Mesoscale Precipitation Discussion 0762
NWS Weather Prediction Center College Park MD
1135 AM EDT Tue Sep 04 2018

Areas affected...Florida Panhandle

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

Valid 041534Z - 042010Z

Summary...Peripheral rain bands from Tropical Storm Gordon may
impact portions of the Florida Panhandle into the afternoon. These
rain bands would be likely to produce very heavy rain rates
approaching (or locally exceeding) 3 in/hr, which could lead to
flash flooding, particularly if they align with urban areas.

Discussion...KEVX and KTLH radars showed a prominent convergent
band of convection extending from near PAM to AAF, and on into the
eastern Gulf of Mexico to near the buoy 42022 west of TPA. The
radars were showing ZDR values just over 1 dB coincident with Z
values reaching 50 dBZ, implying very large concentrations of
smaller, more spherical droplets. This is a common feature of
tropical rain bands, and suggests that 3 in/hr rain rates would be
achievable if the rain band can persist in a particular area for
that length of time. A WeatherSTEM mesonet site near Port St. Joe
recorded 0.61 inches of rain in 15 minutes, but the convective
band was somewhat disjointed in that area and thus the rain rates
were not sustained. Therefore, rainfall could be limited by a
non-continuous line of convection, and would also be limited by a
drift in a particular direction. There are some hints on GOES-16
IR satellite that a slow northward drift of the band was
occurring, and this could prevent the convective rain band from
locking in to a particular location.

Hi-res models were struggling with depicting this rain band, and
thus the rainfall and flash flood potential may be a little higher
than they would suggest. There is an obvious convergent pattern to
the surface wind observations in this area, and that could help
sustain a relatively solid convective band for at least a few
hours. MRMS and KTLH dual pol estimates showed rain rates peaking
around or just over 3 in/hr offshore, and given the observed rain
rates by mesonet sites, 3 in/hr rain rates would be possible over
land as well if the convective band is sustained for a sufficient
period of time. With the limiting factors already mentioned, flash
flooding may be localized where the overall mesoscale setup and
storm-scale processes combine to briefly sustain heavy rain rates
for a longer period of time, particularly if that occurs over an
urban area.

Lamers

ATTN...WFO...TAE...

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

LAT...LON   30758596 30688488 30378411 29738439 29478520
            29948574 30258635