FOUS30 KWBC 270820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Mon Jul 27 2026

Day 1  Valid 12Z Mon Jul 27 2026 - 12Z Tue Jul 28 2026

...THERE ARE MARGINAL RISK OF EXCESSIVE RAINFALL AREAS ACROSS THE
FOUR CORNERS STATES ONTO THE CENTRAL PLAINS AND GREAT LAKES/OHIO
VALLEY INTO THE NORTHEAST...

...Four Corners States onto the Central Plains...
The center of an upper high will shift southward into New Mexico...which
will direct more ridge topping diurnally- driven activity from the
central Rockies onto the central Plains later today and through
much of the upcoming night. Favored focus areas for QPF are again
along the Mogollon Rim in Arizona north/east into Colorado. Only a
minor expansion to the Marginal Risk is noted with the HREF and
RRFS which would favor the central Plains activity into
southwestern South Dakota.

...Great Lakes and Ohio Valley into the Northeast US...
A shortwave that helped trigger convection over parts of the
Minnesota Arrowhead and nearby portions of northwestern Wisconsin
overnight leading into the the start of the Day 1 period at 27/12Z
will continue to shift towards the southeast across portions of the Upper
Great Lakes this afternoon and evening. This is in northwest flow
downstream of the upper level ridge axis stalled over the western
Dakotas. Increasingly difluent flow ahead of the shortwave will
lead to the consolidation and the strengthening of a low pressure
system later tonight...which in turn increases the forcing for
heavier rainfall over the Great Lakes/Ohio Valley. No major changes
were made to the Marginal Risk area,

...Tampa Bay...
Troughing occurs over the western side of the Florida peninsula
which shifts the focus for repeating thunderstorm with locally
heavy rainfall towards the greater Tampa Bay from later this
morning into the afternoon.  Monday morning into the afternoon.
Consensus average QPF is 2-4" with potential for 6" from both the
HREF and RRFS. A Marginal Risk for flash flooding was maintained
especially due to urbanized drainage concerns.

Bann/Jackson
 

Day 2  Valid 12Z Tue Jul 28 2026 - 12Z Wed Jul 29 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
NORTHERN MID-ATLANTIC INTO SOUTHERN NEW ENGLAND...

...Southern Appalachians into New England including northern Mid-Atlantic...

The amplification of a mid- and upper-level trough moving out of
the Great Lakes/Ohio Valley on Monday leads to the development of
surface low pressure over the eastern Great Lakes by the start of
the Day 2 period. However...the low stalls over interior NY/PA on
Tuesday. There is has been growing certainly on the interior
position of this low which then maintains the warm sector over the
northern Mid-Atlantic coast into southern New England. Precipitable
water values of 2 inches is highlighted through this warm sector
which should also have sufficient instability for higher rainfall
rates along with divergence aloft to promote growth and repeating
activity. A Slight Risk remains over the northern Mid-Atlantic
into southern New England with additional focus now across parts of
northern New Jersey into southern New York (inland of New York
City) into western Connecticut considered to be higher-end Slight.

Elsewhere, warm sector moisture and instability is present down
the Mid-Atlantic through North Carolina with potential for south
moving activity to repeat from reinforcing shortwaves rounding the
new low ot the north.

...Four Corners States across the Central Plains...
Further suppression of the upper high center south through New
Mexico opens the opportunity for additional downstream activity
from the Four Corners states across the central Plains. The
Marginal Risk was expanded east to the KC metro. The RRFS still
highlighted the High Plains of both northeast and southeast
Colorado with more typical Monsoonal activity over terrain in the
Four Corners states.

Bann/Jackson
 

Day 3  Valid 12Z Wed Jul 29 2026 - 12Z Thu Jul 30 2026

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

...Northeast...
A dynamic coastal system takes earlier in the week along/near the
New England Coast per model guidance. Strong upper level jet
energy upstream diving toward the trough base will amplify the
upper trough -- to the tune of 2+ standard deviations below late
July means at 500 mb-- which enhances the downstream QG forcing as
it encounters deep moisture at low- to mid-levels being drawn in
from the Atlantic Gulf stream, with a swath of ~1.75" PWs. As
previously noted...given the persistence of favorable cool season
dynamics with warm season thermodynamics (high deep- layer
TPW/theta-e values), have maintained the Slight Risk area pretty
much unchanged across parts of the Northeast, which fits the most
recent global guidance rather well.

Operational guidance shows 2-4 inches of rainfall across the
Slight Risk area from Wednesday into late Wednesday night/early
Thursday morning. Frontogenesis of the comma head could on its own
lead to efficient heavy rainfall; not much MU/ML CAPE will be
necessary to lead to flash flooding concerns.

...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. The Marginal Risk area here only
needed some cosmetic changes to better fit the most recent QPF
guidance but the differences did not reflect a change in the
forecast reasoning.

...Central Gulf Coast Region into parts of the Southeast...
Numerical guidance continued to show heavy...to potentially
excessive...rainfall being forced/focused by a cold front
associated with the cyclone impacting New England. Still little
reason to question whether the moisture will be ample at the same
time that the effective bulk shear is sufficient for convective
organization. The inherited Marginal Risk areas for Wednesday into
Thursday night saw only a few cosmetic changes to better fit the
most recent QPF guidance.

Bann/Roth
 

Day 4 and Day 5  Valid 12Z Thu Jul 30 2026 - 12Z Sat Aug 01 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER MISSOURI AND MID TO UPPER MISSISSIPPI VALLEY...

...Lower Missouri Valley into the Mid-Upper Mississippi Valley...
On Day 4 (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. There's near to
above average discrepancy amongst the 00Z global guidance. The GFS/
GEFS mean/AIGFS are a bit farther east compared to the ECMWF/ECENS
mean/EC AIFS, with the CMC camp basically in the middle in terms of
eastward progression. In addition, there remains latitudinal
differences with the heaviest QPF, with the 00Z UKMet the farthest
north. Given the MCS/upscale growth potential with this system,
considering the influx of 2-2.25+ PWs per the guidance,
concentrated areas of heavy rain are expected, especially across
the Corn Belt given the deep-layer moisture profile and high
surface dewpoints. Certainly an event that would warrant a Slight
Risk, which for now we've confined across eastern SD, far eastern
NE and northeast KS, into northwest MO, western 2/3 of Iowa, and
southern MN, where the global ensemble probabilities of >3" in
12/24 hours are highest.

On Day 5 (Friday-Friday night), have essentially shifted the
Slight Risk area farther downstream (east-southeast) per the global
guidance consensus. Globals all show spotty heavy 24hr rainfall
totals, i.e. 3-5+ inches, though again with some spatial spread in
the guidance. ECENS QPF exceedance probabilities have come up from
the previous run, and are certainly higher compared to the day 4
period (more expanded area of 3 to 5 inch probabilities).

...Four Corners States into the Central/Northern Plains...
Monsoonal flow and anomalous deep-layer moisture will maintain a
Marginal (isolated) excessive rainfall risk over eastern AZ and
much of MN (except SE portion) into southeast UT and through much
of CO. The expanding mid/upper level ridge (600dm heights at 500
mb) will likely limit the coverage compared to recent days,
especially by Friday.

...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. The inherited Marginal Risk areas for Thursday into
Friday night saw only a few cosmetic changes to better fit the
most recent QPF guidance.

Hurley
$$
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