Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
808 PM EST Fri Nov 27 2020
 
Day 1
Valid 01Z Sat Nov 28 2020 - 12Z Sat Nov 28 2020 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
SOUTH-CENTRAL TO SOUTHEAST TEXAS...

...Central/Eastern Gulf Coast...
One round of locally heavy rain continues to advance east across 
areas of southern LA and stretching east across areas of southern 
MS/AL and soon to be moving into portions of western GA. This 
activity is associated with a broken axis of showers and 
thunderstorms focused along and out ahead of a cold front along 
with a wave of low pressure and supporting shortwave trough that 
is crossing the central Gulf Coast region this evening. Generally 
the rainfall rates have been well below FFG values, with 
occasional rates over last couple of hours reaching upwards of 1.5 
inches/hr over parts of central AL. Generally the strongest areas 
of convection right now are across portions of central/southern AL 
closer into the wave of low pressure. Overnight, this activity 
will all continue to advance off to the east, and will result in 
some additional areas of heavy rain, but generally the amounts 
should continue to remain well below FFG values. A Marginal Risk 
of excessive rainfall is maintained across portions of the central 
and eastern Gulf Coast region, with only a very isolated concern 
for any runoff problems.

...South-Central to Southeast Texas...
An upper-level trough and associated closed low ejecting out 
toward the southern High Plains will increase the threat of heavy 
showers and thunderstorms overnight (especially 06Z to 12Z) across 
areas of south-central to southeast TX. This will occur as 
increasingly divergent flow aloft interacts with a very 
slow-moving frontal zone and a pool of fairly deep moisture with 
PWs of 1.5 to 1.75 inches. A modest degree of instability, with 
much of it elevated, will be also pooled along the front. The 
latest hires model consensus, including the last few runs of the 
HRRR and the HRRRp support locally as much as 2 to 3 inches of 
rain with isolated heavier amounts. Overall, much of the rainfall 
should be in the sub-FFG category owing to dry antecedent 
conditions, but there will be some potential for some west/east 
bands of convection to set up with some periodic training of cells 
possible in vicinity of the front. Some of the more urbanized 
areas may potentially see some increase in runoff problems as a 
result. Given the increase in moisture transport and stronger 
forcing that is expected here later in the night, the strongest 
convective potential and heaviest rainfall will likely be in the 
pre-dawn hours leading up to 12Z. A Slight Risk of excessive 
rainfall is denoted across portions of south-central to southeast 
TX as a result.

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