FOUS30 KWBC 260828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sun Jul 26 2026

Day 1  Valid 12Z Sun Jul 26 2026 - 12Z Mon Jul 27 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL
PORTIONS OF THE SOUTHEAST US...

...Southeast into Florida...
Above average moisture will remain over the southeastern U.S. with
low level flow near a lingering quasi-stationary front with
continuing to feed moisture into the area while poorly defined
waves of low pressure stretching across the base of an eastern
U.S. trough will focus/enhance the convection near coastal South
Carolina/Georgia. Given how slow the front has been to depart the
area...opted to extend yesterday's Slight Risk area into the new
Day 1 period.

A Marginal Risk area extends southward along much of the east
coast of Florida. There is abundant deep-layer moisture in place as
shown by the 90+ percentile values of precipitable water which
suggests cells should be efficient rainfall producers with
localized rainfall rates in excess of 3 inches possible and
rainfall totals generally 2 to 4 inches. Urban corridors can
increase sensitivity to high rainfall rates but overall a Marginal
Risk covers the threat of isolated flash flooding.

...Four Corners States onto the Central Plains...
Monsoonal flow from the Desert Southwest to the central Rockies
continues into early next week. Above average precipitable water
values and available instability will again support isolated to
widely scattered thunderstorms capable of producing intense rain
rates capable of producing flash flooding and a multiple weak
shortwaves to help focus the daily convection. Some signals remain in
the convective allowing models to focus the greatest coverage of
heavy rain into western/central Colorado. Held off on any Slight
Risk upgrade again given the uncertainties in the impacts.

...Upper Midwest-Upper Great Lakes-Ohio Valley...
Global, CAM, and AI guidance continue to show convectively-aided
shortwave activity riding down the top side of the upper ridge axis
in northwest flow. A northwest to southeast oriented stationary
front could also provide a convective focus as slightly above
normal PWs but very strong instability come in. Models continue to
show considerable spread in terms of where these ridge riders form
and track. The location of convection will be affected by
placement of upper level convergence associated with a broad
arching anticyclonic flow at the jet level. The potential for flash
flooding will be mitigated by the progressive nature of individual storms.

Bann/Tate
 

Day 2  Valid 12Z Mon Jul 27 2026 - 12Z Tue Jul 28 2026

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL MONDAY ACROSS
THE FOUR CORNERS STATES TO CENTRAL PLAINS AND GREAT LAKES/OHIO VALLEY...

...Four Corners States into the Central Plains...
The outlook changed little this cycle given the amount of run to
run continuity shown by the guidance. Elevated amounts of moisture
will linger over the Four Corners region given a persistent
monsoon pattern, although guidance still points to a gradual
southward shift in the placement of the upper high center. This
should result in a subtle shift in the location of the heaviest
rainfall potential but not enough to make more than nudges to the
Marginal Risk area in the Four Corners states. Circulation around
the northern periphery of the high will transport the moisture and
chance of showers and thunderstorms eastward into the central High
Plains. The ECMWF precipitable water plots have been pointing to
this for several runs so the possibility was already well covered.

...Great Lakes and Ohio Valley into the Northeast US...
By Monday a more potent shortwave will move southeastward through
the Upper Great Lakes region rounding the Rockies/High Plains high.
Increasingly difluent flow ahead of the shortwave will eventually
lead to the consolidation/strengthening of a low pressure system
during the period which in turn help increasing the forcing for
heavier rainfall over the Great Lakes/Ohio Valley. A Marginal Risk
remained in place.

Bann/Tate
 

Day 3  Valid 12Z Tue Jul 28 2026 - 12Z Wed Jul 29 2026

...Central Appalachians-Mid Atlantic through New England...
Introduced a Slight Risk area over portions of New York southward
into a portion of the Mid-Atlantic on Tuesday. Precipitable water
values over the region begin close to 2 inches as an area of low
pressure begins to take shape in response to the amplification of
an upper trough. Divergence aloft increases notably as the winds at
jet-stream level become increasingly difluent. Whenever favorable
(typical) warm-season thermodynamics are present ahead of a more
robust, cooler-season trough with strong deep-layer shear, chances
for more widespread convection increase. Although a Slight Risk
was introduced..a realignment or expansion is possible as the
evolution becomes more clear. The 26/00Z guidance still has a few
pieces of guidance that advertise 2-4" on Wednesday into Wednesday night.

...Four Corners States into the Central/Northern Plains...
Monsoonal flow of moisture, likely to be enhanced by Genevieve
moving offshore Baja California,looks to elevate moisture over the
Desert Southwest through the central Rockies and into the central
and northern Plains with time. The more suppressed ridge will
allow multiple mesoscale/convectively aided shortwaves to push over
the top/across the Rockies into the Plains.

...Southeast...
A continued threat for heavy rainfall lingers along the New
England cyclone's trailing front near the FL/GA border. Moisture
should be ample and there could be enough effective bulk shear for
convective organization. Since to some signal seen in the model
guidance in this region, maintained a Marginal Risk area.

Bann
 

Day 4 and Day 5  Valid 12Z Wed Jul 29 2026 - 12Z Fri Jul 31 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHEAST WEDNESDAY...

...Northeast...
Quite a dynamic coastal system taking shape Wed-Thu along the New
England Coast per the global models and ensembles. Strong upper
level jet energy upstream diving toward the trough base will
amplify the upper trough -- to the tune of 2+ standard deviations
at 500 mb per the latest GEFS -- which in-turn will enhance the
downstream QG forcing (upper diffluence/divergence and deep-layer
ascent). Strengthening southerly low-level flow will draw in
moisture-rich air off the Atlantic Gulf stream, with a swath of
1.8-2.0+ inch PWs within the deepening TROWAL pivoting around the
slow-moving comma head. Given the persistence of favorable "cool
season" dynamics with "warm season" thermodynamics (high deep-
layer TPW/theta-e values), feel confident in hoisting a Day 4
Slight Risk area across parts of the Northeast to include the Tri-
State area and southern New England.

Even without the benefit of high-res CAM runs, the operational
global runs (and several ensembles to boot) show areas of 3-4+
inches within 24hrs across the Slight Risk areas, while their
perspective ensemble probabilities of these QPF thresholds have
come up considerably with the 00Z 07/26 runs. Instability will
likely be capped at 1000 J/Kg if not lower ("tall/skinny" CAPE
profile); however considering the persistent deep-layer forcing and
moisture/TPW anomaly, the amount of CAPE or lack thereof should
not inhibit a Slight Risk for excessive rainfall within this area,
which is supported by the 00Z 7/26 CSU First-Guess ERO fields (UFVS
verified version). 12Z 7/26 ECMWF EFI noted a broadening area over
0.7 with a shift-of-tails to 1. Will be keeping an eye on these
trends over the next few days prior to this event, as an upgrade to
Moderate somewhere in/near the Sight Risk area will be possible.

...Four Corners States into the Central/Northern Plains...
Monsoonal flow, likely to be enhanced by Genevieve moving offshore
Baja California, brings elevated moisture over the Desert
Southwest through the central Rockies and into the central and
northern Plains. The more suppressed ridge will allow multiple
mesoscale/convectively aided shortwaves to push over the top/across
the Rockies into the Plains.

...Lower Missouri Valley into the Upper Mississippi Valley...
On Day 5 (Thursday-Thursday night), convectively-aided shortwave
energy progressing east from the central-northern Plains will
amplify a bit (dig a bit farther south) upon reaching the Lower
MO-Mid MS Valleys) during this time as the ~600DM 500mb high
becomes centered farther west toward the Four Corners. Still some
discrepancy amongst the global guidance (GFS most vigorous with the
deep-layer forcing and low-level moisture transport), however all
the models are trending towards a wetter solution with the
likelihood of an MCS/upscale convective growth within an area of
expanding PWs of 2-2.25+ inches. Could certainly see a bump up to
Slight Risk at some point, but for now given the latitudinal spread
in the guidance, have opted to go with a Marginal Risk in the D5
ERO for now.

...Central Gulf Coast Region into parts of the Southeast...
Heavy rainfall is expected along the New England cyclone's
trailing front near the FL/GA border. Moisture should be ample and
there could be enough effective bulk shear for convective
organization. Since to some signal seen in the model guidance in
this region, added a Marginal Risk area.

Hurley/Roth
$$
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