Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 301430

Area Forecast Discussion
National Weather Service San Juan PR
1030 AM AST Fri Jul 30 2021

Partly cloudy skies prevailed over most of the islands this
morning, with few showers embedded in the easterly trade winds
moving over some areas of eastern and northern Puerto Rico. This
afternoon, showers will develop mainly over the northwestern and
the interior areas of Puerto Rico and produce light rainfall
amounts. Saharan dust particles have filtered into the region and
will remain through the weekend. Currently, temperatures are near
the upper 80s across the metro areas of San Juan, temperatures
will continue to claim over the next several hours and reach the
lower 90s in lower elevations. Winds will be from the east up to
20 mph with higher gusts.


VCSH expected across western and northwestern PR by 30/17Z, which
could affect the vicinity of the TJBQ terminal. HZ is present but
VIS expected to remain P6SM. Winds will be from the east ranging
from 15 to 20 kts with higher gusts and seas breeze variations.


Choppy seas will continue through Saturday for most of the local
waters due to winds up to 20 kts. Seas will range from 3 to 5 feet
across the outer and near-shore waters. There is a moderate risk
of rip current over north, southwest and southwestern local
beaches of puerto Rico, as well most of the local beaches at
Culebra, Vieques and St. Croix.


.PREV DISCUSSION... /issued 426 AM AST Fri Jul 30 2021/


For the weekend, an improvement in weather conditions is expected,
as a drier air mass with Saharan dust particles filters into the
region. Weather conditions will turn variable as the trade winds
push patches of moisture over the forecast area. Tranquil marine
conditions are expected over the local waters over the next
several days.

SHORT TERM...Today through Sunday...

Below normal moisture is moving in and is expected over the local
islands for much of today. There is a brief patch of moisture that
will pass through this afternoon, but for the most part, drier than
normal available moisture is expected. There is also some Saharan
dust, which could cause hazy skies. Currently there are isolated
and brief showers across the local waters, USVI and eastern PR, but
only trace amounts of rain are being observed. This activity is
expected to remain the same or diminish after sunrise. The mid and
upper levels are starting to show high pressure over the local area,
causing stable conditions. That said, locally induced showers are
possible across western PR this afternoon, thunderstorms are not
expected, but will not rule out a brief and isolated thunderstorm.
Based on the current forecast, some ponding of water in areas of
poor drainage is possible across some areas of western PR. The
daytime temperatures will be in the upper 80s to low 90s across the
lower elevations, heat indices may reach 100-105 across portions of
north central PR.

For Saturday and Sunday, the drying trend is expected to continue,
especially for Saturday as much drier air is expected to move in
with a higher concentration of Saharan dust. For that reason, very
limited shower activity is expected. The model guidance also is
suggesting daytime temps in the 90s. Then un Sunday, the general
pattern remains the same but a small patch of moisture may move in
briefly. This quick patch of moisture may be enough to cause some
shower development across western PR when combined with the diurnal
heating and the local effects.

LONG TERM...Monday through Saturday...

A wetter pattern is expected for most of the long term period. At
the surface, a wind surge with precipitable water values near
normal will reach the local islands, increasing the potential for
shower development. In the upper levels, a retrogressing TUTT-low
will continue to moves westward. For Monday, model guidance
suggests the subsident side of the TUTT will move over the local
area. Regardless of the stability of the conditions aloft, the
available moisture, along with the local effects can erode the
trade wind cap and result in afternoon convection over western
Puerto Rico. By Tuesday, at the surface, moisture embedded in the
east- southeasterly wind flow is expected to move in. Meanwhile,
better upper level dynamics are expected, both the GFS and ECMWF
model guidance are suggesting a TUTT to be located just at north
of the forecast area. If model guidance is correct, the TUTT will
result in unstable conditions on Tuesday into early Wednesday.
Therefore, shower activity is expected across the forecast area,
possibly resulting in urban and small stream flooding each
afternoon over the first half of the workweek.

The moisture will slowly decrease over the region on Thursday,
as a narrow slot of drier air moves in. This will limit shower
activity through the late evening hours on Thursday. By Friday, a
tropical wave is expected to move over the lesser Antilles into
the local area. The bulk of the moisture is expected to remain
to our south, however, above normal moisture is expected over the
area. So given the expected pattern, scattered showers are
expected across the local islands. As the building surface high
pressure moves into the central Atlantic on Saturday, a mixture
of patches of moisture and drier air will result in isolated to
scattered shower activity across the local waters, USVI, and
eastern PR in the morning and overnight hours, while afternoon
convection would be expected somewhere across western PR.

AVIATION...VCSH expected this morning across the local terminals
today as ISOL -SHRA affect the local area. SHRA is possible
across W-PR this afternoon, which could affect the vicinity of the
TJBQ terminal. HZ is present but VIS expected to remain P6SM.
Winds will be from the east at around 10KT or less through 30/13Z,
but will increase to around 15KT with gusts up to 25KT and sea
breeze variations thereafter.

MARINE...Tranquil marine conditions will prevail over the local
waters with seas up to 5 feet and winds up to 15 knots. A few
passing showers are posibles over the local waters. There is a
moderate risk of rip current over north, southwest and
southwestern local beaches of puerto Rico, as well most of the
local beaches at Culebra, Vieques and St. Croix.


SJU  89  79  89  79 /  20  20  20  10
STT  89  80  89  80 /  20  20  20  10




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