487
FXCA62 TJSJ 121955
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
355 PM AST Fri Jul 12 2024

.SYNOPSIS...

Tonight into Saturday, a tropical wave and its moisture field will
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 is likely persist.

&&

.SHORT TERM...Tonight through Sunday...

By the early morning hours, passing showers filtered across
eastern Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands,
but accumulations were minimal. However, as the day progressed,
some showers filtered once again across eastern Puerto Rico,
Saint. Thomas and Saint John, leaving rainfall accumulations of
around 0.05 to 0.80 inches. During the afternoon, the shower
activity migrated to the interior and western sections of Puerto
Rico. The wind flow was mostly out of the east, with variations in
sea breezes. Temperature-wise, they remain in the mid-90s to
upper-80s, particularly along the coast and across the mountains,
they stayed in the 80s. Official weather stations reported heat
indices from 102 to 111 degrees Fahrenheit. Overall, a mixture of
humid and hot weather conditions prevailed across the islands.

Tonight into tomorrow, we anticipate a gradual increase in moisture
and rainfall activity as a tropical wave near the Lesser Antilles
approaches the islands. The latest model guidance suggests the
moisture field associated with this wave stretching to the mid-
upper levels of the atmosphere. The tropical wave is foreseen to
depart late Saturday, followed by a dense Saharan Air Layer that
will promote hazy skies. However, at low levels, trailing moisture
will linger. Therefore, occasional passing showers could filter.

On Sunday, a tropical upper tropospheric trough(TUTT) will position
to the northeast of the islands, and this feature will increase
instability aloft. The 500 mb temperatures are expected to cool
down, while the mid-level lapse rates are forecast to increase. As
mentioned in previous discussions, some Saharan dust will be
mixed in with the wave. The Galvez-Davison index also shows the
potential for widespread showers, with heavy rain and
thunderstorms. 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.

&&

.LONG TERM...Monday through Thursday...
//from previous discussion//

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...
(18z TAFs)

HZ due to Saharan dust could lead to brief MVFR conditions, with VIS
as low as 6SM. SHRA/VCTS could reach TJBQ/TJSJ at times thru 12/22z.
A tropical wave will result in SHRA/TSRA aft 13/10z promoting
possibly MVFR or even brief IFR for TIST/TISX/TJSJ and mountain
obscuration for the Cordillera Central. Winds will remain from the
ESE at 14-20KT with higher gusts and sea breeze variations. Lighter
and variable winds will return after 12/23Z, but increase once again
after 13/13Z.

&&

.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 and its associated tropical
wave will arrive tonight, gradually increasing moisture and rainfall
activity. The axis of the wave 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 in Puerto
Rico, Vieques, Culebra, and the U.S. Virgin Islands. 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 until 5 PM AST this afternoon for PRZ001>003-005-
     007-008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...Small Craft Advisory until 6 PM AST Saturday for AMZ711.

     Small Craft Advisory until noon AST Saturday for AMZ723.

&&

$$

SHORT TERM...MMC/CAM
LONG TERM...YZR
AVIATION...CVB
MARINE/BEACH FORECAST...MMC
PUBLIC DESK...GRS