FOUS30 KWBC 192320
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
720 PM EDT Wed Aug 19 2026

Day 1
Valid 2316Z Thu Aug 20 2026 - 12Z Thu Aug 20 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF INDIANA AND OHIO...

...2320Z Special Update...

A Moderate Risk upgrade was introduced with this update for
portions of Indiana and Ohio. Training storms across those states
have developed south of a stationary front over the northern tier
of those states. This area has been very hard hit with heavy rain
over the past several days, so all of this renewed heavy rain will
translate into additional runoff and flash flooding. The storms
will slowly sag southward, following the greatest moisture and
instability. The urban areas of Indianapolis, Dayton, and
Columbus are all threatened with additional flash flooding as these
storms approach from the north.

Wegman

...Previous Discussion...

...16Z Update...

Only some minimal areal adjustments were needed for the inherited
Slight Risk covering portions of the Missouri to Middle Mississippi
and Ohio Valleys after review of the updated 12Z guidance, with the
CAMs well clustered within the risk area. Storm coverage and
rainfall amounts indicated by the guidance have notably come up
along the southward progressing cold front stretching generally
west to east from the Middle Missouri to Upper Ohio Valleys. This
includes portions of central Illinois, Indiana, and Ohio which
remain more sensitive to additional rainfall given very wet
antecedent conditions remaining after the past week's rainfall and
ongoing river flooding. Locally heavy rainfall totals in the 2-4"
range will easily lead to the risk of additional scattered
instances of flash flooding.

Further south, the guidance also continues to depict a separate
focus area for heavy rainfall over the Middle Mississippi to Lower
Ohio Valleys as storms develop and then progress to the southeast
along a similarly oriented frontal boundary. The updated CAMS
continue to show the potential for totals of 2-4", possibly as high
as 6". The HREF/RRFS also both show moderate probabilities
(40-60%) for rainfall greater than 3" with low end probabilities
(20-30%) for rainfall greater than 5". Despite the potential for
these heavier totals and some backbuilding to help contribute to
longer duration rainfall, the storms are expected to remain
relatively progressive overall which should lead to mainly
scattered instances of flash flooding. Antecedent conditions also
remain rather dry and FFGs generally high despite some initial
rainfall from storms this morning.

The prior forecast also remains on track for the Southwest to
Central Plains for locally heavy rainfall from increased Monsoonal
storms with at least an isolated risk for flash flooding. Two more
focused areas continue to be noted over south-central Arizona,
where a Slight Risk was introduced, and along the High Plains of
southeastern Colorado/northeastern New Mexico where shortwave
timing/terrain influences will support locally greater storm
coverage. The latest 12Z CAMs continue to show greater potential
for locally heavy totals of 1-2"+ for south-central Arizona
compared to southeastern Colorado/northeastern New Mexico, and FFGs
are generally lower, so have maintained the inherited Slight for
south-central Arizona with a Marginal Risk into the central High
Plains.

Putnam

...Previous Discussion...

...Central Plains to Ohio Valley..

Current satellite and radar composite depicts the initial stages of
our shortwave ejection from the High Plains making headway into the
Missouri Valley as we move through the evening. Expectation is for
this maturing area of convection to propagate southeastward into
the Mid-Mississipi regions of MO, leading to the first round of
convection being the remnants of the overnight MCS with an
expected decay of the complex later this morning as we lose the
nocturnal LLJ and we approach the diurnal instability minimum
across the western Ohio Valley. A cold front currently analyzed
over the Northern Plains extending east into the Great Lakes will
also continue its progression south and southeastward through the
morning hours with sights on the Ohio and Missouri Valley during
the period with a bit more latitudinal loss further west compared
to downstream. By Wednesday afternoon, we'll see the introduction
of another potent shortwave into the center of the CONUS leading
to broader amplification of the longwave trough positioned east of
the Mississippi allowing for greater large scale ascent focused
over the western Ohio Valley back into the confluent area of the
Ohio and Mississippi River between the MO/TN/IL area. This has been consistently depicted over the past series of forecast runs with
the current 00z CAMs suite maintaining this particular zone as one
of the more favorable areas of organized convection as step into
the second half of the forecast period (Wednesday evening into
early Thursday AM).

After 00z, nocturnal LLJ will kick in across the Mississippi Valley
with the nose of the jet acting as a low-level convergent feature
and aiding in an increasing regional bulk shear to provide a
necessary element to organize and sustain convective development.
The added bulk shear (30-40kts) and increasing upstream forcing
with the addition of the next shortwave is typically a signature
that correlates to a focal point for heavy convection with back-
building potential until the eventual frontal passage is complete
leading to some heavier rainfall lingering for several hours before
the threat subsides. Considering PWATs between 2-2.25" centered
across the tri-state region outlined above, and the added ascent
factors in both the boundary layer and mid-levels (sfc-500mb),
there will likely be an area that sees a period of 1-2"/hr rainfall
for at least 3-6 hrs. leading to flash flood prospects carrying
overnight into early Thursday morning. Latest 00z HREF 1" EAS probs
depict the area of far southeastern MO through southern IL and
western KY as the primary region of focus coming through with
30-60% probabilities, a threshold typically reserved for more
appreciable organized convective setups leading to higher
confidence in this area seeing at least a scattered flash flood
threat when you couple with the current FFG indices. Overwhelming
consistency in the signal has allowed for a continuation of the
previous SLGT risk with the main area of interest lying between
southeastern MO into southern IL and western KY/TN. Some areas are
liable to see 2-4" of rainfall with a max of up to 6" plausible
considering the environment at large.

The last few iterations of the CAMs have added another wrinkle to
the new D1 as the frontal progression and the preceding environment
have increased the overall threat of heavy rain across areas of the
Central Plains over into the Ohio Valley as thunderstorm genesis in
these areas are liable to cause scattered flash flood instances,
especially in those more prone areas across the Ohio Valley from
eastern IL to OH. Latest HREF neighborhood probabilities for >2" of
rain has increased significantly over the past 24 hrs. with the
newest set of probs firmly settled between 50-80% for the threshold
with even a "bullseye" of closer to 90% over portions of eastern NE
near Omaha. The upstream areas across NE and MO are a combination
of both the frontal ascent posed by the addition of the cold front,
as well as the added mid-level diffluence provided by the trailing
shortwave trough descending out of the Northern High Plains on the
backside of the mean trough. There has been a consistent signature
of locally heavy rainfall materializing between 15-18z today
upstream over northeastern NE, moving southeast towards the
Missouri River Basin by mid-afternoon to impact the Omaha area and
points southeast into northwestern MO. Rainfall rates in some of
these cells could eclipse 2"/hr at times, especially when you
couple the intra-hour rate variables that ultimately cause a
majority of the flash flooding incidents. Local totals of 2-4" are
certainly viable in this setup over the above locations leading to
an addition of a SLGT risk now across those area of eastern NE down
through northern MO and far southern IA.

Further east into the Ohio Valley areas stretching from eastern IL
through IN/OH, antecedent soil moisture anomalies remain into the
90-95th percentile over a large area leading to a lower threshold
necessary for any convective elements to cause trouble, especially
ones that linger for multiple hours at a time. We still have
hydrologic concerns aplenty for this region of the CONUS as
several Flood Warnings remain due to the swollen rivers and streams
in recovery from last weeks onslaught of heavy rainfall. The latest
HREF is keen on developing a long line of thunderstorms along the
advancing cold front with some of the activity likely to push rates
between 1-2"/hr by late-afternoon as we see a slow increase in
regional shear (25-35kts) as the upstream pattern evolves and
provides a better environment for sustaining mesocyclone
development. Mean flow will also run fairly parallel to the
boundary, a signature that typically acts as basis for repeated
convection for a time until the cold front finally trudges past the
area of interest. This signature is anticipated to extend from IL
through IN/OH, eventually with the eastern fringes possibly
reaching as far as western PA where the front will be located by
the evening hours. Considering the sensitivity of so many areas
bounded within this region, and in coordination with the local Ohio
Valley WFO's, a SLGT risk was added to encompass a broad area of
the Ohio Valley with emphasis on those sensitive areas spanning
eastern IL, central IN, and much of central/eastern OH.

Kleebauer

...Southwest into the Central High Plains...

The previous forecast remains on track with the latest 00z CAMs
maintaining agreement on scattered thunderstorm activity likely to
transpire later this afternoon, carrying into the evening hours.
Areas of the Front Range and southern AZ are the most notable for
seeing more organized convection in the setup thanks to added
shear and regional ascent attributed to a shortwave proxy off the
Central Rockies to the north, and a deeper moisture layer and
instability maxima near the International border. The general
synoptic pattern has remain persistent in its presentation the
last several forecast cycles with the consensus unchanged on where
convection will occur.

The expected evolution of the setup across the Southwest and Front
Range begins with an upper ridge centered over the Four Corners,
re- centering and amplifying while an inverted trough pushes west
on Wednesday. This will promote a more favorable setup for
monsoonal convection as a surge of thermodynamics (PWs above 1.25"
overlapping MUCAPE of 1000-2000 J/kg) lifting northward behind the
inverted trough. At the same time, weak shortwaves embedded within
the flow and rotating around the ridge will produce areas of
locally enhanced ascent, which into the improving thermodynamics
suggests more widespread convective activity is likely on
Wednesday. While a more substantial risk for greater coverage and
intensity exists just beyond D1, a large MRGL risk for excessive
rainfall appears supported from the Deserts of Arizona northeast
into the Central High Plains of CO/KS. With rainfall rates of
0.5-1"/hr likely at times, and mean storm motions of just 5-10 kts,
(higher in CO but with stronger bulk shear for organization) this
will support at least isolated instances of flash flooding,
especially over vulnerable terrain, urban areas, or recently
saturated soils.

In coordination with the Tuscon WFO in southern AZ, a SLGT risk was
added over the areas along and west of the Huachuca Mtns through
the I-19 corridor into the eastern sections of Pima/Pinal counties
due to a more robust axis of convection expected across far
southern AZ by later today. Forecast is for a cluster of
thunderstorms maturing and migrating off the Huachucas, advancing
westward through Santa Cruz and southeastern Pima counties as we
head into the evening. HREF probs for >1" have ballooned upwards of
60-80% over the I-19 corridor in southern AZ, including over the
city of Tuscon lending a favorable potential for flash flooding,
even into the metro area. Rainfall rates between 0.5-1"/hr are
anticipated in the strongest cells, plenty aggressive enough to
exceed the already low FFG thresholds in place across the southern
AZ deserts. Expecting scattered flash flood potential given the
current signal, more than sufficient for a targeted SLGT risk
upgrade.

...Florida into Far Southeast Georgia...

A small mid-level vorticity maxima will slowly organize over
central and northwestern FL with a proposed surface trough likely
situated very close to the northeastern coast of the area. The
combination of the two features and generally moist and unstable
airmass presence will allow for quite a convective burst to occur
by the afternoon hours, especially as you add the typically diurnal
components of the sea breeze as a focal point for convection. HREF probabilities are pretty aggressive for >3" locally (60-90%)
extending from Orlando up through just south of Jacksonville. We
saw some isolated flood instances crop up from storms earlier in
the previous period from an even less favorable setup compared to
what will transpire today. Hourly rates of the heaviest cells will
likely be between 3-4"/hr after assessing individual deterministic,
which makes sense considering PWATs easily pushing 2.1-2.4" over
the entire stretch of the east coast of FL and all of North FL into
the Panhandle. Given the signal, a MRGL risk was added across that
northeastern extent of FL into far southeastern GA, south of
Brunswick.

Kleebauer


Day 2
Valid 12Z Thu Aug 20 2026 - 12Z Fri Aug 21 2026

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

...2030Z Update...

The main update for this forecast cycle was to modestly expand and
then extend the inherited Slight Risk over the Mid-Atlantic
northward to include more of the I-95 corridor from Philadelphia to
the New York City metro. As outlined below, a line of
thunderstorms is expected to develop along the eastward moving cold
front as it reaches the I-95 corridor. The now available CAMs show
considerable rainfall potential (local amounts of 3-6") from the
Virginia Tidewater north-northeastward through DelMarVa and into
far southeastern PA, most of NJ, and the NYC metro. This is backed
up by moderate (40-70%) HREF/RRFS probabilities of rain totals
greater than 3", and low end (20-30%) probabilities of greater than
5". There remains some uncertainty with the timing/progression of
the cold front and when storms form along/ahead of it relative to
the I-95 corridor before moving eastward, but again the latest CAM
guidance did show notable agreement at least for the northern
extent of the risk. The risk remains just to the east further south
from Baltimore to DC except for some of the more eastern
suburbs/exurbs and towards Annapolis/Chesapeake Bay. In the
Southwest, the forecast remains unchanged for the inherited
Marginal Risk covering additional Monsoon activity.

Putnam

...Previous Discussion...

...Mid Atlantic...

Shortwave over the Ohio Valley will continue its progression to
east, then northeast as it migrates into the Central and Northern
Mid Atlantic by Thursday afternoon. At the surface, a cold front
will remain steadfast on its advancement through the Piedmont and
the eastern sections of PA by late-Thursday morning leading to a
favorable scenario for a line of thunderstorms to develop along and
east of the front itself. Increased dynamical forcing and
favorable diffluent flow within the RER of a 130kt jet over the
Northeast U.S. will lead to a blossoming of afternoon convection
within proximity of the front itself. Latest CAMs are quite
aggressive on their interpretation of both the magnitude and
coverage of the heavy convective signals, especially for areas
extending from Northern NJ down through eastern PA, the Delmarva,
and the VA Tidewater. Thunderstorm initiation will be very close to
the metro areas of DC-Balt-Philadelphia with the majority of CAMs
developing cells almost overhead before advancing steadily
eastward, away from the I-95 corridor. The threat will be close for
the urban centers with the Philadelphia metropolitan area the
closest for more substantial impacts from convection when you
correlate with the mean QPF output in the region.

PWATs between 2-2.3" coupled with a strong theta_E ridge in place
ahead of the front promotes a formidable environment capable for
producing significant rainfall rates in any convective core that
materializes. Analysis of multiple soundings from both the global
and hi-res suite at 00z indicated a textbook signal for heavy
rainfall with a tall, skinny CAPE signature with moisture layers
present from surface to the tropopause. Typically these types of
environments favor, at least scattered flash flood prospects in
areas where thunderstorms reach their mature phases, which seems to
align with areas along and east of I-95. The most notable heavy
rain depictions favor NJ/DE/VA Tidewater for the time being as the
frontal progression times well with the late-afternoon period, a
time frame when diurnal instability has reached its maximum. The
signal is pretty close to the NYC metro, but the heaviest may lie
just south of the metro area, so it will bear watching the trends
over the next 24 hrs to see if a higher risk is necessary for New
York to western LI. For now, a MRGL risk encompasses this
particular zone with areas just south within the added SLGT risk.
The DC/Balt metro area is currently in a MRGL with the new SLGT
just to the east over Prince Georges and Anne Arundel counties.
Areas east of the Chesapeake are firmly into the SLGT risk, as it
stands as the signal is the most robust in that particular
corridor.

...Southwest U.S. into the Southern Rockies...

Relatively textbook monsoonal pattern across the west will lead to
scattered thunderstorm development across much of the Desert
Southwest extending north into parts of the Great Basin and through
the Southern Rockies. As of now, the signal is mainly seasonal with
regards to the PWAT anomalies and general expectation as the most
notable areas will likely be in those terrain focused zones across
AZ (Mogollon Rim), NM (Sangre de Cristos and Sacramento Mtns.), and
UT (Zion National Park). Low probs for >1" on the latest
ECENS/GEFS suites and 01z NBM allow for a standard MRGL risk
issuance for the area as the setup is genuinely favored for pulse
convection and slow cell motions tied to the terrain as a broad
597dm ridge centers over the Four Corners and makes little
progression in the D2 window. No changes were necessary to the
previous forecast given the consistency within guidance over the
last series of model runs. Most favorable areas of impact will
include the more flashy streams and arroyos across the Southwest,
as well as the slot canyon areas of southwest UT and the burn scars
across each of NM and AZ.

Kleebauer


Day 3
Valid 12Z Fri Aug 21 2026 - 12Z Sat Aug 22 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
VIRGINIA AND THE CAROLINAS...

...2030Z Update...

Only one main areal adjustment was made to the inherited Slight
Risk based on the latest guidance, with the risk area extended a
bit further to the northeast over the Virginia Tidewater region and
to the Maryland Eastern Shore. QPF amounts have trended up through
this region and this area will also likely have seen an initial
round of heavy rain during the day 2 period. Storm coverage overall
will shift southward compared to Thursday with development focused
along consolidating frontal boundaries through the southern Mid-
Atlantic and into the Carolinas. The latest 12Z deterministic
guidance continues to depict locally heavy rainfall totals in the
3-5" range. The AI guidance (notably the ECMWF AIFS) is less
bullish on the northward extent of the heavy rainfall, leading to a
bit more uncertainty into Virginia and the Maryland Eastern Shore.
However, as noted, the other deterministic guidance has actually
come up in amounts through this region.

As during the day 2 period, Monsoonal activity is expected to
continue throughout much of the Southwest with at least an isolated
risk for flash flooding. The inherited Marginal Risk was mostly
left unchanged except for a small expansion over portions of
eastern New Mexico to cover more burn scar regions where the latest
guidance does depict some storm potential.

Putnam

...Previous Discussion...

...Virginia to Carolinas...

Cold front will slowly advance south into the Carolinas during the
end of the D2 into D3 time frame with an eventual transition to a quasi-stationary front as it reaches the latitude of north GA into
the Piedmont of SC. Moisture pooling along the front with continued
forcing driven by another shortwave advancing through the
Tennessee Valley will lead to another round of heavy convection in-
of the Southern Mid Atlantic with the primary axis likely to occur
from the southern Delmarva through the VA Tidewater into the
Carolina's. Environment across the region outlined will remain very
buoyant with PWATs between 2-2.3" for the entire area along and
south of the front. Models are consistent with their signature of
1-2" of rainfall on an areal average basis with some of the
deterministic outputs approaching upwards of 3-5" where storm
materialize and settle as the steering pattern weakens
significantly as we step into the D3. Rainfall rates between
2-3"/hr with intra-hour rates upwards of 4"/hr are forecast across
the aforementioned area, plenty aggressive enough to cause at least
scattered flash flood prospects, especially in more urbanized
zones along I-95 and points east. The most notable locations in the
guidance are over the Columbia, SC area up through Florence into
southeastern NC. With the signal maintaining consistent in the
heavy QPF placement and with a favorable environment for locally
enhanced rainfall, the SLGT risk was generally maintained from the
previous forecast.

...Southwest...

Monsoonal activity will continue for another day as the Southwest
CONUS lies underneath a broad ridge centered over the Four Corners
with multiple mid-level vorticity maxima pivoting under the ridge
axis. Seasonal to slightly elevated PWATs will extend throughout
the Southwest, even reaching a bit further north into the Great
Basin by Friday leading to some locally heavy rainfall even making
its presence into portions of NV and central UT while also
impacting those same areas that saw activity the prior period.
Local totals of 1-2" are forecast, but areal QPF average is pretty
much between 0.25-0.5" which is typical for these types of setups.
A broad MRGL remains in effect for much of the Desert Southwest up
into the Great Basin, extending east into the Southern Rockies
located over southwestern CO into NM.

Kleebauer


Day 4 and Day 5

Valid 12Z Sat Aug 22 2026 - 12Z Mon Aug 24 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE EAST COAST, SOUTHWEST AND INTERMOUNTAIN WEST...

...2030Z Update...

Only small changes were needed to the previous forecasts for this
weekend. The Marginal on Saturday was expanded north up the New
England coast. While there is uncertainty with both the timing of
the rain (whether it will be fully off the coast by Saturday) and
how heavy it will be (instability will be a limiting factor), urban
areas further east, including Providence, RI and Boston, MA both
have a higher chance of seeing at least steadier rainfall, as noted
by the higher QPF. Given the sensitive nature of those urban areas
regarding onset of flash flooding, and potential for at least some
instability to support periods of heavier rainfall, the Marginal
risk was extended.

A Slight Risk may need to be considered for coastal areas of North
Carolina as well with future updates if delays that fail to move
the front along in that area persist.

Monsoon rains continue in the afternoons of both weekend days
across much of the West. A bit of trimming was done on the western
side, to include western Arizona and southern Nevada due to lack of
appreciable rainfall forecast in those areas on Saturday.

Transient MCSs may impact portions of the Plains and lower to mid
Mississippi Valley this weekend, which may require at least a
Marginal Risk to be issued with future updates, but large
uncertainty about where they will be precluded that issuance at
this time.

Wegman

...Previous Discussion...

...East Coast...
The mid to upper trough deepens further over the eastern U.S. on
Saturday, but the northern stream energy currently appears
separated from the deeper moisture plume further south and east.
While adequate instability and upper level forcing will support
scattered thunderstorm development across portions of MI, OH and
NY, PWs appear insufficient to support flash flooding, particularly
given expected quick cell motions. Thus the Marginal risk was
removed across this region.

The better threat for heavy rainfall will remain focused from the
coastal Carolinas into the Mid-Atlantic, where a lingering
stationary front interacts with the best overlap of instability
and high PW values. Forecast confidence in a continued flash flood
threat is highest along the coastal Carolinas, where stronger
instability is more likely, and antecedent rainfall in prior days
may lower FFG. Confidence is lower across the coastal Mid-
Atlantic, as the axis of instability and moisture could get shunted
offshore. However, the Marginal risk was maintained here as a few
models still indicate locally heavy rain potential and the CSU
machine learning ERO guidance highlights a small Slight risk area.

No Marginal risk is delineated on day 5 (Sunday) given spatial
uncertainty and the higher probability that the digging trough
does not tap into the better moisture pool. However, if the upper
trough digs more and takes on a negative tilt, deeper moisture
could get drawn back into the system, resulting in some flash flood
risk over New England. Thus we will continue to monitor trends.

...Southwest and Intermountain West...
Broad Marginal risks remain in place across portions of the West
this weekend. PLentiful monsoonal moisture and daytime instability
will promote localized flash flooding both days. Because convective
development will remain largely tied to terrain driven forcing,
overall coverage is expected to stay below Slight risk thresholds.

On Saturday the highest PW anomalies are forecast to be centered
over UT, WY, ID and southern MT, where values are expected to
exceed the climatological 90th percentile. Sufficient instability
should be in place, along with some large scale forcing on the
periphery of the upper ridge...to result in scattered convection
and localized flash flood potential. The Marginal risk was
expanded northward to include these areas.

Chenard


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt__;!!DZ3fjg!8dbTCLmmre5OFOhuZ2at28QpvU1Aje23SY3O7stfBXTGbAivEDtF0P8BlYlCNxEb9n7pdLpXgu86UiLqIdqhEvzGhdQ$
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt__;!!DZ3fjg!8dbTCLmmre5OFOhuZ2at28QpvU1Aje23SY3O7stfBXTGbAivEDtF0P8BlYlCNxEb9n7pdLpXgu86UiLqIdqhXOx_bJ0$
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt__;!!DZ3fjg!8dbTCLmmre5OFOhuZ2at28QpvU1Aje23SY3O7stfBXTGbAivEDtF0P8BlYlCNxEb9n7pdLpXgu86UiLqIdqhJbtN3d0$



$$

= = =
To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM.
--- scorpsmtpd
* Origin: capitolcityonline.net * KY, USA (1:2320/107)