Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 PM EDT Thu Aug 6 2026

Day 1
Valid 01Z Fri Aug 07 2026 - 12Z Fri Aug 07 2026

...THERE IS A BROAD MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST THROUGH THE CENTRAL/SOUTHERN PLAINS, MIDWEST, AND EAST...

...Central Plains into the Lower Missouri Valley and Midwest..

0100Z Update: Any additional organized convection overnight along
and south of the quasi-stationary front is expected across
southeastern KS into northeast OK given the latest CAM trends,
including the 18Z HREF QPF exceedance probabilities. This given the
nocturnal uptick in the 925-850 mb flow (increase in 0-6km Bulk
shear), healthy deep-layer instability (MUCAPEs 2000-3000+ J/Kg),
and PWs approaching 1.75". However, given the degree of low-mid
level shear along/near this quasi-stationary surface front
(especially directional), strong northerly MBE or Corfidi Vectors
will allow for a fairly swift southerly motion of any organized
convective segments exhibiting upscale growth, thus limiting the
risk for cell training overnight. The latest HRRR simulated
reflectivity guidance supports this.

...Eastern U.S....

0100Z Update: A slow-moving frontal boundary expected to persist
as shortwave energy track east along the southern edge of the
Westerlies, helping to accentuate lift in the high PW axis in the
vicinity of this front. Moderate to locally heavy rain totals
expected with hourly rain amounts up to 2.5" possible, supporting
at least isolated runoff issues in urban areas and places with some
degree of soil saturation. The loss of peak heating however will
substantially reduce the risk (i.e. to "sub Marginal") after
03-05Z, given the weak 0-6km bulk shear and weak mid-level lapse
rates (~6 C/Km).

...Southwest/Southern High Plains...

Monsoonal convection will continue through this period thus the
Marginal Risk was maintained for much of the Southwest and Southern
Rockies. While there is continued low confidence with details,
locally heavy rains with hourly amounts up to 2" could lead to
isolated runoff issues.

Hurley


Day 2
Valid 12Z Fri Aug 07 2026 - 12Z Sat Aug 08 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE LOWER AND MID MISSISSIPPI VALLEY EAST AND NORTHEAST INTO THE
MID ATLANTIC AND NEW ENGLAND, ACROSS THE UPPER MISSISSIPPI VALLEY,
AND OVER THE SOUTHWEST...

21Z Update..

Very minor changes were needed for the risk areas. The main change
of note was a minor reduction to the MRGL area in central AR.
Latest run of CAMs guidance suggest a progressive line of
thunderstorms in the northern and central portions of AR, where
rainfall will be heavy but relatively short-lived. FFGs are also
high (~3"), and the progressive nature of the system should limit
flash flooding potential.

Blanco-Alcala

Previous Discussion...

...Lower Lakes, Ohio Valley into New England...

During this period a PW plume of 1.50-1.75 inches will be pooled
across much of the central states to New England within the
southwest flow sandwiched between the upper ridge over the Atlantic
and a broad mean trough over the Great Lakes and Ohio Valley
region. Shortwave energy in this southwest flow will help
accentuate lift in this high PW axis, supporting widespread
scattered convection across these areas. There may be better
organization to the convective activity across the Lower Lakes/Ohio
Valley region in the vicinity of a slow moving frontal boundary
expected to stretch west southwest to east northeast across this
area.

...Southern to Central Appalachians...

PW values in the 1.5 to 1.75 inches range will hold steady across
the region aiding in the local enhancement of the widespread
scattered convection. A broad Marginal Risk covers the Central to
Southern Appalachians where localized hourly rain amounts of 1-2"+
could lead to runoff issues, particularly in urban areas and within
areas of steep terrain.

..Southwest...

While there continues to be low confidence with QPF details,
locally heavy rains and isolated runoff issues will continue in the
axis of above average PW values expected across this area. A
Marginal Risk area remains in effect for much of Arizona and New
Mexico.

Campbell


Day 3
Valid 12Z Sat Aug 08 2026 - 12Z Sun Aug 09 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST, FROM PORTIONS OF THE PLAINS/MISSISSIPPI VALLEY
AND ACROSS THE LOWER LAKES AND INTO THE NORTHEAST AND ACROSS THE
CENTRAL TO SOUTHERN APPALACHIANS...

21Z Update..

A minor reduction of the MRGL Risk area was needed for Iowa and
northern Illinois as the QPF core becomes better defined. A
progressive convective system is forecast to move through Iowa
along the periphery of an upper ridge. PWAT anomalies, particularly
in the ECMWF, are quite high at around 3 standard deviations above
normal. SREF Spaghetti plots are also in agreement in a heavy
precipitation core located in central to southern Iowa. As such, a
targeted Slight upgrade may be possible in the area in future
updates, but due to global model disagreement in terms on the
location of the heaviest precipitation, a MRGL was maintained until
CAMs guidance becomes available.

Blanco-Alcala

...Lower Lakes/Upper OH Valley into northern New England...

A frontal boundary will persist along the southern edge of the
westerlies on the south side of a broad trof. Embedded shortwaves
in this broad cyclonic flow will accentuate lift in the vicinity of
this front where PW values are expected to rise to 1.50-1.75"
ahead of each set of shortwave energy. Height falls moving along
the southern edge of the westerlies will support widespread
scattered convection from the Lower Lakes/Upper Ohio Valley, east
northeastward into much of New York state and northern New England.
Locally heavy rains and isolated runoff issues will be possible,
thus maintaining a Marginal Risk area.

...Upper Mississippi Valley into the Mid-West...

This will be followed by a second area shortwave energy pushing
east southeast from the Northern Plains into the Upper MS
Valley/Mid West region Saturday night into early Sunday. The low
level southerly to southwesterly flow is expected to strengthen
into this front, increasing PW values to 1.50-1.75"+ and supporting
heavy to locally excessive rainfall totals in what may be an area
of organized convection. The marginal risk area was drawn to
encompass the current spread in model qpf axes.

...Southern to Central Appalachians into the Southeast...

A broad Marginal Risk remains in effect for widespread convection.
Although confidence in the location of higher amounts remains lower,
PW values in the 1.5-1.75"+ range will support localized hourly
rain amounts of 1-2"+ leading to potential runoff issues,
particularly in urban areas and within areas of steep terrain.

..Southwest...

A broad Marginal Risk area was maintained across the Southwest for
monsoonal convection. Low confidence with QPF details persist
however locally heavy rains and isolated runoff issues will
continue in the axis of above average PW values across this area.

Campbell


Day 4 and Day 5

Valid 12Z Sun Aug 09 2026 - 12Z Tue Aug 11 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST, NORTHERN PLAINS, GREAT LAKES, MIDWEST, &
NORTHERN MID-ATLANTIC STATES...

1930Z Update: The only notable change to both the Days 4-5 EROs was
to cut back a little across the western portions of the Marginal
Risk areas over the Northern Plains, specifically the Dakotas. This
based on the 12Z guidance trends, including the 12Z AIGFS and ECMWF
AIFS, along with the QPF exceedance probabilities from the 12Z
GEFS and ECENS means.

Hurley

...Northern Plains/Great Lakes/Midwest/Northern Mid-Atlantic
states...
Early next week, broad cyclonic flow expected to persist
across the northern tier of the Lower 48 from the Northern
Plains/Upper MS Valley, eastward through the Midwest/Great Lakes
and into the Northeast, to the south of a closed mid-level low in
Hudson Bay. A frontal boundary will lie along the southern
periphery of this broad trough. Embedded shortwaves in this broad
cyclonic flow will accentuate lift in the vicinity of this front
early next week where PW values are expected to rise to ~1.75"
ahead of each set of shortwaves. When looking at the ingredients,
organized convection looks most likely south of the Great Lakes
within portions of the Corn Belt, where hourly rain amounts to 2.5"
are possible. However, there is a great deal of latitudinal spread
in the axis due to differences between where the diffluent
1000-500 hPa thickness exists and where the instability
gradient/better 850 hPa confluence/convergence could set up, which
appears more poleward into the Great Lakes. When the guidance gets
in better agreement at some future time, a Slight Risk or greater
could be considered, but at the moment there's too much spread in
the heavy rain signal to place anything more than a Marginal Risk.

..Southwest...
Broad Marginal Risk areas exist both days across portions of the
Southwest, Rockies, and Great Basin. While there continues to be
low confidence with the details, locally heavy rains with the
potential of 2" an hour amounts and isolated runoff issues will
continue in the axis of above average PW values expected across
this area. As usual, urban areas, arroyos/dry washes, box canyons,
and burn scars would be most at risk for flash flooding.

Roth


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt