FOUS30 KWBC 071555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Mon Sep 7 2026

Day 1
Valid 16Z Mon Sep 07 2026 - 12Z Tue Sep 08 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
NORTHWESTERN MONTANA...

...16Z update...

A Marginal Risk was added for the central Gulf Coast from far
southeastern Texas into southwestern Alabama and the western
Florida Panhandle. Monday morning blended TPW imagery and the 12Z
LIX sounding sampled anomalous moisture of 2.0 to 2.3 inches from
the upper Texas coast and eastward into northern Florida, which
was weakly anomalous for this time of year (standardized anomalies
up to +1). While not particularly anomalous, the moisture combined
with mostly clear conditions should allow for ample instability to
support scattered afternoon/evening showers and thunderstorms.
Slow movement, especially near the Texas/Louisiana border with less
than 10 kt of motion, will support high rainfall rates of 2-3
inches in an hour at times, (locally higher possible). Should these
higher rates overlap with urban or otherwise sensitive locations,
isolated instances of flash flooding could develop.

The only other change with this update was to make a minor
expansion to the Marginal Risk across the Upper Mississippi Valley
based on timing and the latest 12Z hires model guidance.

Otto

...previous discussion follows...

...Northwest Montana...
A well defined shortwave expected to rotate northeastward from the
Northern Rockies into the Northern High Plains Monday. Model
consensus is for the development of a comma head/deformation precip
band to the northwest of this feature across northwestern portions
of Montana. There will likely be a period when this comma
head/deformation band pivots over northwestern Montana, supporting
heavy precip totals. The slight and marginal risk areas across this
area were changed slightly from previous issuances to align better
with the latest HREF and RRFS neighborhood probabilities for 1 and 2"+ amounts.

Marginal risk areas

...Upper Mississippi Valley...
The height falls ejecting northeastward from the Northern Rockies
into the Northern Plains will strengthen the low level southerly
flow along and ahead of the associated frontal boundary, raising
PW values to 1.75-2.00"+. Overall favorable large scale conditions
will support potential for organized pre-frontal/frontal
convection over the Upper MS Valley. At the moment, this convection
should be very progressive, limiting the excessive rainfall
potential to marginal. No significant change made to the previous
outlook area across this region.

...Eastern Arizona into western to central New Mexico...
PW values expected to remain above average day 1 from eastern
Arizona into western to central New Mexico. Widespread scattered
convection possible in this high PW axis,supporting locally heavy
rains and isolated runoff issues.

...Northeast Florida into coastal Georgia and coastal southern South Carolina.

An axis of above average PW values will persist day 1 from the
coastal Southeast into northeast FL. Both the HREF and RRFS
neighborhood probabilities are fairly high in this above average PW
axis for 2 and 3"+ precip totals. Not a lot of confidence in qpf
details, but potential for locally very heavy rains and isolated
runoff issues, especially in more urbanized areas. The previous
marginal risk was reduced along the northeastern Gulf coast to
where the HREF and RRFS probabilities are greatest.

Oravec
 

Day 2
Valid 12Z Tue Sep 08 2026 - 12Z Wed Sep 09 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER IOWA...

...Upper Mississippi Valley into the Upper Lakes...
The surface front pushing east into the Northern Plains day 1 will
remain very progressive day 2 into the Upper Mississippi
Valley/Upper Lakes. The progressive nature of the front will
continue to be a detriment to widespread runoff issues. However,
the southwestern portion of this front will slow somewhat compared
to the farther northern portion in the vicinity of eastern NE
/northeast Kansas, east into Iowa. Across these regions there is a
better chance of a period of training of cells late Tuesday/early
hours of Wednesday. This is shown in the latest guidance that has
consensus for a heavier axis of qpf here. A small slight risk area
was added across portions of central to southern Iowa for this
training potential. RRFS neighborhood probabilities are fairly high
for 1 and 2"+ amounts across the slight risk area, with a smaller
50%+ probability for 3"+ amounts over east central Iowa.

Marginal risk areas

..Eastern Arizona into western to central New Mexico...
No significant changes day 2 to the axis of above average PW
values extending from eastern portions of Arizona into western to
central New Mexico. Continued low confidence with qpf details, but
potential for another round of widespread scattered convection
across this area. Locally heavy rain and isolated runoff issues
again possible. Anomalous PW values also expected to push farther
north from Southern CA into Central CA. At the moment, models are
not showing precip developing day 2 in this anomalous axis.

The previous marginal risk area over northeast Montana was removed
given latest model qpf consensus. The broad marginal was also
removed along the Gulf coast into FL given less organized precip than day 1

Oravec
 

Day 3
Valid 12Z Wed Sep 09 2026 - 12Z Thu Sep 10 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SIERRA
INTO SOUTHEAST CALIFORNIA, THE SOUTHWEST, THE MID MISSISSIPPI
VALLEY INTO THE OHIO VALLEY AND NORTHERN NEW YORK STATE INTO
NORTHERN VERMONT...

...Sierra, Southeast California, Southwest...
A very broad area of anomalous PW values, 2 to 3+ standard
deviations above the mean, will stretch day 3 from central to
southern California, across the southern Great Basin and into the
Southwest. Latest models do show scattered precip in this high PW
axis from the Sierra into Southeast California and over the
Southwest. The heaviest totals continue across similar areas day 1
and 2 from eastern Arizona into western to central NM where a
marginal risk continues day 3. The marginal risk was expanded
northward from day 2 into the Sierra to capture any locally heavy
precip amounts in the much above average PW axis.

...Lower Missouri, Mid Mississippi, Ohio Valley region...
A surface cold front expected to sink southward through the Mid
MS, Lower MO and into the OH Valley region day 3. PW values
expected to remain anomalous along this front with values 2"+,
supporting potential for widespread moderate to locally heavy
totals. The front remains very progressive to the south, which
along with relatively high FFG values, will limit widespread runoff potential.

...Northern NY State into Northern Vermont...
The northeastern portion of the front moving south into the Mid MS
Valley/OH Valley region will also remain very progressive to the
east across the eastern Lakes, northern NY State and northern New
England. PW values expected to be anomalous along this portion of
the front also, with values 1.75-2.00"+.  No changes to the
marginal risk area for far northern NY state into northern VT where
model consensus is for moderate to heavy precip totals. The
expected very progressive nature of the front will be a detriment
to widespread runoff issues, but the anomalous PW values and
potential for a brief period of training may produce isolated issues.

Oravec
$$
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