Area Forecast Discussion
Issued by NWS Miami, FL

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FXUS62 KMFL 201949

Area Forecast Discussion
National Weather Service Miami FL
349 PM EDT Sun Aug 20 2017

At mid-afternoon Sunday: The axis of a tropical wave was moving
westward out of South Florida into the eastern Gulf. GOES-16
derived TPW imagery showed drier air approaching the Peninsula
from the east, but it hadn`t arrived quite yet. After a round of
showers and thunderstorms this morning and early afternoon, some
sunshine has allowed for destabilization of the air mass over the
southern and western parts of the area. A few additional showers
and thunderstorms are developing and will likely continue for a
few more hours into the evening before the drier air moves in,
ascent associated with the tropical wave departs the area, and
nocturnal cooling stabilizes the atmosphere.

Late tonight into Monday: The dry airmass will be moving over the
region, with subsidence between the departing tropical wave and
the approaching next tropical wave (Invest 92L). POPs will be much
below normal for this time of year. Forecast soundings actually
indicate a bit of a capping inversion above the moist boundary
layer, so while some scattered cumulus clouds are likely during
the eclipse viewing period, significant/deep clouds may be
limited. With plenty of heating and moderate easterly flow at
the surface, also expect a sea breeze will also push inland, and
cannot rule out an isolated shower or two along it. In the very
late afternoon and evening hours, the next tropical wave may draw
close enough to begin pushing some clouds and scattered showers
towards the east coast.

Tuesday and Wednesday: Another surge of deep tropical moisture
associated with the next tropical wave (92L) will move into the
area. This wave appears to be slower-moving, and its remnants in
the form of a shear axis will likely stall over or near South
Florida for a couple of days. In addition, height falls over the
northeast US/southeast Canada will induce more of a southerly flow
over the area as a weakening front pushes toward northern
Florida. The combination of all these features points to periods
of rain and thunderstorms, with potentially some local heavy rain
at times.

Thursday to Saturday: The decaying front slowly pushes south into
Florida and absorbs any remnant low-level vorticity from 92L.
Several medium range models hint at cyclogenesis over the Gulf
Stream northeast of our region as this absorption process takes
place. Regardless of whether a surface low forms or not, there
will be a lot of moisture around, a trailing shear axis, and deep
south/southwest flow providing a continuous moisture stream, which
would argue for additional rounds of rain and thunderstorms
(especially east coast). In fact, the NCEP Weather Prediction
Center is forecasting an areal average of 3 to 4+ inches of rain
over much of the region this week. This would indicate the
potential for much heavier amounts locally, driven by mesoscale
processes. This calls for careful monitoring of the flood
potential throughout the week.


Moderate to fresh easterly breezes are likely tonight as a
tropical wave departs the region, and again on Monday evening into
Tuesday morning as another tropical wave approaches. This will
bring a moderate chop to the local waters with significant wave
heights 4 to 5 feet in the open waters. Winds slacken and turn
southeasterly mid week and then southerly late in the week with
seas decreasing.

A few thunderstorms are possible this evening over the local
waters, and then thunderstorm chances increase again Monday night
through the rest of the week. Winds and waves will be locally
higher near any thunderstorms.


Convection associated with a tropical wave is moving across South
Florida today, bringing MVFR and brief bouts of IFR to the east
coast terminals. Convection should spread north and west this
afternoon and evening. Sub-VFR bouts will be possible with a need
for short-fused amendments through the evening. Convection should
begin to trend down as the wave exits later this evening. Eventual
clearing is expected later in the forecast period.


With breezy onshore flow, the risk of rip currents at the Atlantic
Beaches will rise to at least moderate levels later tonight
through Monday night.


West Palm Beach  81  93  81  92 /  20  20  30  60
Fort Lauderdale  82  91  81  91 /  20  20  40  60
Miami            81  92  80  91 /  10  20  30  60
Naples           77  96  79  92 /  20  20  10  50



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