FOUS30 KWBC 060757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Thu Aug 6 2026

Day 1  Valid 12Z Thu Aug 06 2026 - 12Z Fri Aug 07 2026

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

...Central Plains into the Lower Missouri Valley...
Organized convection is expected to fire up along the southward
moving cold front. An increasing influx of moist, southerly flow
over the frontal boundary overnight will lead to locally heavy
rainfall, potentially accumulating around 2.5 inches.

...Lower Lakes, Ohio Valley into the Northern Mid-Atlantic...
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.

...Southern to Central Appalachians...
A broad Marginal Risk covers the Central to Southern Appalachians
for widespread scattered convection. HREF neighborhood
probabilities over the southern
Appalachians indicated medium chances of 24hr rainfall exceeding
3", and low chances of precip exceeding FFGs which may bring
isolated flash flooding. CAM guidance suggests a rather short
period of heavy precip, and there was a general lack of agreement
regarding the location precip core, and QPF totals.

...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.

Campbell
 

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...

...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...

...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...

...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
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