Mesoscale Precipitation Discussion 1074
NWS Weather Prediction Center College Park MD
506 AM EST Fri Dec 28 2018
Areas affected...far southeastern LA, southern MS into southwest
AL and far western FL
Concerning...Heavy rainfall...Flash flooding possible
Valid 281004Z - 281545Z
Summary...Local 2-4 inch rainfall totals expected through 16Z
along with possible flash flooding for portions of the central
Gulf Coast into central AL. Flash flooding will be possible given
wet antecedent conditions.
Discussion...As of 0930Z, heavy rain continued to fall from near
Lake Ponchartrain into central AL with a southwest to northeast
oriented axis of 1-2 in/hr rainfall rates located at the leading
edge of the rainfall swath. The heaviest rain has begun to shift
east over the past 60 minutes partially in response to a shortwave
seen on water vapor imagery translating east into western AR and
far eastern TX as of 0930Z. Flow aloft remains strongly diffluent
over all areas but instability remains best to the south of a
HDC-MGM-VPS line with MLCAPE estimated to be in the 500-1000+ J/kg
range.
As the AR/TX shortwave continues to advance northeastward through
18Z, mid-level height falls should allow steady movement of the
heavy rainfall axis across northern AL but nearly neutral height
falls over the central Gulf Coast may slow eastward movement of
the heavy rainfall axis. While a general movement of the
convergent low levels / leading edge of heavy rainfall is
expected, unidirectional flow favors training segments that may
develop just ahead of the line which may overlap with Baldwin,
Escambia, Santa Rosa and neighboring counties along the central
Gulf Coast which received 3-6 inches of rain within the past 2
weeks. Farther north of the better instability, heavy rainfall
over the past 24 hours has lowered Flash Flood Guidance values
near or below 1 inch in 3 hours which could be surpassed in a few
locations through 16Z despite the weak available instability
(central AL). The flash flood threat appears to have diminished
relative to earlier in the night but will remain a concern heading
through the morning hours for portions of the central Gulf Coast.
Otto
ATTN...WFO...BMX...FFC...HUN...JAN...LIX...MOB...TAE...
ATTN...RFC...LMRFC...SERFC...
LAT...LON 34208562 33978506 33488473 32888492 32258552
31098630 30108745 29298848 28638979 28999085
30159062 31009003 32198898 33068814 33958712
34178634
Last Updated: 506 AM EST Fri Dec 28 2018