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Area forecast discussion
National Weather Service Miami Florida
731 am EDT sun Aug 20 2017

Aviation...
convection associated with a tropical wave is moving across South
Florida this morning, bringing MVFR and brief bouts of IFR to the
East Coast terminals. Convection should spread north and west
today, with the potential to persist a good amount of the day.
Sub-VFR bouts will be possible with a need for short-fused
amendments through the day. Convection should begin to trend down
as the wave exits later this evening.

&&

Previous discussion... /issued 416 am EDT sun Aug 20 2017/

Discussion...

Today and tonight: the axis of the first of our tropical waves is
moving into the southern peninsula early this morning. The
ongoing convection over the local Atlantic and northwest Bahamas has
waned over the past few hours, leaving only isolated light showers
as we approach daybreak. This may potentially be due to ongoing
shear from the upper level low over the eastern Gulf and/or a
weaker tropical wave limiting low level convergence.

Models have fallen into two camps this morning, either backing
off on convective development through mid morning or ramping up
activity along the local Atlantic and East Coast right after
daybreak. While there has been a modest increase in light showers
in the past hour, have leaned towards the slower start based on
satellite/radar trends.

Derived tpw satellite shows pwats of 2.2-2.3 inches encroaching
on the region, which should be firmly in place as we get on the
more convectively favored east side of the wave by mid morning.
Add some daytime heating, and expect rain to pick up across the
area later this morning. Don't foresee a significant flood threat
today, but given the anomalously moist environment, heavy rain
rates can't be ruled out as stronger convection develops. Expect
impacts to be limited to local flooding of streets and poor
drainage areas.

Showers with embedded storms will shift west later in the
afternoon as the wave tracks into the Gulf, with activity waning
from east to west later today and drier conditions tonight.

Monday: no change in the forecast thinking as the work week and
eclipse day starts out with the driest day of the week. Wave vapor
satellite shows well the area of subsidence and dry air in the mid-
upper levels to our east in between the departing tropical wave from
today, and the next wave (invest 92l). There will still be enough
low level moisture for typical afternoon boundary layer cumulus, but
storm chances look isolated at best for most of the region. With
breezy east winds in place, the East Coast sea breeze should be able
to get going and push pretty far inland, likely providing the main
focus for any isolated convection.

Overall partly cloudy skies look to be in place for the eclipse
Monday afternoon, though anyone in/near any storms will see
mostly cloudy conditions. Cloud cover from the next tropical wave
begins to invade from the east during the late afternoon, but
there is a reasonable chance thick clouds and showers will hold
off until after sunset for East Coast viewers.

Tuesday through the end of the week: weather for the middle of the
weak across South Florida will still be driven by the arrival of
invest 92l, or its remnants, moving west across the region on
Tuesday. Conditions are only expected to be only marginally
favorable for this system to develop any further, and model trends
support this with a somewhat drier, and weaker feature as it
arrives. Satellite data shows pwats 2.1-2.3" pooled around this
feature currently, which models support arriving intact as it moves
into South Florida. So even with perhaps a weaker wave, the return
of tropical moisture as we get on the back side of the wave will
bring a return of numerous showers with embedded thunderstorms for
tues and Wed.

Pattern remains unsettled into late week as deep upper low over
Canada drives digging trough through the eastern US, with an
associated frontal boundary sagging anomalously south into the
Florida Peninsula. With both the low and mid level ridge axis
suppressed to the south, we will be under south-southwesterly flow
that will continue to tap into the tropical airmass to our south.
Numerous showers and scattered storms can be expected through the
period, likely with a diurnal maximum.

Given the potential for several days of active weather in a
tropical airmass, this period will need to be watched for local
flooding problems.

Marine...high pressure to our north will keep prevailing east-
southeast flow in place over the local waters through mid week.
Multiple tropical waves passing through the region will maintain
generally moderate speeds, with fresh winds possible at time
starting late tonight through Monday night, requiring small craft to
exercise caution.

The increase in speeds will bring an increase in wave heights,
building to around 4 feet over the open waters tonight through
Monday night, before gradually decreasing mid week as winds decrease.

Expect scattered to numerous showers and storms over the waters
through today as a tropical wave crosses the region. Coverage will
briefly be lower on Monday, before increasing again Tuesday onwards
with the next tropical wave and an unsettled pattern.

Beach forecast...an increase in east-southeast flow early this week
will raise the rip current threat at the Atlantic beaches through
Tuesday.

&&

Preliminary point temps/pops...
West Palm Beach 89 81 93 81 / 60 20 20 30
Fort Lauderdale 89 82 91 81 / 60 20 20 40
Miami 89 81 92 80 / 60 10 20 30
Naples 92 77 96 79 / 60 20 20 10

&&

Mfl watches/warnings/advisories...
Florida...none.
Am...none.
GM...none.

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