Mesoscale Precipitation Discussion 1058
NWS Weather Prediction Center College Park MD
357 AM EST Thu Dec 20 2018

Areas affected...Central West Coast of FL...

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

Valid 200900Z - 201400Z

SUMMARY...Shortwave approaching enhancing convection/moisture flux
with training leading to possible flash flooding or at least
prolonged inundation flooding.

DISCUSSION...Current GOES-16 WV suite, depicts a potent
tropical-stream shortwave approaching the Gulf coast of peninsular
FL, slowly enhancing at the right entrance to 120-130kt 250,
nicely anti-cyclonically arched jet streak across the eastern FL
panhandle lifting into GA.  This is providing significant upper
level DPVA/Divergence aloft for broad strong vertical ascent. 
Combine this with main height-falls from shortwave digging from
the Northwest out of LA at this time, low level flow will be
tightening and increasing from 40kts SSW to SW 55-60kts from the
SW from 12-15z at 850mb.  CIRA layered PW suggest slug of enhanced
moisture that is slanted from 7-5H currently nearing the west
coast while 850-7H slug angles from the Yucatan channel currently
nearing the main shortwave axis.  The leads to 1.75-2" total PWat
analyzed in the ascent region currently projected to reach the
coast by 13-15z.

At the surface, cooler shallow coastal shelf waters are inhibiting
the advection of higher theta-E air toward the coast, but a strong
FGEN convergence band as setup generally parallel to the coast
from near 27.2N83W to 25N83.2W along this bathymetric feature. 
The stronger mid-level deep layer convergence angles from just
west of the mouth of Tampa Bay to 24.6N86.7W.  This will allow for
a combined confluence of convection to be directed toward Tampa
Bay. Between the two lines, SBCAPEs are increasing to 1000-1500
J/kg, which will be isentropically lifted for near 1000 MUCAPEs to
support strong vertical growth through the early morning.  Given
the moisture flux, rates up to 2"/hour are possible.  Given the
orientation of the tropical-stream wave lifting north, there is
some likelihood for training convection particularly near the
coast and overall totals to be in the 2-4" range through 15z. This
potentially reaches the high FFG values that would suggest flash
flooding is possible. 

However, even if the cooler waters keep the convection from
eastward advancement decreasing vertical growth...and more
slantwise convection, rates of 1"/hr for a few hours still pose
longer-term slower rise inundation particularly from Sarasota to
Sun City, FL where up to 1.5" fell within the last 24hrs as well
as, locales from Tampa Bay north to Cedar Key into the interior
peninsula were 7 day totals are running 400-600% of normal and
river flooding is already occurring. Strong onshore flow is likely
to further compound flooding issues eliminating river outflow to
compound flood waters in the river basins. 

Gallina

ATTN...WFO...JAX...MLB...TAE...TBW...

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

LAT...LON   29818276 29718240 29348204 28778170 28238175
            27938181 27508197 27268209 27098227 27108255
            27368270 27678295 27948298 28268285 28528277
            28798282 28968304 29188326 29448345 29808324