Mesoscale Precipitation Discussion 0764
NWS Weather Prediction Center College Park MD
457 PM EDT Tue Sep 04 2018

Areas affected...Southern Alabama; Western Florida Panhandle,
Southern Mississippi

Concerning...Heavy rainfall...Flash flooding likely

Valid 042056Z - 050200Z

Summary...Tropical Storm Gordon continues to approach the
north-central Gulf coast, and heavy rain bands associated with the
center of the storm will begin to move onshore in the next few
hours. This will likely bring very heavy rain rates to the
coastline from Mississippi, to Alabama, to the western Florida
Panhandle, and increase the risk of flash flooding into the
evening. The greatest risk through 9 PM CDT should be within 30
miles of the coast from Pascagoula, MS to Pensacola, FL.

Discussion...The 21Z advisory from the National Hurricane Center
for Tropical Storm Gordon discusses a vertical tilt to the
cyclone, with the mid-level center displaced to the E/NE of the
surface circulation. This is also evident when overlaying radar
mosaic loops with the 21Z surface center, as the largest core
convective band was focused around a mid-level center.
Extrapolation of radar trends closely matches available hi-res
models, particularly the last few runs of the HRRR, and takes the
comma head portion of the curved convective band into the
north-central Gulf Coast around 22-23Z. The HRRR focuses some of
the highest rain rates in this area as well, and this is supported
by current radar trends. From the KEVX radar, the ZDR values in
this band were almost entirely below 1 dB with KDP values around
1-2 deg/km, implying very large concentrations of small droplets
(and a high likelihood of efficient rainfall processes and heavy
rain rates). Further to the northeast, there was a peripheral
convergence band inland over the Florida Panhandle with ZDR values
closer to 2 dB and higher reflectivity values, suggestive of
larger droplets but less efficient rainfall processes.

Therefore, the greatest flash flood risk through 02Z is expected
to be generally within 30 miles of the coast from PQL to PNS.
Heavy rain bands with some element of training motion will be
possible further inland or to the east, but should have slightly
lower rain rates and are more likely to drift off a particular
axis. This should limit the flash flood threat elsewhere in the
outlined area through 02Z, but it cannot be ruled out entirely.

As the evening progresses, the HRRR does show a small area of high
rain rates being maintained closer to the center of Gordon, and
that would be likely to propagate gradually to the northwest with
time. However, it also shows a large convective band developing on
the east side of the cyclone and feeding into an area centered
around Pensacola. This will likely extend the flash flood threat
beyond the time period of this discussion, and could lead to a
small area of more significant rainfall totals overnight near the
coast but well to the east of the low-level center.

Lamers

ATTN...WFO...LIX...MOB...TAE...

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

LAT...LON   31418809 31348659 30768517 29798448 29428516
            30108652 30038827 30188905 30618917 31078891