to thermodynamics characterized by PWs around 2 inches collocated
with SBCAPE during peak heating that will approach 2000 J/kg. While
convection is likely to be scattered, any storm which trains along
the boundary with intense 1-2"/hr rain rates may result in rapid
runoff capable of producing instances of flash flooding (with
training/repeating of efficient rates less likely into the Plains
and Lower MS Valley, but will come down to mesoscale details that
are difficult to discern in the 24-48 hour timeframe).

Churchill/Weiss

...Central Plains...

A shortwave rotating along the northern periphery of a ridge
centered over the Southwest U.S. will eject out of the northern
extent of the Front Range leading to focused ascent in-of NE into
the northern Missouri Valley and adjacent Midwest. HREF
neighborhood probabilities for >3" are fairly modest (20-35%)
across the aforementioned area as convection fires with the
introduction of a low-level jet Wednesday evening. Sufficient
ascent and increasing regional 0-6km bulk shear should enhance
convective initiation and maintenance through the evening Wednesday
into Thursday morning with the heaviest precip located along the
surface trough bisecting NE down into Northeast KS. Jury is still
out on whether this thunderstorm development can grow upscale and
migrate southeast as more of a developed MCS which has been implied
by some of the latest CAMs. If that case, a more organized complex
would certainly lend favor to at least an isolated flash flood
threat. A MRGL risk was added across the 4 state area of
NE/MO/IA/KS for the risk of nocturnal flash flooding potential.

Kleebauer


Day 3
Valid 12Z Thu Jul 10 2025 - 12Z Fri Jul 11 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL MID-ATLANTIC STATES, AS WELL AS FOR PORTIONS OF THE CENTRAL
PLAINS AND UPPER MIDWEST...

...Tennessee Valley into the Mid-Atlantic and New England...

20Z Update: SLGT risk for the Mid Atlantic was maintained with an
expansion up into portions of Southeast VA as the area will likely
be impacted for a third day in a row with heavy rainfall. Soil
moisture anomalies leading in will be on the high side with
perhaps some regionally compromised soils located over the region
between Richmond and the mouth of the Chesapeake. It will not take
much rainfall in these areas to cause problems and with the
environment remaining favorable, adjusted to reflect the prospects.
The rest of the forecast remains on track with good run to run
continuity from ensembles and associated deterministic.

Kleebauer

..Previous Discussion..

A warm and anomalously moist air mass looks to remain in place
across much of the Southeast and Eastern U.S. into Thursday
continuing a broad flash flood threat into yet another day.
Although upper-level forcing looks less impressive overall relative
to Wednesday, still expect another round of primarily diurnally
driven convection focused along the terrain and associated leeward
trough of the Appalachians, perhaps becoming best organized a bit
farther south than prior days as the combination of best forcing
and moisture/instability looks concentrated across the southern
Mid-Atlantic into portions of the Southeast. Maintained an
inherited SLGT risk area for more vulnerable portions of central NC
and surrounding far south-central VA, but this area will likely be
modified over subsequent forecast cycles with dependencies on both
trends in the guidance and how the forecast ultimately pans out for
days 1 and 2 (as antecedent conditions will be a big factor). The
latest trends in the guidance suggest the threat will be more muted
from much of VA northward (with subtle height rises aloft from
sub- tropical ridging tries to offset the more favorable parameter
space for heavy rainfall).

Churchill

...Northern and Central Plains into the Upper Midwest...

20Z Update: Consensus continues to grow with the expected approach
of a potent shortwave trough ejecting out of WY and the Central
Rockies, flattening the ridge and making headway into the Central
and Northern Plains. A well-defined axis of diffluence ahead of the
approaching trough should entail a pretty stout convective pattern
within a very favorable environment in the grand scheme. Despite a
relatively consistent forward propagation of the disturbance, low-
level inflow within the southern flank of the shortwave and
attendant low will likely enhance from potential for back-building
along the southwest side of the disturbances progression. As of
now, this has been noted over Southern and Eastern NE, a potential
overlap with some areas that will have seen rainfall the period
prior. The correlation would likely enhance flash flood prospects,
in a local sense, but the coverage this go around should be more
favorable for scattered flash flood occurrences. The SLGT was
maintained and expanded to account for the recent trends within
guidance.

Kleebauer

..Previous Discussion..

By Thursday the persistent ridge aloft over the Southwest U.S.
begins to break down, as an upper-low off the northern CA coast
opens up into a shortwave trough on Wednesday and ejects eastward
ahead of a digging longwave trough over western Canada. These two
features look to interact over the Northern and Central Plains, but
there are still substantial differences between models in how these
features evolve and interact. Maintained an inherited SLGT risk
area in the vicinity of where the best QPF signal overlaps (west-
central IA into adjacent portions of NE/SD/MN). PWs of 1.5"+ are
expected (near or above 90th percentile for the region) with ample
instability and dynamics for organized convection and subsequent
high rainfall rates.

Churchill


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