Special Marine Warning 248, NWS Miami, FL

Area Forecast Discussion
National Weather Service Miami FL
405 AM EDT Mon Jul 12 2021

AVIATION(09Z TAFS)... SCT SHRA/ISOL cores of TSRA affecting the Atlc
S-FL coast due to TROF under a TUTT low, causing VCSH across the
eastern terminals overnight. Expected TSRA across S-FL after 12/13Z
will cause at least VCTS across the local terminals through 13/00Z.
Due to uncertainty on exact timing and location of TSRA near the
terminals, AMD and TEMPO will be used as TSRA approaches the
terminals. Winds from the E-ESE at 10 KT or less overnight,
increasing to 10-15KT after 12/13Z but gusty near SHRA/TSRA. KAPF
winds will become SE between 12/17Z and 12/22Z. Trop wave well
represented in 700 mb Theta-E will move onto SE FL coast begg 13/00Z.

.SHORT TERM (Today through Tuesday)...

Main Points:

1) For today, isolated strong to severe thunderstorms will be
possible across South Florida, with the primary concerns being
locally damaging wind gusts and marginally severe hail. Present
indications are that the strongest activity will develop over the
interior and western portions of South Florida during the
afternoon and early evening hours.

2) For overnight tonight into Tuesday, periods of heavy rainfall
from numerous showers and thunderstorms could lead to localized
flooding mainly across the urbanized areas of South Florida. The
risk of excessive rainfall will initially be over the east coast
metro areas tonight, generally spreading westward through the day
on Tuesday.

Subtropical high pressure remains centered over the western
Atlantic waters, with a low/mid-level ridge axis extending
westward across portions of north-central Florida into the eastern
Gulf of Mexico. To the south of the ridge axis, GOES-16 water
vapor imagery depicts a large mid/upper-level low located over the
Bahamas, which will continue advancing westward across South
Florida today. Associated with this feature are seasonably cold
mid/upper-level temperatures (e.g., 500-mb temperatures near -12C)
which are colder than the daily minimum value for this day
according to the SPC sounding climatology. The cold mid/upper-
level temperatures will support steep mid-level lapse rates of 7
to 7.5 C/km overspreading South Florida today, resulting in large
normalized CAPE values and strong upward accelerations in
convective updrafts. This thermodynamic environment will support
a conditional risk of isolated strong to locally damaging
downbursts with the tallest convective cores that develop today
along with marginally severe hail. A lack of strong deep-layer
shear will generally support pulse multicell clusters, with brief
organization being possible with cell mergers over the western
half of South Florida. The threat of severe weather appears
greatest over the interior and western portions of South Florida
during the afternoon hours when diurnal heating will destabilize
the moist boundary layer below the steepening mid-level lapse
rates.

For tonight, the aforementioned mid/upper-level low will lift
northwestward away from South Florida, and the northern portion
of a weak tropical wave over the western Atlantic waters will
track west-northwestward across South Florida. This feature will
lead to deep-layer saturation across South Florida as rich
subtropical moisture overspreads South Florida (i.e., PWs greater
than 2.0 inches). The low-level mass response accompanying the
weak tropical wave will result in strengthening easterly lower
tropospheric flow and the potential for frictional convergence and
related mesoscale ascent along the east coast of South Florida
overnight tonight into the early morning hours. This could support
repeated rounds of locally heavy rainfall and the potential for
isolated flooding along the east coast of South Florida, before
the risk spreads westward toward the Gulf Coast during the day.
There is plenty of uncertainty regarding the timing of potential
impacts, so be sure to keep up with the latest information from
NWS Miami.

.LONG TERM...

Wednesday-Sunday...The tropical wave that arrived in the area on
Tuesday will be shifting west into the Gulf of Mexico early on
Wednesday. Subsidence behind the wave in combination with at least a
thin Saharan Air Layer (SAL) reaching the area should result in some
lower rain chances, compared to early in the week. However, with the
uncertainty in how potent this SAL event will be and lingering low-
level moisture have left scattered to numerous showers and storms in
the forecast for Wednesday. Another unknown variable at this time is
that the "leading edge" of that SAL could act as an elevated mixed
layer and enhance updraft strength, creating the potential for a few
strong storms. Isolated strong wind gusts and localized flooding
will remain the primary concerns.

By Thursday it looks like drier air indeed moves into the area,
bringing rain chances down to expected climatological values for
this time of year. Generally looking at 40-50 percent chance Friday.
The Subtropical Ridge axis remains well north of the area, with deep
easterly flow prevailing. This favors the highest coverage of
diurnal showers/storms across the interior and west coast each
afternoon. Heading into the weekend, models show a renewed increase
in low-level moisture and higher rain chances for next weekend.

Temperatures are forecast to remain near normal as afternoon highs
reach the upper 80s along the east coast, and low 90s across the
interior and west coast. Overnight lows settle into the mid 70s
inland/west coast and upper 70s to low 80s for east coast.

&&

.AVIATION(06Z TAFS)...

ISOL SHRA affecting the Atlc S-FL coast, causing VCSH across the
eastern terminals overnight. Expected TSRA across S-FL after
12/13Z will cause at least VCTS across the local terminals through
13/00Z. Due to uncertainty on exact timing and location of TSRA
near the terminals, AMD and TEMPO will be used as TSRA approaches
the terminals. Winds from the E-ESE at 10 KT or less overnight,
increasing to 10-15KT after 12/13Z but gusty near SHRA/TSRA. KAPF
winds will become SE between 12/17Z and 12/22Z.

&&

.MARINE...

High pressure over the western Atlantic waters will continue
building westward through the week as a weak tropical wave moves
through the area on Tuesday. East to southeasterly winds will remain
elevated across the local waters through the middle of the week
resulting in cautionary conditions for small craft operators,
especially across the Atlantic waters where winds could approach
advisory levels from Monday night into Tuesday morning. Scattered
to numerous showers and thunderstorms are possible each day
through the middle of the week, with the greatest coverage of
showers and storms late Monday through Tuesday. Locally higher
winds and seas will be possible with showers and storms.

&&

.BEACHES...

There is a high risk of rip currents along the Atlantic beaches of
South Florida due to breezy easterly winds. The elevated rip
current risk will last through at least mid-week.

&&

.PRELIMINARY POINT TEMPS/POPS...
Miami            89  77  86  78 /  50  70  80  30
West Kendall     89  77  86  77 /  50  70  80  40
Opa-Locka        90  77  86  77 /  50  70  80  30
Homestead        88  77  86  77 /  50  70  80  40
Fort Lauderdale  88  78  86  78 /  50  70  80  30
N Ft Lauderdale  88  77  86  78 /  50  70  80  30
Pembroke Pines   88  76  86  76 /  50  70  80  30
West Palm Beach  89  77  86  77 /  50  70  80  30
Boca Raton       89  78  85  78 /  50  70  80  30
Naples           90  74  89  76 /  70  60  80  40

&&

.MFL WATCHES/WARNINGS/ADVISORIES...
FL...High Rip Current Risk from 7 AM EDT this morning through Tuesday
     evening for FLZ168-172-173.

AM...None.
GM...None.
&&

Today through Tuesday and Marine...Weinman
Tuesday Night through Sunday...Rodriguez
Aviation...WFO SJU

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