Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 232046

Area Forecast Discussion
National Weather Service San Juan PR
446 PM AST Sat Sep 23 2023


A tropical wave will increase shower and thunderstorm activity
across the region from Sunday into Monday. Model guidance
continues to lean towards the now Tropical Depression Seventeen
staying over the tropical Atlantic next week. Please continue to
monitor the latest updates from the National Hurricane Center
(NHC) and WFO San Juan regarding this system.


.SHORT TERM...Tonight through Monday...

The weekend began with variable conditions across the islands. In
the morning, mainly clear skies prevailed over the western
quadrant of Puerto Rico. However, cloudy and rainy skies prevailed
over eastern Puerto Rico, Culebra, Vieques, and the U.S. Virgin
Islands. Rainfall across the east left around 0.30 to 0.50 inches
of rain in the morning. Then, the activity moved across the
interior and remaining parts of Puerto Rico. Despite that, an
Excessive Heat Warning was issued for the north-central and
northwestern sections of the island due to heat indices above 115
degrees Fahrenheit based on reports from the weather stations in
those areas. Daily maximum temperatures along the coastal regions
remained in the upper 80s and low 90s, while in the mountains,
they stayed in the 80s. By the afternoon hours, showers spread
across most areas in Puerto Rico. However, the heaviest rainfall
and isolated thunderstorm activity formed over the northwestern
sections of PR.

Overnight, some occasional passing showers will filter across eastern
PR and the USVI. The minimum temperatures along the coastal
regions are forecast to range in the upper 70s to low 80s, while
in the mountains, they will remain in the upper 60s to low 70s.

The tropical wave in the central Caribbean basin will continue to
increase the moisture content across the forecast area from Sunday
to Monday. Therefore, expect the variable weather conditions to
remain throughout the period, along with some showers and isolated
thunderstorm activity, mainly across the western sections of PR
each afternoon. The wind flow by late Sunday will become more
easterly as a perturbation approaches the area. The rainfall
activity during this period will largely depend on the local
effects and diurnal heating.


.LONG TERM...Tuesday through Saturday...FROM previous

Northeasterly winds are forecast to prevail during most of the long
term forecast period. Current model guidance indicates a drying
trend on Tuesday as a tropical wave, and moisture associated with
it, moves away from the islands. Patches of moisture will continue
to filter into the region on Wednesday through Saturday, maintaining
precipitable water (PWAT) values at up to normal values. During the
night and early morning hours, passing showers affect windward
sectors of the islands, particularly across northern and eastern
Puerto Rico, and across Vieques, Culebra and the USVI. NE wind flow
will help promote afternoon shower activity, induced by sea breeze
convergence, diurnal heating and local effects, mainly over sectors
of interior to southern Puerto Rico each day. Afternoon activity
could also be observed downwind of El Yunque, Vieques, Culebra, and
the USVI. Maximum temperatures should be in the upper 80s to low 90s
across lower elevations of the islands. Minimums should range in the
upper 70s to low 80s across lower elevations of the islands, and in
the upper 60s to lower 70s across higher elevations of Puerto Rico.
At least a limited excessive heat risk will persist each late
morning to early afternoon across urban, coastal and lower elevation
areas. A tropical wave (AL90), currently several hundred miles west
of the Cabo Verde Islands, has a high chance of development. The
NHC`s 2 AM EDT TWOAT mentions that this system will likely become
a tropical depression during the next couple of days. With model
consensus now leaning towards this system staying over the
Atlantic, the main hazard from this system would be swells
reaching our local waters and coastlines. Please continue to
monitor the latest updates from the NHC and WFO San Juan regarding
this system.


.AVIATION...Afternoon TSRA/SHRA could result in brief MVFR
conditions, mainly across TJSJ and TJBQ through 23/22Z, followed by
VCSH after 23/22Z for TJSJ and USVI terminals. Tropical wave could
promote TSRA/SHRA across TJPS 24/08z resulting in brief MVFR.
Surface winds will remain from the east-southeast at 10-15 knots,
becoming light and variable after 23/22Z, and increasing to 10-15
knots after 24/14Z. Higher wind gusts likely near TSRA/SHRA.


.MARINE...Seas should remain favorable for small craft at 2 to 5
feet through early next week. Light to moderate east winds will
continue before becoming more northeasterly by mid next week. The
next tropical wave is expected by tomorrow into Monday and we
anticipate shower and thunderstorm activity across the regional




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