Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
221 PM EST Sat Jan 28 2023

Day 1
Valid 16Z Sat Jan 28 2023 - 12Z Sun Jan 29 2023

The probability of rainfall exceeding flash flood guidance is less
than 5 percent. Per collaboration with WFOs HGX, LCH, and SHV,
have opted to remove the Marginal Risk that was in place across
the Upper TX Coast-Eastern TX into western and northern LA.
Guidance trends, including the 12Z CAMs, have really backed off on
the timing of the more organized convection (and setup more
favorable for training) along/hear the Gulf Coast, i.e. mainly
after 12Z. 12Z HREF exceedance probabilities have followed suit as
well, with probs of >1"/hr rates no higher than 30-35% through 12Z
Sun (where current 01 hr FFG is >2"/hr).

Hurley

Day 2
Valid 12Z Sun Jan 29 2023 - 12Z Mon Jan 30 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND CENTRAL GULF COAST...

2000 UTC Update -- Very few notable changes with the latest
forecast update, based on the latest (12Z) guidance QPF trends
(deterministic and probabilistic). A slowly strengthening upper
level jet streak will put much of the Slight Risk area in the
right entrance region of the jet streak. Small disturbances riding
along the jet may locally trigger convection across the Slight
Risk area as they race northeastward with the jet. Much of the
dynamics for this rainfall event will be in the lower levels of
the atmosphere as broad southerly flow covering the whole of the
Gulf peaks in intensity as far as the LLJ is concerned over
southern LA midday Sunday. The LLJ will reintensify as the
convection moves east across the FL Panhandle and southern GA
Sunday night, including values as strong as 60 kt over the FL
Panhandle, far SE AL and far SW GA. Thus, expect convection to be
quite efficient at producing heavy rainfall as it moves eastward
Sunday and Sunday night. To be sure, the greatest threat within
the Slight Risk area will be across western areas from TX through
MS, with slightly lower threats for flash flooding further east.
Peak storm total rainfall may exceed 3 inches in portions of
southern LA through southern MS-AL-GA, within a more favorable
thermodynamic environment with PWs close to 1.75" along with
better deep-layer instability (MUCAPEs 500-1000 J/kg). The 12Z
HREF in fact indicates clusters of 40-60+ percent probs of
>3"/24hrs within the Slight Risk, with the highest probs (60-80%)
just north of the Upper TX Coast through much of southwest LA.
There continues to be good model consensus however that the axis
of heaviest rain will remain north of New Orleans.

While main-stem rivers and soil moisture across all of SC remain
high, with a few of the main-stems in flood stage, the flashier
creeks and smaller tributaries are quite dry, and are likely to
handle the 1 to 2 inches of rain expected during the overnight
hours Sunday night in this area. Model guidance is in excellent
agreement on the swath of heaviest rain falling in the new Slight
Risk area. Additional trimming of the Slight Risk was done on the
western end with good agreement that metro Houston may largely
miss out on the heaviest rain, and also that the heaviest rain
will miss the Mississippi River Delta to the north. Confidence
wasn't quite there to remove Houston from the Slight given some
variability, but extreme southeastern LA was downgraded to a
Marginal with this update.

Hurley/Wegman

Day 3
Valid 12Z Mon Jan 30 2023 - 12Z Tue Jan 31 2023

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

Hurley


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