Area Forecast Discussion
Issued by NWS Miami, FL

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FXUS62 KMFL 181157
AFDMFL

Area Forecast Discussion
National Weather Service Miami FL
757 AM EDT Fri Aug 18 2017

.AVIATION...
A drier day is expected today, though APF and some of the inland
east coast terminals could see convection around. Sub-VFR is
possible around storms, which could prompt short-fused amendments.
Things should quiet down for the evening, though an approaching
wave could bring increasing rain chances beyond the TAF period.

&&

.PREV DISCUSSION... /issued 406 AM EDT Fri Aug 18 2017/

DISCUSSION...

TODAY AND TONIGHT: An interesting synoptic setup will be in place
across South Florida today. Satellite shows a well defined upper
level low moving through the Central/NW Bahamas and eastern Cuba
this morning. Entrained within in it is a considerable amount of dry
air, a Saharan Airmass, that is suppressing most convection despite
the cold mid level temps of -8 to -9C.

Models are showing that this dry air will be the dominant feature,
holding storm chances generally in the 30-50% range despite an
otherwise favorable dynamic profile with moderately steep lapse
rates and a 40-50kt upper level jet as the low tracks west
through the FL Straits later today. As with the past few days we
should see storms getting going around midday as both seabreezes
develop and move inland, focusing towards the western interior
later in the afternoon and evening. The presence of the
aforementioned upper level features may help enhance storms, with
a few more strong storms with gusty winds likely, especially with
boundary collisions.

With limited cloud cover and storm coverage, temperatures will rise
well above normal with highs climbing into the low to mid 90s. A
few upper 90s are likely for the interior portions of Collier into
Hendry counties. See the climate section below for today`s record
highs.

THIS WEEKEND: An unsettled and stormy weekend looks to be in store
for South Florida. The upper level low and the coldest mid level
temperatures will move into the Gulf Saturday morning, but this will
place a robust southerly upper level jet across the region with
speeds in the 40-50kt range. Moving into the region behind the upper
level feature will be a tropical wave (NOT any of the features
currently being monitored for development by NHC), with its deeper
tropical moisture spreading across the area Saturday afternoon
and Sunday.

Models show it potentially taking most of Saturday morning for
the deeper moisture to advect across the region, which may give us
a period of morning sunshine to help destabilize the atmosphere.
With temperatures a little cooler aloft, a few more strong storms
may be possible.

The wave axis looks to cross the region Saturday night, putting us
on the ascending side on on Sunday. With temperatures warming aloft
and the deep tropical airmass in place, activity on Sunday will
likely be more in the form of rain with embedded thunderstorms.
Moderate east-southeast flow, around 15 knots, may also help enhance
convergence and rainfall along the east coast, mainly early in the
day. Heavy rainfall will be a concern both days, most likely in the
Saturday afternoon to midday Sunday timeframe. WPC has already
highlighted most of South Florida for a slight risk of excessive
rainfall for 12Z Sat through 12Z Sunday.

NEXT WEEK: Model forecasts for next week suggest the wet and
unsettled pattern will continue through the week.

Upper level low and tropical wave will depart to the west on Monday
as a strong upper level ridge builds across North Florida/GA. Ridge
center will migrate westwards through midweek to our north, keeping
South Florida under deep east-southeast flow. Monday looks to be the
driest day of the week with near normal storm coverage as dry air
and subsidence move in behind the low/tropical wave.

The exact details of the weather Tuesday through Thursday will hinge
on the exact development and movement of the tropical disturbance
currently located about 1000 miles east of the Leeward Islands. As
of 2 AM, NHC currently gives this feature a 70% change of developing
into a tropical cyclone in 2 days and 5 days respectively. It is
expected that this feature will continuing moving northwest and
potentially be in the Bahamas and South Florida as we head into mid
next week.

Regardless of development, at this point signs are pointing towards
another period of heavy rainfall across the region for at least
Tuesday and Wednesday. Which, especially depending on this weekend`s
rainfall, may raise flooding concerns across the area. All residents
and visitors or South Florida need to continue to monitor this
tropical disturbance through the weekend.

MARINE...East-southeast winds will prevail for the weekend and
through the upcoming week across the local waters. The main concern
will be wind speeds, with several passing tropical waves
periodically bumping up speeds to cautionary levels. Right now,
expect overall speeds to increase to around 15 knots, with 15-20kts
possible with the first wave Saturday night into Sunday, and again
Monday night into Tuesday. Seas 2ft or less today, building to 3-
4ft, especially in the Atlantic by Sunday.

Convection will remain more isolated over the open waters today,
favoring the the interior/Lake Okeechobee this afternoon. Expect
scattered to numerous storms over all the waters for the weekend
with the first tropical wave.

The threat of rip current could increase this weekend along the
east coast beaches of South Florida, due to the increase in
easterly wind flow.

CLIMATE...High temperatures today may be within a few degrees of
record.

      Forecast Highs     Record Highs
Miami       94             95 - 1995
Fort Lauderdale   94             98 - 1921
West Palm Beach    95             96 - 1949
Naples       94             97 - 2001

&&

.PRELIMINARY POINT TEMPS/POPS...
West Palm Beach  95  81  91  80 /  20  10  60  60
Fort Lauderdale  94  82  90  81 /  20  20  60  60
Miami            94  81  90  80 /  20  20  70  60
Naples           94  76  92  78 /  50  30  40  40

&&

.MFL WATCHES/WARNINGS/ADVISORIES...
FL...None.
AM...None.
GM...None.

&&

AVIATION...02/RAG


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