Special Marine Warning 13, NWS Key West, FL

Area Forecast Discussion
National Weather Service Key West FL
1104 PM EST Fri Jan 31 2020

...A SEVERE THUNDERSTORM WATCH REMAINS IN EFFECT UNTIL 3 AM...

.DISCUSSION...
An upper-level composite analysis this evening shows a large-scale
trough from the Great Lakes to Mexico, with an expansive area of
cyclonic west-southwest flow over the Gulf of Mexico basin. Large-
scale ascent east of the trough axis, induced by geopotential
height falls and transient mid-tropospheric cyclonic vorticity
maxima has destabilized tropospheric columns around the Florida
Keys. In addition, a developing complex area of low pressure over
the eastern Gulf of Mexico and Florida Peninsula has helped draw
tropical maritime air northward out of the western Caribbean and
Yucatan Channel area. The air mass that developed around the Keys
by early evening as a result of these interactions was moist,
moderately unstable, and highly sheared. Intense clusters of deep,
moist convection developed during the afternoon over the Loop
Current, and moved eastward. By sunset, a classic bow echo
structure approached the western marine zones, and then evolved
into a stratiform-trailing quasi-linear convective system with
prolific, vivid lightning displays, blinding downpours of rain,
and strong, but mostly non-severe wind gusts. The most intense
part of this northeast-southwest-oriented squall line is
approaching Florida Bay, the middle Florida Keys, and the Florida
Reef tract near Sombrero Key. We expect the convective system to
migrate eastward during the next few hours, delivering rainfall
coverage of nearly 100 percent to Florida Keys island communities.
Thereafter, rain should diminish in intensity, and then end in the
dark of overnight. The potential for severe thunderstorms should
end abruptly as the squall line clears the Keys to the east, with
cancellation of the Severe Thunderstorm Watch likely before the
listed expiration time of 3:00 a.m. EST.

We have updated the digital forecast database and derived
products, based on our evening analysis which accounts for the
latest trends in both radar and surface observations.
&&

.MARINE...
A Severe Thunderstorm Watch remains in effect until 3:00 a.m. EST.
A Small Craft Advisory likely will be required on Saturday. A
complex frontal system will shift eastward from the Florida
Peninsula to the Atlantic Ocean overnight and early Saturday.
Widespread rain and thunderstorms across the Florida Keys and
adjacent coastal waters will end toward daybreak Saturday. A cold
front is expected to move swiftly eastward across the Gulf of
Mexico, and then sweep across Florida Keys coastal waters late
Saturday afternoon or early evening. In wake of the cold front,
high pressure will build in from the west.
&&

.AVIATION...
A Severe Thunderstorm Watch is in effect for EYW and MTH until
01/08Z. Through 01/24Z, MVFR/IFR episodes are likely at both EYW
and MTH, with a stretch of VFR conditions between 01/07Z and
01/21Z. Thunderstorms have moved away from EYW, but are imminent
at MTH. The best chance for thunderstorms Saturday at both EYW and
MTH will be after 01/21Z preceding an approaching cold front.
Sometime after 01/21Z, winds will shift from about 320/20G27KT
resulting in northerly crosswinds persisting Saturday night.
&&

.CLIMATE...
In 1983, Key West experienced its wettest January ever with
17.64" of rain recorded, with 12.11" of this total falling on the
22nd and 23rd. despite being in the heart of the climatological
dry season, this is the seventh-wettest month on record for Key
West, with more than twice as much rain as the next-wettest
January. The winter of 1982-1983 was strongly influenced by El
Nino conditions in the Pacific.
&&

.KEY WATCHES/WARNINGS/ADVISORIES...
FL...Severe Thunderstorm Watch in effect until 3:00 a.m. EST.
GM...Severe Thunderstorm Watch in effect until 3:00 a.m. EST.
&&

Public/Marine/Fire...Kasper
Aviation/Nowcasts....Haner
Data Collection......KBL

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Hazardous Weather Outlook
National Weather Service Key West FL
505 AM EST Fri Jan 31 2020

FLZ076>078-GMZ031>035-042>044-052>055-072>075-011100-
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-
505 AM EST Fri Jan 31 2020

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

.DAY ONE...TODAY AND TONIGHT

Scattered thunderstorms will be possible across the Florida Keys
and adjacent waters this afternoon and tonight. The primary
thunderstorm hazards will be cloud to surface lightning strikes
and strong gusty winds. A few thunderstorms may become severe,
producing damaging winds. An isolated tornado or two will also be
possible tonight. The Storm Prediction Center is forecasting
a slight risk for severe thunderstorms across the Keys for this
afternoon and tonight.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

Isolated thunderstorms will be possible across the Florida Keys
and adjacent waters on Saturday. The primary thunderstorm hazards
will be cloud to surface lightning strikes and strong gusty
winds. A couple of thunderstorms may become strong to severe,
producing damaging winds. The Storm Prediction Center is
forecasting a slight risk for severe thunderstorms across the Keys
for Saturday.

A Small Craft Advisory will likely be required for all of the Keys
waters from late Saturday through at least Sunday morning, due to
strong northwest to north winds and building seas behind a cold
front. Winds and seas should decrease below advisory levels by
Monday.

For Monday through Thursday, no hazardous weather is expected at
this time.

.SPOTTER INFORMATION STATEMENT...

Spotters should report any hazardous weather observed to the
National Weather Service in Key West according to standard
operating procedures.

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Special Weather Statement
National Weather Service Key West FL
1058 PM EST Fri Jan 31 2020

FLZ076-077-010500-
Monroe Upper Keys FL-Monroe Middle Keys FL-
1058 PM EST Fri Jan 31 2020

...SIGNIFICANT WEATHER ADVISORY FOR MIDDLE KEYS IN MONROE AND UPPER
KEYS IN MONROE COUNTIES UNTIL MIDNIGHT EST...

At 1058 PM EST, National Weather Service Meteorologists detected
strong thunderstorms along a line extending from 10 miles southeast
of Mahogany Hammock to 10 miles southeast of Flamingo to Grassy Key
to 14 miles south of Bahia Honda Key. Movement was east at 35 mph.

Winds in excess of 45 mph will be possible with these storms.

Locations impacted include...
Marathon, East End Of Long Key Viaduct, East End Of Channel 5 Bridge,
West End Of Channel 5 Bridge, Jewfish Creek Bridge, Long Key Viaduct,
Key Largo, Channel 5 Bridge, West End Of Long Key Viaduct and
Plantation.

This includes US 1 between mile markers 51 and 112.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with these storms, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

&&

A Severe Thunderstorm Watch remains in effect until 300 AM EST for
Florida.

LAT...LON 2528 8025 2513 8034 2493 8055 2479 8078
      2478 8086 2466 8106 2467 8111 2477 8103
      2487 8077 2493 8071 2495 8065 2508 8050
      2524 8050 2522 8048 2526 8043 2523 8038
      2529 8039 2528 8033 2535 8026
TIME...MOT...LOC 0358Z 269DEG 29KT 2521 8077 2502 8081 2476 8097 2446
8121

Haner