Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
158 PM EST Fri Nov 21 2025

Day 1
Valid 16Z Fri Nov 21 2025 - 12Z Sat Nov 22 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA...

16z update:

...Southern California/Lower Colorado River Valley...

GOES-W shows solid Atmospheric River has turned westward into the
TROWAL north of the initial closed low/occluded surface low across
southern California. This has directed enhanced (95th-99th
percentile) moisture and moisture flux across the Imperial Valley,
Mojave Desert and Lower Colorado River Valley to maintain enhanced
rainfall potential. Some filtered insolation may allow for
increased instability later this afternoon especially within the
Lower Colorado, for some scattered thunderstorm development capable
of .5-1"/hr rates and localized 1-1.5" totals.

The strong upstream digging shortwave on the western side of the
upper-level closed low will elongate the pattern from NNE to SSW
over the California Bight which will stall the frontal zone and
moisture plume. While most of the flux will be directed toward NW
Baja California, this will also maintain the placement of the AR
across the areas that longer duration light to moderate showers
could result in longer duration FFG exceedance (6+hr) as well. As
such, the the current Slight Risk in place remains solid. Have
expanded the Marginal Risk a bit further northwest into the
southern San Joaquin Valley to account for the longer term trends
of persistent TROWAL moisture flux across the area.


...Lower Missouri, Ohio and Tennessee River Valleys...

12z CAMs along with 06z/12z HREF probabilities continue a show a
downward trend with only a few locations of 1"/hr or 1"/3hrs
reaching above 25% across Middle TN and S KY. Additionally,
observational trends depict the deeper moisture and more
unstable/warmer air has been trending south and eastward with a
broadening of the drier EML extending through the Delta Region
of the Lower MS Valley, further severing needed moisture
connection to the the Gulf. In coordination with local forecast
offices and river forecast centers, it was determined to remove the
Marginal Risk area from the eastern third of the U.S. Please note,
this does not remove the potential for an isolated incident or two
of flash flooding/FFG exceedance across the area, but the coverage
potential is considered less than 5%.

Gallina



~~~~Prior Discussion~~~~
Showers and thunderstorms are expected across southern California,
southern Nevada and southwest Arizona as lobes of vorticity rotate
to the east of the low offshore. Instability appears to be on the
weak side which will likely be a limiting factor, however some
brief heavier rates are still possible from short training segments
within the deep layer southerly flow. A Slight Risk for excessive
rainfall remains in effect for southern California and southwest
Arizona and a Marginal Risk spans from the California coast to
south-central Arizona.

For the Missouri/Ohio/Tennessee Valleys there will be an elevated
threat for locally excessive rainfall. During this period a steady
stream of above average moisture will be transported northward and
encounter an east-west boundary. Isolated areas of moderate to
heavy rainfall that leads to spotty problems from run-off can be
expected. However...the flow aloft becomes increasingly flat and
confluent during the day which will reduce the risk of any
excessive rainfall. A Marginal Risk area was maintained from
eastern Kansas to West Virginia/western Virginia/western North
Carolina.

Campbell


Day 2
Valid 12Z Sat Nov 22 2025 - 12Z Sun Nov 23 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill/Campbell


Day 3
Valid 12Z Sun Nov 23 2025 - 12Z Mon Nov 24 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS...

...20z Update...

Maintained the inherited Slight Risk with only minor adjustments
made, primarily to shift the axis slightly south to southwest and 
tighten the gradient along the north and south. This reflects the 
latest trends in QPF, which are fairly tightly clustered for this 
time range (relatively high spatial confidence). This is somewhat
concerning, as this brings the forecast axis of QPF even more into
alignment with areas that realized heavy rainfall in prior days. 
However, the categorical outlook is still held at a Slight Risk 
due to a combination of meaningful time between the prior event
and the upcoming event (three full days inbetween should allow for
soil moisture and streamflow recovery) and uncertainty with regard
to the amount of training and resultant QPF (with most of the new 
guidance suggesting localized 2-3" totals, while some solutions,
including the downscaled 12z ECMWF, depict localized totals of up 
to 4-5").

Churchill


...Previous Discussion...

The low pressure system that brought heavy rainfall for the West
Coast during the Day 1 period will be exiting into the Southern
Plains and the warm, moist air will support scattered to widespread
heavy rainfall. There is an increasing signal that this setup could
be a significant event, especially for north-central portions of
Texas and into southern Oklahoma where a couple of solutions are
suggesting a narrow swath of 4 to 6 inches possible. A Slight Risk
for excessive rainfall is in effect for for much of north-
central/northeast Texas and south/east Oklahoma. A broader Marginal
Risk spans from the Big Bend vicinity to southwest Missouri and
western Arkansas. This part of the country will continue to be
monitored for a possible upgrade to a Moderate with future updates.

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