Mesoscale Precipitation Discussion 1038
NWS Weather Prediction Center College Park MD
1251 AM EST Mon Dec 03 2018

Areas affected...Central Florida Panhandle and South Georgia

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

Valid 030550Z - 031150Z

Summary...Rain and thunderstorms will persist through the
remainder of the night from the Florida Panhandle into South
Georgia, and repeated waves of thunderstorms in some locations
will lead to localized swaths of heavy rainfall. Rain rates should
peak in the 1-2 in/hr range and this may lead to flash flooding,
particularly where rain rates of that magnitude intersect prior
swaths of heavy rainfall from the past 12-24 hours.

Discussion...A relatively stagnant pattern over the Southeast
continues to support a quasi-stationary band of rain and
convection stretching from southern Georgia back to the southwest
into the Florida Panhandle and the northern Gulf of Mexico. This
band of convection was situated on the poleward (northwest)
periphery of a plume of PWs around 1.7 to 1.9 inches and MLCAPE of
around 1000-1500 j/kg. This should maintain organized convective
clusters and lines through the remainder of the night, especially
given the convection is embedded in a region of about 50-60 knots
of deep layer shear. The combination of moderate instability with
relatively high PWs should yield rain rates into the 1-2 in/hr
range, and KVLD reported 1.46 inches in the 04-05Z hour. 1hr flash
flood guidance is generally above 2 inches across the region,
which means that flash flooding would be most likely to develop
(1) in areas that have received the heaviest rainfall over the
past 12-24 hours, or (2) due to persistence of heavy rain rates
for 2-3 hours.

Over the past day, the heaviest rainfall in the outlined areas has
fallen from near the Florida-Georgia border around Valdosta, ENE
to Waycross and Brunswick. NSSL QPE shows over 3 inches of
rainfall in these areas, with localized areas receiving more than
double that. It just so happens that the ongoing band of
convection intersects some of these areas with very saturated
soils. Hi-res models show the strongest convection through around
10Z being maintained in the same area, further northeast into
portions of Georgia where the ambient flow in the 925-850mb layer
is stronger. It makes sense that the strongest convection would be
maintained where the LLJ is strongest, especially given the slight
inversion indicated on the 00Z TLH sounding. Thus we may see
elements of both heavy rain rate persistence and antecedent
conditions creating some flash flooding in south-central and
southeast Georgia.

Further to the southwest, there is a significant cluster of strong
convection (as of 0530Z) pushing into the central Florida
Panhandle from the Gulf of Mexico just to the northwest of Cape
San Blas. This convection may train for an hour or two and lead to
additional swaths of heavy rainfall from Mexico Beach northeast
toward Tallahassee. However, these areas have received slightly
less rainfall as compared to south Georgia, and thus antecedent
conditions will be initially less favorable for flash flooding;
this is reflected in higher FFG numbers. Nevertheless, given the
vigor of the convection, flash flooding will be possible in these
areas as well given the favorable training pattern. As we approach
sunrise, 09-12Z, RAP forecasts show increasing convergence over
the north-central Gulf of Mexico as winds over the central Gulf
Coast states begin to veer with a weak wave pushing in. This may
reinvigorate upstream convection over the Gulf, perhaps displaced
slightly to the left of ongoing convection relative to the mean
storm motion vector, and this could lead to a continuing threat of
flash flooding into the morning hours over the Florida Panhandle.

Lamers

ATTN...WFO...CHS...JAX...MOB...TAE...

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

LAT...LON   32128065 31808079 31288115 30768127 30278251
            29868375 29618455 29478520 30058584 30328671
            30688625 30898496 31268336 32108165