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Area Forecast Discussion (AFD)

Issued by Key West, FL (KEY)

FXUS62 KKEY 212016

Area Forecast Discussion
National Weather Service Key West FL
316 PM EST Tue Nov 21 2017

A line of semi-persistent showers linger across the Middle Keys, 
bayside waters and near the Dry Tortugas, with some lighter 
activity off the Florida Straits. Temperatures are in the upper 
70s to low 80s. Dew points have risen to the low 70s. ASOS and 
CMAN stations report winds onshore and offshore to be 5 to 10 mph,
variable in direction but generally out of the southeast. A weak 
warm front is lifting from Central Florida to North Florida, and a
weak upper level trough is remaining relatively stationary across
the Central Gulf. 

As the warm front continues to lift northward, a dry air mass (evident
on MIMIC) will move into the southern gulf and across the Florida
Keys, filtering down below the 850 mb level tonight and dropping 
PWAT accordingly. While this will be a brief event, we feel
reasonably confident with maintaining the current forecast with
lower shower coverage overnight and Wednesday. In the boundary
layer however, dew points near and above 70 degrees will continue,
so expect temperatures to be a little warming in the morning. A 
surface trough digging across the Central Plains will interact 
with a shortwave overnight, and cyclogenesis of a low pressure 
system may begin in the west- central gulf may begin as early as 
Wednesday morning.


The main focus in the long term portion of the forecast will be
the expected cyclogenesis in the Gulf of Mexico, with the non-
tropical low approaching the Florida Peninsula Thursday, crossing
the state Friday, and exiting into the western Caribbean 
Saturday. The subtropical jet positioned across the Florida Keys 
will aid in providing reasonable support for scattered showers and
isolated thunderstorms, a few strong, in time for Thanksgiving 
and the Friday thereafter. As the system sweeps out during the 
weekend, a cooler continental air mass will build in behind it, 
with a reinforcing dry frontal boundary behind it early next 
week... ushering dew points into the low 60s, and max/min temps 5 
degrees below normal. Stay tuned.


No watches, warnings, or cautionary headlines are in place across
the coastal waters adjacent to the Florida Keys. Expect gentle
breezes and pleasant marine conditions through Thursday as a 
broad low pressure system forms in the eastern Gulf of Mexico. The
low will shift across the Florida peninsula Friday, moving into 
the western Atlantic Saturday. Uncertainty exists as far as the 
track of this non-tropical system. A broad high pressure system 
shifts into the southeastern US Sunday night and Monday.


VFR conditions will prevail at EYW and MTH this afternoon and most 
of tonight. MVFR ceilings, based at around 2,500 feet, are 
conceivable at EYW and MTH late tonight and Wednesday morning. This 
possibility is not great enough to include in the terminal forecasts 
at this time. Sustained southeast to south winds of 10 to 15 knots 
from just off the surface to about 3,000 feet will decrease to 5 to 
10 knots tonight. 


On this date in Florida Keys weather history, in 1914, the high 
temperature in Key West was 61 degrees. This set the daily record 
for coolest maximum temperature measured in Key West on November 
21st, a record which still stands 103 years later. In addition, this 
is tied as the coolest maximum temperature ever observed in Key West 
during the month of November. Temperature records at Key West date 
back to 1872.


Key West  73  81  73  81 / 20 20 20 50 
Marathon  74  82  74  83 / 20 20 20 50 




Data Collection......Chesser

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