AWUS01 KWNH 291535
FFGMPD
FLZ000-291945-

Mesoscale Precipitation Discussion 1205
NWS Weather Prediction Center College Park MD
1034 AM EST Sun Dec 29 2024

Areas affected...FL Keys

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

Valid 291533Z - 291945Z

SUMMARY...There is increasing potential for torrential rainfall to
impact the FL Keys over the next few hours. Rainfall rates of 2 to
4 in/hr will be possible, though significant uncertainty remains
with the evolution of rainfall.

DISCUSSION...15Z visible satellite and local radar imagery from
KBYX showed a WSW to ENE axis of thunderstorms about 10 miles
north of the lower and middle FL Keys. The storms were slow
moving, located along a long-lived outflow boundary which has been
slowly dropping south over the past few hours. The environment was
favorable for organized cells with 40+ kt effective bulk shear and
500-1000 J/kg MLCAPE and about 2 inches PWAT (SPC mesoanalysis and
12Z KEY sounding). KBYX estimated rainfall rates were 2-4 in/hr
across the offshore waters of FL Bay. Area surface observations
along the Keys showed southerly winds of 10-20 kt with gusts to
near 30 kt, within a broadly confluent low level wind pattern just
above the surface, allowing overrunning of the slow moving outflow
boundary with cells generally training from west to east. There
were embedded mesocyclones with a fairly large mesocyclone/mesolow
located about 15 miles northwest of MTH. Upper level flow was
diffluent ahead of a southern stream upper jet max over the Gulf
of Mexico.

While hires models are unable to handle the mesoscale details of
the ongoing convective axis, loops of satellite/radar suggest the
outflow will continue to slowly sink south with heavy rain
impacting the Keys over the next 1-2 hours. Cyclonic flow around
the mesolow northwest of MTH may cause the outflow to
progressively sink south through the lower Keys over the next
couple of hours, with only a glancing blow of heavy rain to land
areas. However, this scenario is only speculation and continued
upstream convective development and strong southerly flow may
offset any stronger southward push of outflow, allowing for
stalling and torrential rainfall to persist a bit longer. Even if
rainfall overlaps the island chain for "only" an hour or two, that
could still result in several inches of rain which may produce
flash flooding.

Otto

ATTN...WFO...KEY...

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

LAT...LON   25258024 24738027 24338119 24408180 24688195 
            24878117 

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