647
FOUS30 KWBC 040849
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
449 AM EDT Thu Sep 4 2025

Day 1
Valid 12Z Thu Sep 04 2025 - 12Z Fri Sep 05 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST CALIFORNIA AND CENTRAL AND SOUTHERN ARIZONA...

...Southwest...

The signal is clear within all CAMs leading to a strong
probability assessment for 3 and 6hr FFG exceedance intervals, each
carrying 50+% over a large area extending from Tuscon all the way
to the Colorado River basin between the 18-06z time frame in the
period. Rates of 1-2"/hr will be plausible in this setup as PWATs
remain ~2 deviations above normal, a signal favoring locally heavy
rainfall and heightened flash flood concerns over flashy
soil/terrain features. Rather impressive instability for this part
of the country is forecast, with values upwards of 2000-3000 J/KG,
and this combination of above average PWs and instability should
support intense rainfall rates. Always worry about cloud cover
impacting instability, which could happen in spots, but the overall
model signal certainly favors areas of robust instability. In a
part of the country where model QPF struggles, CAPE and PW overlap
often ends up being a better flash flood potential indicator. A
Slight risk for much of central and southern Arizona into southeast
California was maintained for this period.

...South Florida...

A persistent wet pattern to continue through the weekend thanks to
stationary front in the low levels, an upper jet to the north
providing some right entrance region ascent, and broad mid level
troughing. Rounds of convection will occur each day, and with PWs
around 2.3", heavy rainfall rates are likely. The exact location
of the axis of heaviest rainfall is a bit uncertain, but most of
the guidance is favoring the Keys and southern portions of the
peninsula. Heavier convection may impact the urban corridor of
southeast Florida at times as well. A Marginal Risk area remains in
effect for southern Florida and the Keys.

...Kentucky...

Isolated flash flooding possible across eastern Kentucky and points
eastward this morning. Individual storms will be capable of
producing 1-2 inches/hour, particularly over a part of the country
that is vulnerable to these intensities. Please refer to Mesoscale
Precipitation Discussion #1045 for additional details.

Campbell/Chenard/Kleebauer


Day 2
Valid 12Z Fri Sep 05 2025 - 12Z Sat Sep 06 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
ARIZONA AND SOUTHWEST NEW MEXICO....

...Southwest...

The exact track of the circulation from Lorena as it approaches
Sonora from the California Baja remains a bit uncertain within the
latest guidance. Some of the model guidance remains particularly
aggressive with holding the center of circulation to the
International border, just south of southeastern Arizona leading to
multiple solutions (GFS/RRFS/NAM/HAFS suite/HWRF) indicating a
swath of heavy rainfall impacting at least portions of southeast
Arizona into southwest New Mexico. ECMWF/CMC and mean ensemble
output remains not as aggressive as the previous models referenced
putting a little more emphasis on the low degrading prior to
reaching northern Sonora leading to the remnant moisture field not
advecting further north.

Assessment of the guidance compared to reality is important in
this scenario as major discrepancies early on would very well
inhibit model outputs through the course of the run. As of now, the
cluster with the greater outputs over Arizona/New Mexico are
handling the system a bit better which trends more in favor of the
wetter solutions as we move forward in time. Ensemble bias
corrected QPF was also particularly wet aiding to a little better
consensus on what could transpire across the high deserts near the
border. Ensemble totals between 1.5-3" with locally higher are
being forecast in those more substantial outputs, a range that
would cause some flash flood concerns, especially when you add in
the more effective warm rain processes being depicted when looking
at forecast soundings over parts of the aforementioned area. PWAT
anomalies closer to 3 deviations above normal would be common in
the current expected evolution, plentiful for more locally
significant impacts.

The large Marginal risk area spans from Oregon and as far east as
Colorado and New Mexico. The overall synoptic pattern favors
isolated to scattered convection over this entire area, and the
forecast PWs and instability would support locally heavy rainfall
rates.

...South Florida...

The overall pattern described during the Day 1 period is not
expected to change much and locally heavy rain between 2-4" are
forecast in the period, however its location will certainly be
important with the urban centers the most favorable for at least
scattered flash flood prospects. The large scale pattern becomes
more meridional by Friday, with upper level flow becoming more
southerly over south Florida and the upper jet orientation favoring
a bit more upper level divergence. Thus could potentially see a
pattern that may favor more of a stationary or training convective
threat along the urban corridor of southeast Florida. A Marginal
Risk remains in effect for southern Florida and the Keys.

Campbell/Kleebauer/Chenard


Day 3
Valid 12Z Sat Sep 06 2025 - 12Z Sun Sep 07 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM TEXAS TO
THE PACIFIC NORTHWEST, SOUTHERN FLORIDA AND NEW ENGLAND...

...West, Southwest and Southern Plains...

During this period a frontal boundary will stretch from the Gulf
westward into Texas. Some enhanced moisture left over from Lorena
and instability will be present over the Southern Plains, which
could allow for locally heavy rainfall to occur. A broad Marginal
is in effect for the western half of Texas into the southern High
Plains, Colorado, Southwest, Great Basin, Idaho and Oregon.

...Florida and New England...

The frontal system over Florida and northeast in the western
Atlantic will provide a focus for moisture to pool, leading to at
least scattered diurnal convection each day. Excessive rainfall
across Florida will likely focus across the southeast peninsula in
Miami-Dade County and vicinity. The Marginal Risk up north covers
most of Vermont, New Hampshire and Maine.

Campbell


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