881
FXCA62 TJSJ 120908
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
508 AM AST Fri Jul 12 2024

.SYNOPSIS...

Another pulse of suspended Saharan dust particulates will reach
the region today and prevail through Sunday. Tonight and Saturday,
a tropical wave and its moisture field is forecast to move across
the region leading to an increase in showers and thunderstorms
throughout the weekend. On Monday, a TUTT will approach the region
enhancing instability and maintaining the wet and unstable pattern
in place through at least the first part of the week. An elevated
heat risk will also likely persist.

&&

.SHORT TERM...Today through Sunday...

The Doppler radar loop since last night showed a few lines of
showers reaching the municipalities of eastern Puerto Rico, as
well as the Virgin Islands. These showers were weak to moderate,
and rainfall accumulations were minimal.

A surface high pressure will maintain the wind flow from the east,
and enhanced, at speeds of 17-20 knots at the low levels. In this
flow, a small patch of moisture will continue to cross the region,
resulting in passing showers for eastern Puerto Rico, and for the
Virgin Islands. Wet roads and ponding of water can be anticipated
with this activity. Saharan dust will also move in today, and skies
will turn gray due to the haze. Additionally, the available moisture
and the local effects will aid in the development of showers this
afternoon, from the islands into portions of eastern Puerto Rico,
but with the strongest activity in the interior and west, and from
El Yunque into portions of the San Juan metro area, and the
municipalities of the north-central. Urban and small stream flooding
is likely in these areas.

On Friday night and Saturday, a tropical wave, currently located
well to the east of the Lesser Antilles, will reach the islands. The
moisture field of this wave stretches into the mid-levels of the
atmosphere. At the upper levels, a trough will be positioned to the
northeast of the islands, and this feature will increase instability
aloft. In fact, the 500 mb temperatures are expected to cool down,
while the mid-level lapse rates will increase. Some Saharan dust
will be mixed in with the wave too. The Galvez-Davison index also
show the potential for widespread showers, with heavy rain, and
thunderstorm. Taking all these factors into consideration, heavy
showers and strong thunderstorms are possible. The risk of frequent
lightning, gusty winds, and flooding will be present too.

The wave departs late on Saturday, and again, a dense Saharan Air
Layer will follow. Visibilities are expected to be reduced, and
skies will be hazy. However, at the low levels, trailing moisture
lingers. Therefore, it will not be dry, as some passing showers and
isolated thunderstorms are expected to move in at times across the
waters, the eastern half of Puerto Rico and the Virgin Islands.
Also, enough heating should trigger another round of strong
convection for the interior and west, where the risk for flooding
will be elevated.

&&

.LONG TERM...Monday through Thursday...

The latest models suggest the wet and unstable pattern will
prevail early next week. The Saharan Dust concentrations decrease
significantly. A TUTT low is forecasted to move north of our
region, inducing a surface perturbation. Also, the 500 mb
temperatures should remain cooler, around -7 to -8 degrees
Celsius. The high pressure across the Atlantic Ocean will weaken
somewhat, and low-level winds will become lighter on Monday and
Tuesday. With the proximity of the aforementioned TUTT low, those
other factors in place, and residual moisture from the departing
tropical wave (above-normal PWAT values; around 2 to 2.2 inches),
we can expect the development of showers and thunderstorms that
will likely generate frequent lightning, strong gusty winds, and
heavy downpours, and flooding concerns early next week. If the
rain materializes, lighter winds at the surface may result in
slow-moving showers, which are not beneficial for flooding
concerns. By mid-week, the surface high pressure across the
Atlantic will reinforce, winds should veer from the ESE and become
slightly stronger. That will continue to push tropical moisture
into our area. By Thursday, we should see some improvement in
weather conditions as models are showing patches of drier air
mixing and moving into our forecast area and a more typical
weather pattern may return.

In terms of temperatures, warmer-than-usual conditions are
expected for most of the period. On Monday, if the rain
materializes, the temperatures should stay a few degrees lower.
Generally, daytime highs will range from the lower 90s in lower
elevations to around 80 degrees in higher elevations. With
abundant moisture in the air, heat indices in coastal and urban
areas may soar above 108 degrees Fahrenheit.

&&

.AVIATION...
(06Z TAFS)

Passing SHRA are expected to continue to affect the USVI and TJSJ
terminals through the period. Hz due to Saharan dust is also
expected, but with VIS remaining at P6SM. After 16Z, SHRA and TSRA
are expected to develop in Puerto Rico, with periods of reduced
VIS and low ceilings anticipated for TJSJ and TJBQ. Mountain
obscuration is also expected for the Cordillera Central. Winds
will be from the ESE at 16-22 kts, and stronger gusts.

&&

.MARINE...

A surface high pressure across the Atlantic Ocean will promote
moderate to fresh easterly to east southeasterly winds through
Saturday night. A wind surge associated with a tropical wave will
arrive this afternoon, increasing moisture and convective activity.
The wave`s axis will cross the region by Saturday, promoting
unsettled weather conditions throughout the weekend due to residual
moisture.

&&

.BEACH FORECAST...

There is a moderate risk of rip currents for most beaches of
Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands today
and tonight. The risk will likely remain moderate through the
weekend. For more details about the latest forecast, please refer
to the Surf Zone Forecast (SRFSJU).

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>003-005-007-008-010>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.

AM...Small Craft Advisory from 6 PM this evening to 6 PM AST Saturday
     for AMZ711.

     Small Craft Advisory from 6 PM this evening to 6 AM AST Saturday
     for AMZ723.

&&

$$

SHORT TERM/AVIATION...ERG/CAM
LONG TERM/MARINE/BEACH FORECAST....YZR