Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
149 PM EST Tue Dec 2 2025

Day 1
Valid 16Z Tue Dec 02 2025 - 12Z Wed Dec 03 2025

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

Otto

Day 2
Valid 12Z Wed Dec 03 2025 - 12Z Thu Dec 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA...

Mid-level flow will amplify dramatically across much of Texas ahead
of an approaching mid-level wave near the Four Corners. Meanwhile
at the surface, 850mb flow will increase dramatically into a small
part of southeast Texas and adjacent areas of the northwestern Gulf
and southwestern Louisiana. Ascent on the nose of this low-level
flow will combined with modest increases in low- and mid-level
instability to force mostly elevated deep convection initially
across southeastern Texas around 03-06Z Thursday. These cells will
will form in an environment favoring appreciable storm motions, 
although the development of convection relative to an east-west 
oriented warm frontal zone across the region will likely support 
areas of training. CAMs are depicting local 3+ inch rainfall totals
in areas southwest of Houston that could be realized especially if
storms can become rooted in the boundary layer and/or training 
becomes focused along a local corridor in the region. A Marginal 
Risk/5% area was added to the outlook to account for these model 
trends. This risk is likely to begin after 03Z Thursday and persist
through 12Z and into the D3 forecast period.

Cook

Day 3
Valid 12Z Thu Dec 04 2025 - 12Z Fri Dec 05 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL 
GULF COAST...

...21Z Outlook Update...
The previous discussion and forecast philosophy is still on track,
with an extensive overrunning event expected to produce widespread
areas of 2-4 inch rainfall totals especially across southern
Louisiana. Urban/flood prone areas will be most susceptible to
flash flooding in this regime. The notion that most convection
will be elevated atop a cool stable layer precludes any
introduction of Slight Risk probabilities for this outlook.
Marginal Risk areas have been trimmed/confined southward to
locations expected to experience the highest rainfall totals.

Cook

...Previous Discussion...

A strong warm advection regime of deep Gulf moisture will have
developed along the central Gulf Coast on Thursday morning. A polar
high over the Midwest will suppress the northward extent of the
precipitation. A unidirectional WSW flow regime will set up over
all of the Southeastern U.S. with a 170 kt jet streak aligned
roughly along the Ohio Valley, with a longwave positively tilted
trough over the Southwest. Weak impulses of shortwave energy will
eject out of the trough and along the jet streak, locally enhancing
any convection. The primary forcing for precipitation however will
be overrunning, as warm Gulf air tries to lift over the much colder
air mass over much of the eastern U.S. The combination of
unidirectional flow at all levels out of the WSW, overrunning of
anomalously moist air, and some, albeit far from impressive
instability will support training thunderstorms capable of 2 inch
per hour rates. These storms will align in lines parallel to one
another. Those lines will be slow moving, supporting multiple
rounds of training storms over much of southern/southeastern
Louisiana and into coastal Mississippi. NASA Sport imagery shows
soils in much of southern Louisiana to be bone dry, with moisture
levels at single digit percentages as compared with climatology.
Thus, much of the rainfall, especially in areas where it remains
light, will be beneficial for these areas. The Drought Monitor also
shows this area to be in D2/Moderate Drought. Thus, it will take
some time for enough rain to fall to support flash flooding.
However, guidance suggests the New Orleans metro to be in the
bullseye with the greatest chances for prolonged heavy rain. Given
all of the above, the area remains in a Marginal Risk, but the area
in and around New Orleans to the urbanized Mississippi Gulf Coast
will be evaluated for the need for a Slight Risk upgrade with
future updates.

Wegman


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