Mesoscale Precipitation Discussion 0801
NWS Weather Prediction Center College Park MD
315 PM EDT Mon Sep 28 2020

Areas affected...Southern and Central Florida Peninsula

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

Valid 281915Z - 290100Z

Summary...Increasing coverage of showers and thunderstorms across
the Florida Peninsula will create a flash flood risk into the
evening. Rainfall rates may exceed 2"/hr at times. Slow motion of
these rain rartes to the E/NE could produce 2-4" of rainfall with
locally higher amounts. Flash flooding is possible.

Discussion...GOES-E 10.35 micron IR imagery this aftn indicates
pockets of rapidly cooling cloud tops across the southern and
central Peninsula of Florida. These cooling tops are associated
with expanding and deepening thunderstorms evident on the regional
radar mosaic. The environment across much of FL is extremely
favorable for heavy rainfall. PWs as measured by GPS observations
are approaching 2.25", above the 90th percentile according to the
NAEFS ensemble table, and SBCape as analyzed by the 18Z RAP has
eclipsed 3500 J/kg across much of the southern half of the state.
This environment are being impinged upon by a 50kt jet streak
across N FL developing ahead of a deepening and eastward advecting
long wave trough, producing modest height falls and a weak
divergence maxima overhead to drive deep layer ascent.

The extremely favorable thermodynamics are allowing for heavy
rainfall rates despite overall modest forcing. Recent rain rate
estimates from KMLB and KAMX are exceeding 1.5"/hr, and the HREF
hourly probabilities indicate these may climb above 2"/hr at times
this aftn. Although 850mb inflow is weak at 5-10kts from the Gulf
of Mexico, this is roughly equal to the 0-6km mean wind, and with
synoptic flow out of the W/SW, a pinned east coast sea breeze is
likely where storm mergers may occur. Propagation vectors aligned
with the mean flow indicate the likelihood for cell training, and
with slow motions expected, and cell mergers likely near the east
coast, heavy rainfall amounts of 2-4" are forecast by many of the
available CAMs.

During the past week, much of this area has seen below normal
rainfall, although a few pockets have received >200% of normal.
This has led to FFG which is generally 3"/1hr and 4"/3hrs. With
rain rates rising above 2"/hr and slow storm motions of 5-10 kts
to the E/NE likely, some exceedance of these values are possible
as echoed by the HREF indicating a 50% chance for the 3-hr FFG.
Flash flooding will be most likely should this heavy rainfall
occur atop the pockets of higher antecedent soil moisture or in
any urban areas, but anywhere along the pinned sea breeze where
storms may briefly stall or regenerate in place could produce
flash flooding.


Weiss

ATTN...WFO...JAX...MFL...MLB...TBW...

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

LAT...LON   29688123 29158089 28588060 28178054 27628032
            27228016 26978007 26708009 26498015 26288030
            26218043 26198058 26258079 26358102 26568113
            26798145 26988157 27488173 28048186 28528191
            29108176 29508158