Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Tue Sep 8 2026
Day 2
Valid 12Z Wed Sep 09 2026 - 12Z Thu Sep 10 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER FAR SOUTHEAST
KANSAS INTO CENTRAL MISSOURI AND THE SOUTHWEST...

21Z Update..

A Slight Risk upgrade was introduced in southeast Arizona and far
western New Mexico in coordination with the local forecast offices. The
early availability of CAMs guidance indicated notable FFG Max
Exceedance probabilities, especially in the RRFS. Anomalous mean
PWATs are expected to exceed the 90th percentile throughout the
period. Scattered thunderstorms will form and be fueled by this
anomalous moisture, and will be locally supported in the upper-
levels, with precip occurring along the eastern periphery of a
ridge, promoting some areas of divergence aloft. A building upper-
ridge should confine precipitation to be in the form of scattered
thunderstorms mostly along southern Arizona, but will have the
potential to bring heavy rainfall.

Elsewhere, the inherited SLGT risk was expanded to include more of
eastern Kansas and central Missouri. Updated HREF & RRFS
precipitation probabilities suggest medium to high chances (~55% &
75% respectively) of rainfall exceeding 3" during the day 2
period. In terms of hourly rates, both HREF and RRFS neighborhood
probabilities suggest medium chances ~50-60% of rainfall exceeding
1"/hr over a 3-5hr period. The general storm motion should be
fairly progressive however, which resulted in maintaining the
Slight.

The inherited MRGL Risk areas were also expanded and introduced
through the Ohio Valley and Lower Great Lakes, as well portions of
the Southern Plains, to capture the latest run of global models
and ensembles. An introduction to a SLGT was considered over the
Ohio Valley given higher QPF totals and higher rain rates per the
HREF, but the expected quick progression of storms was the main
deciding factor to not upgrade to a SLGT. Later runs may
necessitate the need for a SLGT, particularly if storm motions
slow.

Blanco-Alcala

...Previous Discussion...

...Lower Missouri Valley...
A surface cold front expected to continue to sink southward
through the Mid MS, Lower MO and into the OH Valley region day 2.
PW values expected to remain anomalous along this front with values
2"+, supporting potential for widespread moderate to locally heavy
totals. A small slight risk area was added over far southeastern
portions of Kansas into central Missouri for potential of training
of cells in this anomalous PW axis. Much of this area has, however,
been very dry over the past month, which may mitigate potential for
runoff issues. RRFS neighborhood probables are high for 1 and 2"+
totals day 2 in the slight risk area and RRFS probabilities of 3
hour precip exceed FFG values are in the 20-30%+ range at 0000 UTC
Sept 10. These probabilities are not available for the 0000-1200
UTC period, but given the overall favorable pattern along this
front for heavy rains, similar probabilities likely will continue
along the front into early Thursday morning.

Marginal risk areas.

...Sierra, Southeast California, Southwest...
A very broad area of anomalous PW values, 2 to 3+ standard
deviations above the mean, will stretch day 2 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 2. The marginal risk was expanded
northward from day 1 into the Sierra to capture any locally heavy
precip amounts in the much above average PW axis.

...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


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