Area Forecast Discussion
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223
FXUS62 KKEY 200806
AFDKEY

Area Forecast Discussion
National Weather Service Key West FL
406 AM EDT Fri Sep 20 2019

.DISCUSSION...
Scattered fast-moving showers were noted on radar this morning
with fresh to strong northeast breezes of 15 to 20 mph over the
islands and 19 to 24 knots measured along the Reef. Strong high
pressure over the Mid Atlantic and relatively low pressures across
the central and western Caribbean Sea have maintained the 7 to 8
mb gradient from Jacksonville to Havana. GOES16 confirmed the
relative minimum in moisture over the Keys; scant above 8,000 ft
from the Thursday evening sounding, and estimated precipitable
water from 1.2 to 1.5 inches through the local area. Aloft, a
sharp trough axis extends through the northwest Bahamas and
central Cuba. Deep moisture and the entrance to a 50- to 70-knot
upper jet was not too far east of the trough axis.

Certainty surrounds the strength of high pressure to the north,
with the upper trough/shear axis slowly moving westward through
the Keys by tonight. This heralds the return of deeper moisture,
although the best upper jet/wind maximum only manages to reach the
eastern Straits by Saturday. Guidance has quite a spread in rain
chances, partly due to a steep moisture gradient with dry air not
too far up the Florida Peninsula and better proponents for upward
vertical motion remaining a bit to the east. We are delaying the
slight chance of thunder until afternoon, with rain chances nudged
upward mainly due to the presence of the sharp trough/upper shear
axis passing through. While coverage of showers may be largely
scattered at any given movement, fast cell motion toward the
west-southwest will mean quick but brief rainfall, with the
geographic alignment of the Keys maximizing coverage of rainfall
given the mean east-northeast flow. Should the upper trough move
far enough (and fast enough) westward, the upper southwest
relative wind maximum may also lead to higher rain chances
Saturday.

Confidence in rainfall dips a bit by early Sunday as some guidance
indicates a temporary push from the east coast high pressure,
thinning moisture depth in spite of continuing fresh to strong
east-northeast winds. Low-to-mid-level troughing moving slowly
west from the Caribbean appears not to bodily translate through
the area, but bringing a potential slug of deep moisture later
Sunday through Monday. Rain chances were boosted a bit to account
for this, however deep drying Monday night occurs right on
schedule with previous forecasts of slight rain chances.

Gradual weakening of east coast high pressure will result in
steadily decreasing breezes by mid-week, with a return to
climatological rain chances near 30 percent by week`s end.

&&

.MARINE...
Small Craft Advisory in effect for all coastal waters of the
Florida Keys. Northeast to east winds of 20 to 25 knots will
likely peak near 25 knots this evening, as well as Saturday
evening. Likewise, large seas continue to build across the Straits
of Florida. High pressure will maintain fresh to strong breezes
through Monday, with some lulls occasionally occurring in the lee
of the Florida Peninsula.

&&

.AVIATION...
Windy, but VFR conditions will prevail at both terminals. Sustained
northeast winds near 15 knots with frequent gusts to around 25 knots
will be the expected throughout Friday. By the late afternoon and
evening, east-northeast winds will increase to 20 knots sustained
with frequent gusts near 30 knots. Brief bouts of passing showers in
the morning will give way to showers and possible thunderstorms by
the late afternoon and evening hours.

&&

.CLIMATE...
On this date, September 20, in 2005, the center of Hurricane Rita
passed 45 miles south of Key West as a category two hurricane.
Key West International Airport recorded sustained winds of 62 mph
and gusts to 76 mph. Flooding 1 to 2 feet above ground level
flooded up to 4 blocks inland in portions of oceanside Key West.
Numerous coastal streets flooded with the Overseas Highway
temporarily impassable at Sea Oats Beach on Lower Matecumbe Key.
Rainfall totalled 4.26 inches at John Pennekamp State Park in Key
Largo.

&&

.PRELIMINARY POINT TEMPS/POPS...
Key West  88  78  87  79 / 50 60 50 50
Marathon  89  78  88  79 / 50 60 50 50

&&

.KEY WATCHES/WARNINGS/ADVISORIES...
FL...None.
GM...Small Craft Advisory for GMZ031>035-042>044-052>055-072>075.

&&

$$

Public/Marine/Fire...JR
Aviation/Nowcasts....Chesser

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