FOUS30 KWBC 240827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Fri Jul 24 2026

Day 1  Valid 12Z Fri Jul 24 2026 - 12Z Sat Jul 25 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST UNITED STATES...

...Southeast...

A stalled front will provide the focus for heavy to potentially
excessive rainfall across the Southeast United States with portions
of the Carolinas having the greatest risk for flash flooding.
Models continue to advertise a steady increase of moisture in the
850-500 mb layer, precipitable water values reaching 2.2 to 2.4
inches later today and deep-layer winds decreasing into the 10 to
15 knot range. This will all work to support slow moving
convection capable of producing very instantaneous heavy rainfall
rates and rainfall amounts this afternoon and tonight along a
weakening front which helps focus the activity. The 24/00Z HREF
and RRFS runs showed the potential for rainfall rates in excess of
3 inches per hour focused initially along coastal portions of South
Carolina and a small portion of adjacent North Carolina. So the
potential for locally significant flash flooding remains.

...Ozarks and Lower Missouri Valley...

Convection may be ongoing at the start of the period somewhere
across portions of eastern Kansas and Missouri. This activity
should taper off as the nocturnal low level jet tapers off.

There is more uncertainty about what happens later in the forecast
period, from late this afternoon into tonight, Most hi-res models
still suggest the convective cluster will progress southeast into
the Ozarks and potentially Mid- Mississippi Valley by 12Z Saturday.
However, they increasingly disagree on placement of the heaviest
rainfall, the convective evolution, and magnitude of the rainfall.

From a pattern recognition standpoint, there is some concern about
an increasingly favorable setup for a backbuilding MCS overnight. The
low-level jet will veer to be more westerly, and the 20-30 knot
low-level inflow will exceed the strength of the increasingly weak
15-20 knot deep layer mean flow. Synoptic evolution would favor
potential for backbuilding storms tonight, but that is contingent
on the convective evolution and whether a stronger, more
concentrated cold pool can develop.

Southwest US,,,
Moisture remains abundant in the Southwest US which should lead to
another round of convection developing in the afternoon and which
persists into the overnight hours. At this point...there appears
to be an isolated threat that rainfall rates or amounts lead to
flash flooding so a Marginal Risk was maintained.

Bann/Lamers
 

Day 2  Valid 12Z Sat Jul 25 2026 - 12Z Sun Jul 26 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE EASTERN
CAROLINAS AND FOR PORTIONS OF NEW MEXICO...

...Southeast through the Tennessee Valley...

The outlook for the Southeast was left fairly consistent with the
previous version with a Slight Risk in the eastern Carolinas, and a
broad Marginal Risk over the rest of the region in the general
vicinity of the lingering front. Hi-resolution guidance has become
more available and continues to place the heaviest rain farther
south. For the time being, the Marginal Risk was positioned in an
intermediate location and close to the corridor of greatest QPF and
along the axis of highest precipitable water values. This also
continued to align with the WPC deterministic forecast. However,
the positioning of the risk area may continue to evolve in future
outlook updates. Overall, the environment will still be favorable
for high rain rates in the most organized thunderstorms, with
precipitable water values still expected to be above 2 inches.

...Southwest...

The monsoon recharges and resets across the Four Corners region on
Saturday. A Slight Risk was introduced primarily across New Mexico
where available high resolution convective allowing models showed localized
heavy rainfall in the same area as elevated values approaching 20
percent on the 12Z CSU machine learning ERO probability guidance.
There is some concern about the potential for a MCV from late
tonight/early Saturday morning to become a player which elevates
the potential for flash flooding later in the day on Saturday. This
would be in addition to the precipitable water values that will
continue to be around or just above the 90th percentile for July,
and supportive of high rain rates in any thunderstorms that can develop.

Bann
 

Day 3  Valid 12Z Sun Jul 26 2026 - 12Z Mon Jul 27 2026

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS STATES, EXTENDING INTO THE CENTRAL PLAINS MONDAY AND OVER
THE GREAT LAKES, EXTENDING INTO THE NORTHEAST ON MONDAY...

...Four Corners States onto the Central Plains...

Monsoonal flow from the Desert Southwest to the central Rockies
continues into early next week. Precipitable water anomalies are
offset with the ECMWF farther west (New Mexico and Colorado as
compared to the GFS being located over Arizona and Nevada).
Maintained the Marginal Risks but will continue to note that
shortwave impulses coming up from Mexico could enhance precip and
warrant a Slight Risk. Was able to remove part of southwest WY
given confidence on anomalous moisture being contained south from
the Uinta Mtns.

...Upper Midwest-Great Lakes-Ohio Valley-Interior Northeast...

Global and AI guidance continue to show convectively-aided
shortwave activity riding down the top side of the upper ridge axis
over the western Dakota border in northwest flow. There was a bit
of a realignment made which focused on the shortwave crossing the
southern portion of Minnesota and Iowa moreso than the area farther
north. In doing so...the QPF tracks along a corridor of lower Flash
Flood Guidance that reaches 1.5 inches per 3 hours and tends to fit
the placement of maximum precipitable water values which peak at
or slightly greater than 1.75 inches. The convection should be
progressive enough to preclude the need for a Slight Risk at this point.

Bann
 

Day 4 and Day 5  Valid 12Z Mon Jul 27 2026 - 12Z Wed Jul 29 2026

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL MONDAY AND
TUESDAY ACROSS THE FOUR CORNERS STATES EXTENDING INTO THE CENTRAL
PLAINS, AND OVER THE EASTERN GREAT LAKES AND OHIO VALLEY EXTENDING
INTO THE NORTHEAST AND MID ATLANTIC...

...Four Corners States into the Central Plains...

Monsoonal flow from the Desert Southwest to the central Rockies and
central Plains continues into early next week. Based on the latest
(00Z) guidance, did not make many changes to yesterday's Day 5
Marginal Risk area. This also includes the latest ECENS/GEFS/CMCE
24 probabilities of 1-2+ inches in 24 hours.

For Day 5 (12Z Tue-12Z Wed), given the persistent pattern, the ERO
is essentially a carbon-copy of Day 4, with the eastern extension
across the Central Plains. This follows along with the boost in QPF
per the AIGFS and ECMWF AIFS, as both of these models show more
vigorous (convectively-induced or enhanced) shortwave activity
rounding overtop the mid/upper level ridge.

...Eastern Great Lakes-Ohio Valley-Interior Northeast...

Global and AI guidance continue to show convectively-aided
shortwave activity riding down the top side of the upper ridge axis
over the western Dakota border in northwest flow. Guidance still shows
these ridge crawlers becoming a bit better organized through the
day 4-5 period, with an uptick in deep-layer QG forcing (Qs convergence)
and increasing low-level moisture transport. By Day 5, strengthening
exit region forcing downwind of advancing upper level jet streak
will help amplify the longwave trough across the Great Lakes-OH
Valley and Northeast, with a decent surge in pre-frontal deep-layer
moisture as PWs climb between 1.75-2.00". Based on the QPF trends
from the global guidance (including AI), and given the progressive
forward speed of the embedded shortwaves, have essentially
propagated the Day 4 Marginal farther downstream into the Eastern
Seaboard from the Mid Atlantic to Northeast Tue into Wed. Given
the progressive nature of the synoptic features, relatively low
24hr probabilities of QPF>2" per the GEFS/CMCE/ECENS systems, and
current lackluster ECMWF EFI values, for now will start with a
broad Marginal Risk, though with the anticipation that at some
point (once in the short range) we may be able to target areas with
a notably higher ERO risk (i.e. Slight).

Hurley
$$
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