Mesoscale Precipitation Discussion 1034
NWS Weather Prediction Center College Park MD
1235 AM EST Sun Dec 02 2018

Areas affected...Florida Panhandle, Southeast Alabama, Southwest
Georgia

Concerning...Heavy rainfall...Flash flooding possible

Valid 020534Z - 021130Z

Summary...Persistent bands of heavy rain and thunderstorms are
expected on the eastern Gulf Coast, and may lead to flash
flooding. Some parts of the area have already received as much as
4-6 inches of rain in the past 12 hours, and if thunderstorms can
persist in these areas for an hour or more, additional flash
flooding might occur.

Discussion...KEOX, KEVX and KJGX radars showed a relatively
stationary band of convection stretching from PNS-DHN-EZM as of
05Z, with maximum rain rates peaking as high as 2-3 in/hr and some
variance between the dual pol and MRMS estimates. This band has
essentially not moved in three hours in the larger sense, but
individual storm motions have varied slightly and the convective
band has had some minor oscillations. Furthermore, the convective
clusters within the line were beginning to show some separation.
These factors were generally preventing a continuous band of heavy
rain from persisting in any one location for many consecutive
hours. However, the convection is impacting a corridor that has
already received heavy rainfall over the past 12 hours, with as
much as 4-6 inches of rain recorded on some mesonet sites. These
antecedent conditions will create a generally favorable setup for
additional flash flooding, but the main question will continue to
be how long high instantaneous rain rates can persist in a given
location.

Convection is expected to continue from the Florida Panhandle into
Southwest Georgia through the rest of the night. In addition to
the band of ongoing convection over land, GOES-16 IR satellite
channels show a narrow ribbon of low-level clouds concentrating
along a convergence zones extending southwest toward the mouth of
the Mississippi River. This larger axis of low-level convergence
is forecast to persist on the western edge of a plume of deeper
moisture, and it is also generally parallel to the deep-layer mean
wind. RAP forecasts maintain fairly strong instability through
12Z, with even a slight increase noted in the warm sector and
inflow region; MLCAPE may exceed 2000 j/kg in some areas. Combined
with precipitable water values around 1.7 to 1.9 inches, this will
easily support organized convective clusters and bands with rain
rates continuing to peak in the 2-3 in/hr range.

The QPF signal on hi-res models in the 06-12Z time frame is not as
strong as it was for some of the past 12 hours. This is likely due
to expected oscillations of the convective band, the relatively
narrow width of the convective line, and the broken nature of the
line; therefore, the heavy rain may not focus in one particular
area for multiple hours. However, given the heavy rainfall that
has already occurred, even 1.5 to 2.0 inches of rain in an hour
could be sufficient to exacerbate ongoing flooding or lead to
redevelopment of flooding.

Lamers

ATTN...WFO...BMX...FFC...MOB...TAE...

ATTN...RFC...SERFC...

LAT...LON   32588363 32238272 31468326 30848433 30248569
            30288668 30168749 30328785 30838754 31338681
            32188529