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FOUS30 KWBC 181600
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Sat Jul 18 2026

Day 1
Valid 16Z Sat Jul 18 2026 - 12Z Sun Jul 19 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
WEST TEXAS THROUGH THE EASTERN GREAT BASIN, AS WELL AS PARTS OF THE
NORTHEAST AND MID-ATLANTIC...

...Western Texas to the Four Corners/Great Basin...
Monsoon ridge will become favorably aligned to draw impressive
thermodynamics northward from Mexico, reaching from West Texas
through much of the Desert Southwest and then northward into the
Great Basin and Intermountain West. Multiple weak shortwaves
embedded within the flow will continue to progress anti-
cyclonically around this ridge, bringing locally enhanced ascent
to the region, with a few of these impulses noted on morning WV
imagery in TX and NM. While any of these impulses will locally
enhance ascent leading to clusters of more organized convection
today, the greatest risk is just the breadth of coverage of
thunderstorms with potentially heavy rainfall today.

PWs this morning are impressive across the region, measured via GPS
and morning RAOBs vary from around 1.2 inches at SLC to as high as
2.1 inches at PHX, at or above the 90th percentile across a large
area (and as high as +3 sigma), and that measured PW at PHX this
morning is a daily record. At the same time instability is progged
to climb to 1000-2000 J/kg, locally even higher, as far as 40N
latitude in the Four Corners, which is impressive and is a typical
discriminator of more widespread monsoonal heavy rain/flash flood
days.

THe CAMs, including the UA WRF, are aggressive with coverage of
their simulated reflectivity across much of the area today, with
hourly rainfall potentially reaching 1.5 to 2.5 inches. This could
easily exceed the 1-hr FFG across the region, and in some areas the
3-hr FFG even has a high chance (40-60%) of being exceeded where
terrain-tying or boundary-collisions can cause a temporal extension
of the heavy rain rates. With storm motions generally expected to
be weak at 5-10 kts and chaotic, an active day is expected, with
scattered flash flooding expected, especially across vulnerable
features like burn scars, slot canyons, and urban areas.

The inherited SLGT risk was modified to account for recent
guidance, and there was some consideration of an increased risk
based on the impressive REFS exceedance probabilities which rise
above 55% for FFG, but the more muted HREF was used more heavily,
especially since the highest FFG exceedance is across the higher
elevations of the Mogollon Rim and not down into the deserts
(although an expansion SW into some lower terrain was made with
this update).

...Midwest to the Mid Atlantic and Northeast...
A cold front will drop steadily southeast from the Great Lakes,
reaching the lower Ohio Valley and the Mid-Atlantic by Sunday
morning. Ahead of this front, impressive moisture will pool from
Kentucky eastward to Long Island and Cape Cod, where PWs are
already 1.75 to 2.0 inches and will climb above 2.0 inches through
this aftn. The overlap of this elevated PW with increasing SBCAPE
(2000-3000 J/kg forecast) will create robust thermodynamics which
will support widespread convection capable of producing intense
hourly rainfall for which the HREF and REFS suggest has a 40-60%
(10-15% chance) of exceeding 1 inch (2 inches).

The excessive rainfall risk will steadily increase through the aftn
and continue into this evening/tonight as multiple rounds of
thunderstorms develop and track across the area. 0-6km bulk shear is
progged to increase to 30-40 kts ahead of the cold front, and as
weak impulses move west to east from the Ohio Valley into the Mid-
Atlantic, this will allow for some organization into clusters
tracking parallel to the front (and pre-frontal trough). Ahead of
the front, the Corfidi vectors and mean flow will align from the
west, leading to an increased backbuilding potential into the
higher thermodynamics with cells then tracking across the same
areas of the Mid-Atlantic states and into southern New England.
This will occur ahead of the front which will produce a second
round of heavy rainfall potential, albeit with more progressive NW
to SE motion to the cells.

The inherited SLGT risk has been adjusted to account for new
guidance, and due to elevated probabilities from both the HREF and
REFS (40-60% for 3"/24hrs, 10-15% for 5"/24hrs) atop sensitive
areas due to urban development and recent rainfall producing compromised
FFG. This included connecting the SLGT risks across PA where the
ARW guidance is most aggressive and there is sufficient potential
for some training (or meso-cyclones to enhance rainfall rates) to
include a larger area. Considered an elevated internal SLGT risk
for the Tri-State area as well, but enough uncertainty in position
and timing of redevelopment after CAPE exhaustion this morning
prevented that category increase.

...Florida Panhandle through Southwest Georgia...
Deep moisture spreading north ahead of a slow-moving mid-to-upper
level low will pool along a surface trough extending out of the
eastern Gulf. This moisture (PWs increasing to above 2 inches)
will fuel slow-moving storms as Corfidi vectors become aligned
anti-parallel to weak 0-6km mean winds of just 5-10 kts. With rain
rates possibly (20-40%) exceeding 2"/hr at times this could create
at least isolated instances of excessive rainfall leading to flash
flooding.

...Western Florida Peninsula...
A convective MCV positioned west of the Florida Panhandle across
the northeast Gulf will drift westward today within a pool of
tropical PWs nearing 2.25 inches. As this feature drifts slowly
westward, it has, at least this morning, temporarily stalled inflow
along the Florida west coast between Naples and Fort Meyers. The
high-res CAMs are not extremely aggressive with rainfall today,
but have been under-forecasting the current activity (MRMS already
2-4" in some areas with hourly rain rates of 2-3"). If this band
continues to stall, continued impressive rainfall rates are likely,
and the recent RRFS, HRRR, and some of the experimental REFS
members show an additional 2-4" of rain are possible. This has
resulted in a narrow MRGL risk addition which aligns well with
recently issued MPD #764.

Weiss


Day 2
Valid 12Z Sun Jul 19 2026 - 12Z Mon Jul 20 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS THE
SOUTHWESTERN U.S. FROM WEST TEXAS TO SOUTHERN UTAH...

...Southwestern U.S....
The overall synoptic configuration remains largely unchanged from
the previous period, with a persistent monsoon pattern locked in
place -- keeping scattered to widespread convection in the forecast
from West Texas into the Great Basin and southern Rockies. Given
that PW anomalies remain notably high at 2-3 std dev above normal,
and soil conditions moist from prior rainfall, confidence was
sufficient to introduce a Slight Risk that extends from West Texas
northeastward across southern New Mexico, central and northern
Arizona, into southern Utah.

An upper-level high centered over northern Utah and Colorado will
support steady easterly flow across the southern tier, effectively
tapping into and beginning to pull mid-level energy that has been
centered over West Texas along this deeper moisture axis. The
highest areal average precipitation amounts are expected to center
along the higher terrain, where slow-moving storms are expected to
develop with afternoon heating, before propagating off of the
terrain along convective outflows. Given sensitive, moist soil
conditions from successive days of widespread storms, much of this
area remains vulnerable to rapid runoff. Complex terrain, slot
canyons, and recent wildfire burn scars will further accentuate the
threat for flash flooding across parts of the region.

...Mid Mississippi Valley east to the Carolinas and south to the
Eastern Gulf Coast...
A more expansive Marginal Risk has been drawn for this period,
reflecting deep moisture atmospheric profiles and an increasing
potential for localized heavy rainfall amounts producing isolated
runoff concerns. This risk is centered along a cold front dropping
into North Carolina and the Tennessee Valley, while is begins to
slow over the lower Ohio and mid Mississippi valleys. Diurnal
heating and prefrontal convergence will tap into this moisture
pool, generating clusters of slow-moving efficient thunderstorms,
capable of producing 1-2 inch totals, with isolated higher amounts.
There remains a greater threat for more widespread heavier amounts
along the eastern extent of this area, centered over southeastern
Virginia and eastern North Carolina, where more defined upper
forcing will help accentuate the threat for amounts over 3 inches.

Additionally, the Marginal Risk has been extended southward down to
the Gulf Coast. This accounts for the deep tropical moisture
arriving ahead of the slow-moving disturbance lingering in the
eastern Gulf, as well as favorable low-level convergence focusing
along a slow-moving surface trough extending north from the Gulf.
Highly efficient warm-rain processes along these features will
maintain a threat for heavy, short-term duration totals and
isolated runoff concerns.

Pereira


Day 3
Valid 12Z Mon Jul 20 2026 - 12Z Tue Jul 21 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ARIZONA AND NEW MEXICO...

...Southwest...
The primary adjustment for this forecast cycle is the introduction
of a Slight Risk across portions of New Mexico and Arizona. For
parts of the region, the setup remains much the same, with
anomalous moisture (2+ std dev above normal) supporting convective
activity and rainfall efficiency.

Mid-to-upper level energy/an inverted trough drifting east across
the region will provide larger-scale ascent to help better
organize monsoon convection. Furthermore, this feature will help
steer and push the convective and heavy rainfall threat a bit
farther west across portions of southeastern Arizona than in the
previous days. Given the combination of persistent anomalous
moisture, enhanced upper-level support, and sensitive soils,
expect localized rainfall rates to easily raise runoff concerns
across parts of the region. A surrounding broad Marginal Risk
remains in place extending up into the Great Basin and central
Rockies -- outlining the risk for less-organized storms, producing
isolated runoff concerns in those area.

...Upper Great Lakes...
A well-defined shortwave interacting with deepening moisture is
expected to produce strong storms capable of producing heavy
rainfall across the region. Guidance shows southwesterly to
easterly inflow supporting PWs increasing to around 2 inches. This
moisture along with strong ascent, afforded in part by favorable
upper jet dynamics, will fuel the potential for heavy rainfall-
producing storms. Training storms along the front may raise the
threat for heavy amounts and flash flooding concerns. Models differ
on the details, but there is some consensus for the heaviest
amounts centering from southern Wisconsin to northern Illinois,
where an upgrade to a Slight may be introduced in a future
issuance, if the trends hold.

...Eastern North Carolina...
A Marginal Risk was maintained where a front and coinciding pool of
deep moisture (PWs at or above 2 inches) are expected to linger,
extending the threat for heavy rainfall and possible flash flooding
into a second day.

...Florida Gulf Coast...
It remains uncertain how the disturbance in the eastern Gulf will
evolve and track. However, regardless of development, it is
expected that heavy rainfall will remain a threat for parts of the
Gulf Coast. Therefore, a Marginal Risk was maintained from the
Tampa Bay region to the Panhandle.

Pereira


Day 4 and Day 5
Valid 12Z Tue Jul 21 2026 - 12Z Thu Jul 23 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHWEST AS WELL AS THE CENTRAL APPALACHIANS ON TUESDAY...

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN NORTH
CAROLINA AND SOUTHEASTERN VIRGINIA ON WEDNESDAY...

...Southwest...

The Southwest Monsoon will remain in full force across Arizona and
into Utah on Tuesday. Moisture anomalies especially to the north
across Utah could approach 5 sigma above normal for this time of
year, while into Arizona, PWATs to 2 inches will be a solid 3 sigma
above normal. With daytime heating, renewed rounds of thunderstorms
are expected to redevelop for the latest in the multi-afternoon
pattern. Soil moisture levels are already saturated over much of
Arizona and into southern Utah. Expect with the thermal low peaking
in intensity on Tuesday, that flash flooding concerns will become
more widespread as rains impact beyond the 4 Corners into Nevada,
Idaho, and Wyoming. A Slight Risk upgrade was introduced with this
update, and it`s possible localized areas may see further upgrades
as Tuesday approaches. By Wednesday, ridging over the Southwest
will push the greater threats for thunderstorms further north into
the Intermountain West, with a low developing over the Plains
allowing for some of the moisture to overspread into portions of
the central Plains. The signal for heavy rain lessens on Wednesday,
so the threat drops to Marginal for now, though a Slight may be
needed into the Intermountain West with continued consistency in
the guidance.

...Central Appalachians...

A Slight Risk upgrade was also introduced for portions of the
Central Appalachians, particularly across West Virginia and into
southwest Pennsylvania. Abundant moisture across the Mid-Atlantic
will interact with a strong cold front pushing southwest across
Ohio during the afternoon. This will allow for numerous
thunderstorms to develop Tuesday afternoon. Upslope flow into the
flash flood prone mountains of West Virginia and Pennsylvania will
support additional thunderstorm development as compared with areas
further west where the front will clear the moisture, and further
east where there will be less forcing. PWATs approaching 2 inches
will also support storms capable of multiple inch per hour rainfall
rates, especially as they run into the mountains.

To the east across the Mid-Atlantic, moisture levels further
increase, with some areas perhaps reaching 2.25 inch PWATs Tuesday
afternoon. The forcing will be a bit more uncertain, but any storms
that form will be capable of very heavy rains. The I-95 corridor
from DC to NYC will have to be closely monitored as these heavily
populated and flood prone areas may have enough widespread activity
to need further upgrades.

...Tidewater of North Carolina and Virginia...

On Day 5/Wednesday, the same cold front that swings through the
Midwest, Northeast, and Mid-Atlantic on Tuesday slows down its
forward speed, perhaps even stalling to the west of the Tidewater
of North Carolina and Virginia. Plentiful tropical moisture
continues riding northeastward along that front, providing ample
opportunity for numerous training showers and thunderstorms to
develop over the region, likely moving out over the Atlantic off
Ocean City, Maryland and points north. Thus, the focus will be in
the highly urbanized Hampton Roads area, extending into much of the
coastal plain of North Carolina. Given the pretty impressive signal
for heavy rain, have started out the Day 5 ERO with a Slight Risk
in this region. There remains some uncertainty as to whether any
tropical development over the northeastern Gulf contributes to the
heavy rainfall expected in this region on Wednesday. Given the
signals for heavy rain expected Sunday/Monday over eastern North
Carolina, expect the soils by Wednesday to be significantly more
saturated than they are now, contributing to the heightened flash
flooding risk.

Wegman


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



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