Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Sat Dec 02 2023
 
Day 1
Valid 12Z Sat Dec 02 2023 - 12Z Sun Dec 03 2023 

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE FLORIDA 
PANHANDLE INTO LOWER SOUTHEAST ALABAMA...

...Gulf Coast and Southeast...

A strong cold front pushing east out of the Plains and into the 
Southeast will draw deep Gulf moisture northeastward along and 
ahead of the front. Meanwhile, a strong southwesterly jet streak 
will continue intensifying from the southern Plains to the 
southern Appalachians. This will put the portions of the Gulf 
coast in the favorable right entrance region of the jet. Helping 
intensify the jet will be a strong shortwave that will also be 
tracking northeastward. While the shortwave is unlikely to have a 
direct impact on the storms, it's influence on the upper jet will 
help support broad scale lift over the area.

Showers and thunderstorms are ongoing across LA and MS this 
morning. Heavy-rain-producing thunderstorms have been embedded 
within the broader rain shield, which fortunately have been moving 
enough to limit flooding impacts somewhat. Nonetheless their 
tendency to produce 3 inch per hour rainfall rates at times has 
resulted in localized flash flooding in portions of southwestern 
LA. Storms and the broader rain shield will push off to the east 
as more storms develop over the Gulf through the morning due to 
the aforementioned large scale environmental lift. The heaviest 
rain of the day and the greatest potential for flooding will 
likely be in the morning hours/first part of the day today. 
Supporting the storms are CAPE values between 500 and 1,000 J/kg 
over the Gulf and the immediate Gulf Coast. Instability generally 
diminishes as one goes further inland, but the increasing 
southwesterly flow supported by the jet is helping advect the 
unstable air mass further inland.

There's likely to be a break in the significant rainfall for much 
of the afternoon through the evening hours between systems. A 
renewed round of storms is expected to develop overnight tonight 
into the early morning hours of Sunday. While the storms will have 
more support in the upper levels as the jet streak intensifies to 
over 150 kt, the time of day, faster storm motion, and lesser 
forcing across the FL Panhandle and nearby portions of AL/GA 
should limit both the storms' intensity and coverage tonight/early 
Sunday morning.

The Slight and surrounding Marginal Risk areas are largely 
unchanged with this update. Given the above reasons, the Slight 
may be able to be cancelled with the midday update depending on 
the evolution of the convection at that time.

...Western Oregon into far Northwest California...

The next in the long string of fronts associated with a persistent 
atmospheric river is already moving into the Pacific Northwest 
(primarily Washington State) this morning. The heaviest rain will 
shift southward and diminish through the morning. A stronger front 
will push into the far northern CA and OR coasts tonight, which is 
when the bulk of the day's rainfall is expected. Expect a soaking 
3 to 5 inches of rain into the coastal ranges of OR and the 
Cascades through tonight, with local amounts approaching 8 inches 
by Sunday morning. The impulse of moisture will be accompanied by 
much warmer air typical of many atmospheric rivers. This will 
rapidly raise snow levels, allowing snowmelt to contribute some to 
rising river levels. Since the area has been relatively dry 
recently, much of the rainfall and snowmelt should be contained in 
the river banks, so only isolated flash flooding in particularly 
sensitive areas is expected through Sunday morning. The Marginal 
Risk was maintained with this update.

Wegman


 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt