Special Marine Warning 108, NWS Key West, FL

Area Forecast Discussion
National Weather Service Key West FL
251 PM EDT Mon Jul 12 2021

.DISCUSSION...
A rather cloudy, breezy, and wet pattern is likely to persist
through Tuesday night across the Florida Keys and adjacent coastal
waters. An intensifying subtropical anticyclone centered near
Bermuda will accelerate gradient flow east to west, from the Turks
and Caicos to the southeastern Gulf of Mexico. This will not only
promote increasing upward fluxes of heat and moisture off the warm
surrounding seas, but will also be associated with an area of mass
speed convergence promoting lift on a meso-alpha scale. In
addition, a nearby TUTT cell is reflected at least down to the
middle troposphere, and is associated with steeper-than-average
lapse rates and reduced static stability across much of the
Florida Peninsula and Keys. Upper-tropospheric flow along the
southwestern flank of the TUTT cell is at least not inhibiting the
convective environment, and at best may be aiding and abetting it
via episodic favored difluent outflow aloft. Forecast rain and
thunder chances are about twice the climatological average for
tonight and Tuesday, and still well above average Tuesday night.
After a lull late this afternoon and early evening, mesoscale
convection allowing numerical weather prediction models suggest an
outbreak of showers and thunderstorms spreading east to west
across the Keys and adjacent coastal waters overnight, persisting
into the early morning hours. This is followed by another lull,
and then another outbreak of showers and storms in the
evening. Such is one of many plausible scenarios. By Wednesday,
the TUTT cell will be moving away/weakening, and near-surface flow
should becoming increasingly difluent with weak large-scale
subsidence returning. This is a more typical rainy season scenario
and will not completely shut off rain chances, but bring them back
to a the more climatological 20-30 percent range.

The local digital forecast database is being tweaked, and all
derived products will be sent and available via normal ways and
means by 1630 EDT.
&&

.MARINE...
Small Craft Should Exercise Caution in Florida Keys coastal waters
tonight because east breezes will be freshening with numerous,
fast-moving showers, squalls, and thunderstorms. Winds and seas
will be higher in and near showers and thunderstorms, and a Small
Craft Advisory may be required in some zones before midnight.
Subtropical high pressure centered over the North Atlantic Ocean
will build westward across the Florida Peninsula tonight and
Tuesday, resulting in moderate to fresh breezes across Florida
Keys coastal waters. Above average rain and thunder chances will
persist through Tuesday night, and winds and seas will be higher
in and near in these scattered showers and thunderstorms. A
decrease to mainly moderate breezes is expected on Tuesday night.
&&

.AVIATION...
Shower activity has all but diminished with a remaining cell between
the EYW and MTH terminals. This too will diminish over the next hour
with a considerable break expected thereafter. Expecting VFR
conditions to prevail through the afternoon and early evening.
Another round of convection may impact the terminals this evening,
and VCSH is introduced again at that time. Surface winds will remain
generally from the E to ESE at 9 to 15 knots with gusts up to 20
knots.
&&

.CLIMATE...
On this day in 1929, the daily record cold high temperature of
79F was recorded in Key West. this is also tied for the coldest
high temperature ever recorded in July. Temperature records date
back to 1872.
&&

.PRELIMINARY POINT TEMPS/POPS...
Key West  79  87  80  89 / 60 60 50 30
Marathon  78  89  80  91 / 60 60 50 30

&&

.KEY WATCHES/WARNINGS/ADVISORIES...
FL...None.
GM...None.
&&

Public/Marine/Fire...Kasper
Aviation/Nowcasts....LIW
Data Acquisition.....DR

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Hazardous Weather Outlook
National Weather Service Key West FL
425 AM EDT Mon Jul 12 2021

FLZ076>078-GMZ031>035-042>044-052>055-072>075-130830-
Monroe Upper Keys-Monroe Middle Keys-Monroe Lower Keys-
Florida Bay including Barnes Sound, Blackwater Sound, and
Buttonwood Sound-
Bayside and Gulf side from Craig Key to west end of Seven Mile
Bridge-
Gulf waters from East Cape Sable to Chokoloskee 20 to 60 NM out
and beyond 5 fathoms-
Gulf of Mexico including Dry Tortugas and Rebecca Shoal Channel-
Gulf of Mexico from west end of Seven Mile Bridge to Halfmoon
Shoal out to 5 Fathoms-
Hawk Channel from Ocean Reef to Craig Key out to the reef-
Hawk Channel from Craig Key to west end of Seven Mile Bridge out
to the reef-
Hawk Channel from west end of Seven Mile Bridge to Halfmoon Shoal
out to the reef-
Straits of Florida from Ocean Reef to Craig Key out 20 NM-
Straits of Florida from Craig Key to west end of Seven Mile
Bridge out 20 NM-
Straits of Florida from west end of Seven Mile Bridge to south of
Halfmoon Shoal out 20 NM-
Straits of Florida from Halfmoon Shoal to 20 NM west of Dry
Tortugas out 20 NM-
Straits of Florida from Ocean Reef to Craig Key 20 to 60 NM out-
Straits of Florida from Craig Key to west end of Seven Mile
Bridge 20 to 60 NM out-
Straits of Florida from west end of Seven Mile Bridge to south of
Halfmoon Shoal 20 to 60 NM out-
Straits of Florida from Halfmoon Shoal to 20 NM west of Dry
Tortugas 20 to 60 NM out-
425 AM EDT Mon Jul 12 2021

This Hazardous Weather Outlook is for the Florida Keys and
adjacent coastal waters.

.DAY ONE...TONIGHT

A Small Craft Advisory may be required tonight for the Straits of
Florida, Hawk Channel, and Bayside waters for easterly winds near
20 knots.

Scattered thunderstorms are expected across portions of the
Florida Keys and adjacent coastal waters today and tonight. The
primary thunderstorm hazards will be lightning strikes and gusty
winds.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

Scattered thunderstorms are expected Monday through Wednesday and
isolated thunderstorms are expected Wednesday night through
Sunday. The primary thunderstorms hazards will be lightning
strikes and gusty winds.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated.

NB

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