SUMMARY...Heavy showers and thunderstorms will continue to
generally increase in coverage and concentration heading into the
overnight hours. There will be concerns for extreme rainfall rates
and some cell-training which is expected to result in scattered to
numerous areas of flash flooding. Some of this may become
particularly significant and life-threatening.
DISCUSSION...Deep layer tropical moisture transport continues into
the Southeast U.S. well to the north of Hurricane Rafael which is
currently crossing western Cuba. The latest CIRA-ALPW imagery
shows strong concentrations of this tropical moisture in the low
and mid-levels of the vertical column, and overall the PWs now up
across the FL Panhandle into central and southern GA have risen
into the 2.25 to 2.5 inch range based on some of the polar
microwave and GPS data.
This moisture transport continues to work in tandem with MLCAPE
values of as much as 1000 to 1500 J/kg and proximity of a surface
trough to promote numerous areas of heavy showers and
thunderstorms. Radar generally shows the greatest concentrations
of convection focused over areas of southern and eastern GA and
into portions of the central SC. The flow aloft continues to be
increasingly divergent which is showing up well in the GOES-E IR
satellite imagery along with the expansion of cooling convective
tops as favorable upper-level jet divergence sets up.
The latest 18Z HREF guidance and HRRR solutions support locally
some additional expansion of convection along with a likelihood
for this activity to become increasingly concentrated and focused
over areas of east-central GA and potentially edging into parts of
the SC Midlands and Lowcountry heading into the overnight hours.
The deeply tropical environment coupled with the level of
instability and low to mid-level shear may favor some particularly
high rainfall rates overnight. Some of these rates may reach 2 to
4 inches/hour and will tend to be maximized with any mesocyclone
activity that occurs near the aforementioned surface trough axis.
The cell-training concerns associated with these rates will favor
additional rainfall totals of 4 to 6 inches by midnight with
locally heavier amounts possible.
Dry antecedent conditions at least in the short-term will continue
to locally mitigate the flash flood threat, but as these extreme
rainfall rates materialize and persist into the overnight hours,
there will eventually be scattered to numerous areas of flash
flooding, and some of this may become significant and
life-threatening.