FOUS30 KWBC 090755
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Sat Aug 9 2025

Day 1 Valid 12Z Sat Aug 09 2025 - 12Z Sun Aug 10 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM FAR NORTHERN
MISSOURI THROUGH MUCH OF EASTERN IOWA AND INTO SOUTHWESTERN WISCONSIN...

...Upper Midwest...
The synoptic setup for heavy rainfall is becoming increasingly
favorable as a near-textbook Maddox Frontal Pattern develops across
the Upper Midwest. This will likely result in numerous instances
of flash flooding from northern Missouri through southern Wisconsin.

The period begins with a closed 500mb low moving across southern Manitoba/Saskatchewan, which will drive an elongated trough
southeast into the Central Plains and Upper Midwest with
accompanying height falls. Embedded shortwaves shedding around the
primary closed low will help keep the trough axis aligned SW to NE
from the Four Corners into the Upper Midwest, and this is a
critical piece to the evolving setup. As the trough lingers back to
the west, the parent upper jet streak rotating into central Canada
will maintain a tail back into the Central Plains, leaving
favorable RRQ diffluence atop the region, and aligned with the
greatest height falls. Since the trough will move little during
this time, this pronounced synoptic support will be quasi-
stationary, resulting in prolonged lift across the region.

At the surface, a cold front will try to push east, but become
generally stationary from MN to KS as the upper trough lingers to
the west. This will enable a plume of elevated moisture to
continually surge northeast ahead of the front, as PWs of 2 to 2.2
inches (above the 97th percentile according to NAEFS) get drawn
northward on 850mb inflow of 20-30 kts, pushing an impressive theta-
e ridge axis northward into IA. This anomalous moisture will
combine with MUCAPE of 2000 J/kg or more to supply extreme
thermodynamics into the region, supporting heavy rainfall rates for
which both the HREF and REFS indicates has a 30-40% chance of
exceeding 2"/hr, and the HRRR (as far out as it goes) suggesting as
much as 1"/15min (4"/hr rates) at times.

The concern for Saturday, especially within the inherited MDT risk
area, then, is that multiple rounds of convection are likely as weak
impulses track aloft and 850-300mb winds remain parallel to the
front. At the same time, Corfidi vectors become increasingly aligned anti-parallel to this mean wind, indicating a high potential for
backbuilding and training, which will support total rainfall above
5" (40-60% chance), with locally as much as 8" possible (10-15%
chance). However, there is still considerable uncertainty into the
exact placement as rounds of convection, especially through any
clusters/MCS, drive the effective boundary (merged outflow with
front) north and south at times. This could result in a slow
southern push of the heaviest rainfall through the day, although
the northward advance of the theta-e ridge at times should again
push heavy rain northward as well. With a lot of spread in place,
the HREF EAS probabilities (which are extremely significant for
2"/24hrs at above 35%) were used to help tailor the MDT risk area,
which aligns quite well with the latest ECMWF EFI above 0.7.

Despite the uncertainty in exact placement, the MDT risk remains
needed for the likelihood of training/repeating intense rainfall
rates of 1-3+"/hr leading to FFG exceedance probabilities that
peak above 40%. The MDT risk was expanded slightly to the SW to
account for the latest high res CAMs as well, and numerous
instances of flash flooding are likely.

...Southeast...
Another active day of showers and thunderstorms is expected across
Florida and the coastal Southeast. The driving factor for this
widespread convection will be a trough of low pressure centered
over the eastern Gulf, with a secondary surface stationary front
draped along the Panhandle of FL to off the SC/GA coast. This
results in generally weak flow within a col in the vicinity of a
mid-level ridge, with modest onshore flow from the Atlantic
becoming subtly accelerated by the trough over the Gulf.
Additionally, weak impulses rotating around the Gulf trough will
push W/NW through the period, providing locally enhanced ascent
across the area.

Although ascent will be generally modest outside of any impulses or
sea breeze/frontal convergence boundaries, the environment will be
favorable for periods of heavy rainfall. PWs above 2.25 inches, or
above the 90th percentile according to NAEFS, will encompass the
region, and overlap with MUCAPE of 1000-2000 J/kg to support robust thermodynamics for heavy rainfall. Any ascent working into this
environment will support convection with rainfall rates of 2-3"/hr.
Cells that develop will likely move very slowly W/NW on 0-6km mean
winds of just 5-10 kts, and as propagation vectors become
increasingly aligned back into the higher moisture and anti-
parallel to the mean wind, backbuilding and repeating cells will
become likely. This could be enhanced by regeneration along sea
breeze boundaries or storm mergers, lengthening the duration of
these heavy rain rates.

Although identifying where the heaviest rainfall will occur today is challenging due to the weakly forced environment and slow-moving
storms, the HREF neighborhood probabilities support generally two
areas of heavier rainfall. The first is along the Gulf Coast of the
FL Peninsula where the W/NW inflow may allow for a more focused
region of heavy rainfall as the sea breeze becomes pinned and cells
moving westward merge with it. The other region is along the SC/GA
coasts where a weak impulse moving onshore within modest WAA will
help deepen warm cloud depths and focus heavier rainfall along the
frictional land/sea convergence boundary. In both of these areas,
HREF probabilities for 5"/24hrs peak above 50%, and this could occur
atop primed soils from heavy rainfall on Friday. After coordination
with ILM/CHS/JAX, a targeted SLGT risk has been added for the
coastal plain of GA/SC where the HREF signal is most impressive and
rounds of storms effectively converging along the coast could
produce repeating rounds of heavy rainfall.

...Southwest...
A mid-level ridge centered across southern NM will gradually
weaken today in response to an impressive trough amplifying across
the Northern Rockies and a weak mid-level impulse rotating westward
from Texas beneath it. This feature is progged to track along the
international border before gradually rotating northwest late in
the period, and locally enhanced ascent in the vicinity of this
impulse will drive renewed and focused convection. As this
shortwave tracks W/NW, it will impinge into a narrow corridor of
elevated PWs above 1.25 inches aligned with a similar narrow
channel of MUCAPE above 1000 J/kg. Together, this will provide an
environment that will support thunderstorms with heavy rainfall as
reflected by simulated reflectivity from the available CAMs
including the HRRR-forced and GFS-forced UA WRF.

Storm motions beneath this weakening ridge will be quite weak and
variable, as expected, with 0-6km mean wind vectors showing a
variety of directions and speeds of just 5-10 kts. With bulk shear
remaining minimal, this indicates that pulse-type storms with
generally limited lifespans will be the primary storm mode, and in
the weakly forced environment this will allow for outflows and storm
mergers to result in additional, and at times more intense,
redevelopment. Rainfall rates could exceed 1"/hr at times (20-40%
chance) which through mergers and slow motions could produce 1-2" of
rain with locally higher amounts. Briefly considered a small SLGT
risk over the area of greatest instability near Tucson, but wide
variation in coverage among the various CAMs combined with recent
dryness across the area precluded the confidence for an upgrade at this time.

...Central High Plains...
The tail end of a front stretching from the Upper Midwest will
linger across the Central High Plains today. As a trough upstream
deepens over the Northern Rockies and sheds spokes of vorticity
eastward, these will interact with the lingering front to produce
scattered showers and thunderstorms late this afternoon through
tonight. Impressive bulk shear of 35-50 kts combining with moist
E/NE 850mb flow will result in slow moving thunderstorms organizing
into clusters, with repeated development likely upstream. Coverage
of storms may be isolated to scattered according to high-res CAMs,
but the slow motion of any cell that develops combined with
anticipated rain rates above 1"/hr may produce isolated flash flooding.

Weiss
$$
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FOUS30 KWBC 100835
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
435 AM EDT Sun Aug 10 2025

Day 1 Valid 12Z Sun Aug 10 2025 - 12Z Mon Aug 11 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
KANSAS AND MISSOURI...

...Central Plains through the Upper Midwest...
The synoptic pattern today continues to be indicative of a Maddox
Frontal rainfall event, although confidence in the exact evolution
has decreased and significant changes are needed to the inherited
ERO. Some of these changes are due to exceptional model uncertainty
at this time range, but also accounting for what occurred Saturday
when the event did not really pan out as expected.

Synoptically, the setup still remains extremely favorable for an
axis of very heavy rainfall. An anomalous mid-level trough (500mb
and 200mb heights locally below the 10th percentile according to
NAEFS) will remain aligned from ND/MN southwest through the Four
Corners. Although the leading impulse embedded in the core of this
trough will advect east across southern Canada, spokes of vorticity
and associated shortwaves rotating through the flow to the south
will force the trough axis to remain to the west. Downstream of this
trough axis, a persistent upper jet streak will arc poleward into
Ontario, placing at least modest but persistent upper level
diffluence within its RRQ. Beneath this evolving setup, a surface
front will oscillate at least subtly NW to SE as a stationary front,
providing additional focus for ascent beneath the weak height falls
and periods of PVA. Additionally, any stronger impulse interacting
with the surface front could result in waves of low pressure
rippling along to the northeast.

East of this front, low-level return flow will continue to advect
from an open Gulf, drawing elevated thermodynamics and theta-e
northward. PWs are progged to climb to above 2 inches as the 850mb
LLJ surges to 20-30 kts, overlapping intense MUCAPE of more than
3000 J/kg to provide an extremely favorable environment for heavy
rain, which is additionally supported by specific humidity values
between 925-850mb above the 97th percentile (NAEFS). At the same
time, the theta-e ridge will blossom and align into the front, with
an intense gradient suggesting heavy rainfall on its NW axis as is
typical with the Maddox Frontal conceptual model. In this setup,
with persistent ascent acting upon impressive thermodynamics, and
mean 0-6km winds aligned to the front, it is likely streaks of heavy
rainfall will occur, and most deterministic models showing 1-4
inches of rainfall through 12Z Monday, with high-res CAMs indicating
locally 5-7 inches, so heavy rainfall is almost a certainty.

However, uncertainty is exceptional (especially for a 24-hr
forecast) with the placement of this heavy rain, and the spread in
placement ranges from southwest WI to southern KS, and almost
anywhere in between, leading to a challenging ERO forecast.

While the front will be the primary focus for convection, along with
a wave of low pressure expected to develop along it, and the LLJ is
likely to be parallel to it, there is increasing concern that
convective clusters (both overnight Saturday night into Sunday, and additionally on Sunday) will leave outflow boundaries aligned more
west-to-east outrunning the front. This could drastically change the
position of the heaviest rain axes, and would support heavy rainfall
south of the global members in a typical bias. If these clusters do
not develop, convection and heavy rainfall would likely remain more
aligned SW to NE, but have noted a trend in the Corfidi vectors to
be more pointed S/SE after SW early, which would additionally
support more W-E heavy rainfall. With all the uncertainty in
placement, the ERO was modified heavily and tailored primarily to
HREF/REFS neighborhood probabilities, but with additional changes
influenced by the EAS probabilities, the CSU first guess fields,
and coordination with the impacted WFOs.

...Southeast...
Another active day of convection is likely from the coastal
Carolinas through Florida.

Broad ridging extending from a Bermuda-type but elongated high
pressure over the Atlantic will be modified by an inverted trough
aligned from the eastern Gulf to the NC coast. Onshore and confluent
flow into this inverted trough will maintain impressive moist
advection into the area, manifesting as PWs above 2.25 inches, well
above the 90th percentile according to the SPC sounding climatology.
This will overlap with MUCAPE of 500-1000 J/kg, modest overall
instability indicative of a tropical airmass, to produce impressive thermodynamics to support heavy rainfall. Additionally, this robust thermodynamic advection onshore will result from 850mb winds as high
as 15 kts, which, while modest, will exceed the mean 0-6km winds
which are progged to be just around 5 kts. This suggests widespread
convergent ascent across the Southeast, with the accompanying WAA
surging warm cloud depths to around 15,000 ft, supporting efficient
warm rain processes within a deeply saturated column noted in
regional forecast soundings.

The CAMs are in generally good agreement in widespread convection
today. Scattered slow-moving thunderstorms with rainfall rates of 1-
3"/hr are expected across much of FL and into GA/AL. However, the
focus for the heaviest rainfall is likely along the coastal plain of
GA through southeast NC. Here, propagation vectors are aligned
against the weak mean wind, and with subtle mid-level impulses
lifting northwest within the weakly forced environment, waves of
repeating rounds of convection with heavy rainfall are likely. In
this region, the HREF indicates a 20-30% chance of 2"/hr rain rates,
with locally more than 3"/hr possible at times, at least briefly.
With backbuilding offshore leading to regenerating and training
rounds of heavy rain, total rainfall could exceed 5" (30-50% chance
from the REFS/HREF), with the greatest potential along the SC coast.
The inherited SLGT risk was cosmetically adjusted, but scattered
instances of flash flooding appear likely today, even across these
areas that are generally more difficult to flood.

...Southwest...
A MRGL risk of excessive rainfall continues for portions of the
Southwest today as monsoonal moisture returns to the area leading
to a more active convective pattern. A mid-level trough extending
down from the Northern High Plains will deepen today, producing
lowered heights across the Four Corners and Desert Southwest,
through which weak spokes of vorticity will rotate. These lowered
heights and locally enhanced ascent will impinge into an
increasingly favorable thermodynamic environment to support
convection with heavy rainfall as low-to-mid level flow backs
gradually to the SW, allowing for a better draw northward of PWs
exceeding 1", which may spread as far north as the Mogollon Rim and
combine with MUCAPE of 500-1000 J/kg across the southern half of
AZ and NM. This will support scattered to widespread showers and
thunderstorms which will move slowly eastward through the day. With
rainfall rates as much as 0.75"/hr to 1"/hr at times as shown by
the UA WRF and HREF neighborhood probabilities , this could result
in at least isolated instances of flash flooding, especially across
more vulnerable soils or terrain. The MRGL risk was curtailed a
bit on the eastern side across NM where CAPE is progged to be much
weaker, but in general changes to the inherited risk were minor.

...Central High Plains...
The inherited SLGT risk was maintained across eastern CO,
primarily for renewed thunderstorm activity on Sunday evening
falling atop soils that will be primed from ongoing (Saturday
night) convection. The elongated stationary front across the
Central Plains early today will drop steadily southward, leaving NE
upslope flow in its wake. This will be most pronounced into
eastern and southeastern CO, where PWs will likely surge to above 1
inch. As convection develops across the higher terrain of CO
during the aftn, it should steadily progress east and then grow
upscale as it moves off the terrain in response to bulk shear of 40
kts. While these clusters should move steadily eastward on mean
0-6km winds of 15-20 kts, rainfall rates of 1-2"/hr are likely
which could be sufficient to overwhelm soils that will be saturated
from antecedent rainfall. Additionally, some local backbuilding is
possible as reflected by CAMs across southeast CO, lengthening the
potential duration of heavy rainfall in a few areas.

Weiss
$$
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FOUS30 KWBC 140754
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 AM EDT Thu Aug 14 2025

Day 1 Valid 12Z Thu Aug 14 2025 - 12Z Fri Aug 15 2025

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

...Southern New England through the Lower Mississippi Valley...
Surface cold front draped from New England through the Ozarks will
drift gradually southeast but generally stall across the region
Thursday as the parent trough begins to fill, leaving gradually
more zonal flow over the eastern CONUS. Although heights will
gradually rise, weak shortwave impulses embedded within the flow
will maintain periodic ascent atop the front to create waves of low
pressure rippling northeast to produce locally enhanced lift, and
scattered to numerous thunderstorms are again expected across the
east on Thursday, primarily from southern New England through the Gulf Coast.

Thunderstorms that develop will all have the potential to produce
pockets of heavy rainfall as thermodynamics remain impressive. PWs
of more than 2 inches will remain in a plume arcing within return
flow around offshore high pressure from the Florida Panhandle
through the northern Mid-Atlantic states. This PW will overlap with
elevated MUCAPE of 1000-2000 J/kg to create the favorable
thermodynamics for heavy rainfall, and the HREF indicates scattered neighborhood probabilities for 2"/hr rain rates across much of the
area supporting the broad inherited MRGL risk.

However, there appears to be at least a signal for some more
organized convection with a greater threat for excessive rainfall
across the eastern Carolinas and into Southeast Virginia. This
region will benefit from additionally focused ascent through the
presence of the cold front and a surface trough, upon which a more
pronounced shortwave is progged by the consensus of the models to
lift northeast during peak heating. This will promote at least
modest bulk shear to support multi-cell clusters across this area,
while most of the rest of the east should generally feature pulse
type thunderstorms. Additionally, mean 0-6km winds and propagation
vectors are aligned and parallel to the boundaries, suggesting an
enhanced training risk as well, which additionally supports the
elevated HREF and REFS probabilities for 3"/24hrs (and lower but
still notable 5"/24 hrs). For this reason the inherited SLGT risk
was adjusted to spread farther south into the eastern Carolinas.

...Southwest...
Mid-level ridge centered over NM to start Thursday morning will
weaken in response to height falls approaching from CA as a trough
digs down the Pacific coast. This will have the two- pronged affect
of lowering heights across the Desert Southwest/Four Corners
region, while also leading to increased cyclonic flow emerging
from near Baja to push a potent shortwave northward across AZ and
into UT. Additionally, this increasingly south flow, especially
between 850-700mb, will transport elevated PWs northward, and a
plume above 1" is progged to surge almost to UT by the end of the
forecast period. Where this combines with MUCAPE above 500 J/kg,
slow moving storms (0-6km mean winds of just around 5 kts to the
north) could become more organized leading to a longer duration of
heavy rainfall rates of 0.5-1"/hr. The total coverage is still
somewhat uncertain so the inherited MRGL risk still appears
appropriate, but this was expanded north to match recent guidance,
and convection is likely to be more widespread Thursday than it was
on Wednesday.

..Upper Mississippi Valley...
A warm front will lift northeast towards Minnesota today,
accompanied by a surge of 850mb winds to 20-30 kts advecting
anomalous PWs of 1.5 to 1.75 inches northeastward (+1 to +1.5
standard deviations above the climo mean). This front will be
driven eastward by a potent but flat and fast- moving shortwave
embedded within the nearly zonal flow aloft, leading to a period
of enhanced ascent aided by the warm front and accompanying
isentropic lift of the 850mb wind. This will result in scattered
thunderstorms across the area, with bulk shear of 25-35 kts
potentially supporting some organization into clusters. The
guidance is highly variable in both the placement and intensity of
convection today, and the QPF footprint and ensemble probabilities
suffer as a result. Still, with the impressive thermodynamics in
place, thunderstorms that develop could support rainfall rates of
1-2"/hr with at least short-term training possible, and the
inherited MRGL risk was curtailed for latest guidance but continued.

...Pacific Northwest...
A rare August atmospheric river (AR) is progged to make landfall
this evening north of 45N into WA state with IVT above 500 kg/m/s
as reflected by both GEFS and ECENS probabilities above 90%. A weak
shortwave embedded within otherwise zonal flow aloft will help
produce ascent into this plume of IVT, but instability will be
negligible and jet level ascent appears to be displaced north of
the core of the AR. While deterministic model QPF output and
accompanying ensembles support a threat for heavy rainfall,
especially in the Olympics, this will be occurring atop extremely
dry soils and should generally be beneficial rainfall.
Additionally, in the AR/ERO verification data set since 2019, there
has never been an impactful (MRGL risk or higher) AR in the month
of August. For this reason, despite some heavy rain spreading
onshore this evening, no excessive rainfall impacts are expected.

Weiss
$$
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FOUS30 KWBC 210719
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
319 AM EDT Thu Aug 21 2025

Day 1 Valid 12Z Thu Aug 21 2025 - 12Z Fri Aug 22 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST, SOUTH-CENTRAL TEXAS, SOUTHERN APPALACHIANS, EASTERN
NORTH CAROLINA, UPPER MIDWEST, AND SOUTHWEST U.S...

...Southwest...

Elevated monsoonal moisture lingering under the western ridge will
lead to yet another afternoon of scattered convective development
across much of the desert Southwest and accompanying terrain.
PWATs running slightly above normal across the region will provide
just enough deep layer moisture to couple with relevant diurnal
buoyancy to account for periods of heavy rainfall in some of the
stronger convective cores that materialize Thursday afternoon into
the evening. Totals generally will fall <1" over the Southwest when
looking at areal averages, however some 1-2" totals are certainly
plausible in the setup with the best threat for impacts likely in
those slot canyon areas, drainage across complex terrain into
adjacent valleys, and remnant burn scar locations. 00z HREF
neighborhood probs for >1" are running between 30-45% across the
central and eastern Mogollon Rim, and across the southern extent of
the Sangre de Cristos in NM. These are the areas with the best
flash flood potential given the anticipated convective pattern and
topographic challenges that accompany these locales. The previous
MRGL forecast was maintained with little to no adjustments.

...Texas through the Southeast and Southern Appalachians...

Our slowly progressing front is currently analyzed across the
Lower Mississippi Valley back into the southern Ohio Valley with a
general south to southeast progression over the past 24 hrs. Cold
front will continue to wander further south with yet another day
of convective firing near and along the front as it migrates
through south-central TX and towards the Gulf coast with an
elongated surface trough running parallel to the immediate Gulf
coast from TX to FL/GA. The front will arc back into the Southern
Appalachians where scattered thunderstorm activity is likely to
occur for at least one more day in the terrain with the northern
extent extending up into Southern WV and Southwest VA. The signal
is still scattered in terms of convective coverage and flash flood
prospects leaning towards isolated to perhaps scattered when
assessing the HREF probs for hourly rates and FFG exceedance probs
in the 12-00z window Thu/Fri. The strongest signature for heavy
rain concerns lies within the ridges and eastern slopes of the
Southern Appalachians across western NC down into the far western
SC escarpment. 3hr and 6hr FFG exceedance probabilities between
35-50% exist over the above area with the time frame of interest
falling in that afternoon and early evening window where diurnal
instability maximum and convective temp breach overlap.

Convection will likely dissipate slowly after sunset with the loss
of diurnal heating and surface decoupling putting a cap on the
threat. Totals between 3-5" are most common among some of the
stronger cores in guidance, aligning well with the modest >3" probs
and low-end >5" probs within the latest HREF iteration. Despite the
totals in question, the flash flood threat will likely be more
comprised of 3 and 6hr FFG exceedance compared to lone hourly
rates, however some instantaneous rates upwards of 2-3"/hr in the
terrain could spur some flash flood hazards, especially as storm
motions remain on the weaker side and generally parallel to the
topography. The previous MRGL risk was maintained given the
signatures across the South and Appalachian front. Higher FFG's
across the above regions were the main deterrent for a higher
risk, but will be monitoring the threat closely to see if a
targeted upgrade is necessary.

...Northern Plains and Upper Midwest...

An energetic upper-level shortwave is forecast to move east across
the northern Plains and Upper Midwest later this afternoon through
the overnight. A strong cold front trailing a strengthening
surface low over southern Canada will aid in surface based forcing
to couple with the improving mid and upper dynamics as the trough
migrates along the Canadian border. Sufficient buoyancy and
moisture anomalies of 1-2 deviations above normal will exist along
and ahead of the front coinciding with a strong, yet progressive
line of thunderstorms that will be capable of isolated flash flood
prospects thanks to hourly rates between 1-2"/hr expected. Trends
of an areal maximum across eastern SD have gained traction within
the latest CAMs and matches the global ensemble means and EC AIFS
Ensemble probs being highest in-of that region. This correlates
with the stronger mid-level vorticity advection entering the area
to couple with the best low-level instability axis along the
southern edge of the front. The previous MRGL risk was generally
maintained with some minor trimming on the eastern edge to account
for lower potential downstream given timing of convective
advancement among the hi-res.

Kleebauer
$$
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FOUS30 KWBC 260802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Tue Aug 26 2025

Day 1
Valid 12Z Tue Aug 26 2025 - 12Z Wed Aug 27 2025

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN ARIZONA, THE NORTHERN GREAT BASIN, THE SIERRA NEVADA, THE
CENTRAL ROCKIES AND THE SOUTHERN PLAINS...

...The West...

The sprawling upper ridge over the Southwest continues to funnel
rich monsoonal moisture northward through the Intermountain West.
ECMWF guidance shows PWATs in excess of 1" advancing as far north
as the Blue Mountains and Columbia River Valley of WA/OR. Some
PWATs are also expected to top 1.25" in portions of the Great Basin
and Snake River Valley. Helping to add some upper-level support
aloft will be the presence of a meandering 700mb low positioned
over the Pacific Northwest and a healthy diffluent pattern at 250mb
thanks to a longwave trough situated along the West Coast. With
such high moisture content, not much in the way of surface based
heating will be needed to spark widespread thunderstorms Tuesday
afternoon and through Tuesday evening. Combined with MUCAPE between
500-1,000 J/kg, thunderstorms will be capable of generating up to
1.5"/hr rainfall rates, which will pose a flash flood threat in
areas of complex terrain, burn scars, and nearby creeks and
streams. The Slight Risk over the northern Great Basin and
portions of the Snake River Valley remains in place. The Slight
Risk area resides within the strongest signal from the ECMWF-EFI
with 0.8-0.9 values, which give support to highly unusual QPF
totals and potential impacts for this time of year. A Slight Risk
was also introduced over the central Sierra Nevada given the
lingering threat for flash flooding over the rugged terrain that is
growing increasingly more saturated and sensitive.

...Southern Arizona...

Fresh off a busy day of thunderstorms on Monday, there is the
possibility that the atmosphere is overworked to where converage of thunderstorms is not as expansive as Monday. However, southeast AZ
was not as active on Monday and should tap into modest instability
(500-1,000 J/kg of MUCAPE) and moisture aloft (PWATs up to 1.5" in
some cases). This continues to support rainfall rates up to 2"/hr
in the more intense storms. While storms will remain more pulse-
like in nature, these potent storms still pose a flash flood
threat, especially in dry washes and in spots that drain poorly.

...Central & Southern Rockies...

Guidance across the board shows a higher concentration of
atmospheric moisture over the region on Tuesday compared to Monday,
and that is notable considering PWATs were already above the 97.5 climatological percentile on Monday. PWATs in the tallest peaks
will generally hover near 1" at their highest values, but along and
just east of the Front Range, PWATs are likely to range between
1.3-1.5". These values are surpassing the 99th climatological
percentile while also having 250-500 J/kg of MLCAPE present. With
soils steadily growing more and more saturated due to recent heavy
rainfall and the favorable atmospheric parameters in place, more
instances of flash flooding are expected with complex terrain and
burn scars more at-risk.

...Southern Plains...

A nearby frontal boundary will be responsible for triggering
thunderstorms this morning in portions of central OK. Following a
brief lull in the action this afternoon, a rejuvenated LLJ will
cause low-lvel WAA and additional storms to fire along the warm
front over the OK/TX Panhandles. PWATs are expected to top 1.8"
(also above the 90th climatological percentile) throughout the day
and into Tuesday night, while some meager MUCAPE values topping out
around 500 J/kg are expected. The 00Z HREF guidance shows low-
chance probabilities (20-30%) for rainfall >2" across central and
southern OK, but soils have been gradually moistening following
recent days of heavy rainfall in the area. Given these factors, the
inherited Slight Risk remains in place with instances of flash
flooding most likely to occur in poor drainage areas and locations
with more sensitive soils.

...Western Florida Peninsula...

A similar setup to Monday is expected to produce thunderstorms
capable of producing torrential downpours over the heavily
urbanized western Florida Peninsula today. An approaching cold
front will act to trigger thunderstorms in an atmospheric
environment that contains up to 2.25" PWATs and MLCAPE around
1,000 J/kg. Surface-500mb mean winds remain fairly uniform out of
the WSW, so there is the potential for training thunderstorms
around the Tampa-St. Pete metro area. The 00Z HREF shows moderate-
to-high chances (50-70%) for 24-hour rainfall totals >3" and low
chances (10-30%) for 24-hour rainfall totals >5". The inherited
Marginal Risk remains on track with the more urbanized areas
containing the greatest concentration of hydrophobic surfaces most
at-risk for flash flooding.

Mullinax
$$
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FOUS30 KWBC 160808
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Tue Sep 16 2025

Day 1 Valid 12Z Tue Sep 16 2025 - 12Z Wed Sep 17 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHEASTERN NORTH CAROLINA AND SOUTHEASTERN VIRGINIA...

...Northeastern North Carolina and Southeast Virginia...

Well-defined surface reflection off the NC coast will continue to
slowly move to the north over the course of the overnight and
morning hours before parking off the VA Capes as the initial
occlusion process takes shape. There's been a bit of a deviation
in the overall precip pattern from what has been forecast over the
past few forecast cycles, a lot of it driven from complex
convective dynamics that thwarted the initial low-level moisture
surge that was supposed to have already occurred according to older
forecasts. The previous convective cluster has since dissipated,
indicated via the rapidly warming cloud tops located just to the
north of the surface low center. As a result, have finally seen
bands of precip rotate inland with relatively modest rates as the
instability pool remains parked just offshore when assessing the
latest mesoanalysis. The trend is for a slow advection of
increasingly favorable instability to penetrate inland with the
alignment of the greatest forecast MUCAPE to be situated over far
northeast NC into the VA Tidewater. This is the area of interest
for the D1 when it comes to potential slow-moving convective
elements within the smaller deformation axis transposed on the
western flank of the cyclone. Some of the CAMs have been way too
aggressive with this feature and has allowed the HREF probs fields
to be heavily skewed into a more robust precip depiction compared
to what is current ground truth.

The HRRR/RAP combo has been handling the current evolution the best
of any CAMs member, and has certainly been outperforming most
global deterministic with the CMC the closest to what is occurring.
Considering the variables and complexity of the forecast, there
were some changes from the inherited SLGT risk across NC where the
western extension being removed allowing only the northeastern
corner of NC up into the VA Tidewater the only remaining locations
within the SLGT. This is subject to change pending on the
instability advection regime as rates will need to pick up in order
to secure a significant flash flood threat with the Hampton
Roads/VA Beach area the most prone to the risk due to urbanization
factors. Anticipating 2-4" with perhaps up to 5" within the
targeted SLGT risk as this remains the area with the best chance to
see heavy rain longevity with less of a threat the further away
from the coastal plain. Hourly rates maxed between 1-2"/hr are the
most likely scenario at this juncture, putting mainly a small area
of potential for flash flooding as sandier soils located outside
the VA Tidewater are likely to negate widespread flash flood
concerns. Considering how poorly the guidance is handling this
setup, near term monitoring will be employed over the course of the
period with the best threat likely occurring between 12-00z,
waning quickly in the evening as the low becomes vertically stacked
and favors a rapid occlusion leading to a degrading heavy precip threat.

...Southwest...

Scattered convective pattern will initiate later this afternoon
and the early evening hours before fading with the loss of diurnal
heating. Current progs are for convection to be tied closer to the
terrain across southeast AZ into southern NM with isolated
signatures of >1"/hr plausible in the stronger cell cores. These
areas generally prone to runoff due to terrain complexities, as
well as remnant burn scars present in some locations. There was
very little change from the inherited MRGL risk with the forecast
running close to general continuity.

...Central High Plains...

Strong shortwave trough ejection out of the interior Northwest will
lead to convective development and regional enhancement across the
Central High Plains. Greatest ascent is aligned with the diffluent
region of the mid-level shortwave with left-exit region dynamics
likely to maintain a prevalent heavy rain signature across the
western third of NE down into neighboring northeast CO and
northwest KS. Guidance has come into alignment with the axis of
heaviest precip centered near and just north of North Platte, NE
with the corridor extending south into southwestern NE and
northwest KS by the second half of the forecast period. 00z HREF
neighborhood probs for >3" are running between 20-40% across the
referenced area near North Platte and points south. This still
aligns well with the previous forecast MRGL risk leading to little
deviation in the current thinking. The areas of highest risk will
likely be smaller towns and urbanized areas, like North Platte that
are more susceptible to runoff.

Kleebauer
$$
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