563
FXCA62 TJSJ 171855
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
255 PM AST Tue Jun 17 2025

.KEY MESSAGES...

* Hazy skies will persist due to the presence of a Saharan Air
  Layer through midweek, gradually diminishing throughout the
  second part of the week. Those suspended dust particles will
  limit nighttime cooling, resulting in warmer-than-normal minimum
  temperatures.
* A surface high pressure across the Atlantic Basin is tightening
  the local pressure gradient, enhancing winds and creating choppy
  wind-driven seas across the regional waters.
* Beachgoers can expect a moderate risk of rip currents due to the
  possibility of life-threatening rip currents.
* Afternoon thunderstorms will remain possible across the western
  third of PR each day, especially on Thursday or Friday, as a
  weak tropical wave moves near the region.

&&

.SHORT TERM...Tonight through Thursday...

We had another day with hazy skies due to the Saharan Air Layer
(SAL) over Puerto Rico (PR) and the US Virgin Islands (USVI). The
Doppler radar detected some showers, mainly across the Atlantic
Ocean. Sea breezes developed, producing convection across western
Puerto Rico. Maximum temperatures ranged from the upper 80s to the
low 90s, with heat indices reaching around 103 degrees Fahrenheit
due to the available moisture, primarily at urban and coastal
sites.

Showers and thunderstorms may result in flooding rains through
this evening, mainly across the west municipalities of PR. The
rest of the islands will remain under hazy skies due to the
lingering SAL. This air mass will limit nighttime cooling,
promoting warmer-than-normal minimum temperatures across Puerto
Rico and the US Virgin Islands.

The subtropical ridge across the Atlantic will maintain breezy to
locally windy conditions under an easterly wind flow throughout
the forecast period. A mid-level high pressure will promote a
drier mid-level air mass moving into the region later tonight,
leading to subsidence aloft and limiting the potential for deep
convection. Additionally, model guidance suggests that low to
moderate concentrations of SAL particles will persist through at
least tomorrow, Wednesday, resulting in hazy skies, reduced
visibility, and poorer air quality. Although below-normal moisture
will predominantly be present over the islands, localized effects,
daytime heating, and sea breeze interactions will likely produce
afternoon showers and thunderstorms, especially across western and
northwestern Puerto Rico. Be aware that a weak tropical wave is
expected to approach by Thursday, leaving its bulk of moisture to
the south.

.LONG TERM...Friday through Tuesday...prev discussion...

Variable conditions are still expected for the long-term forecast.
Moisture associated with a tropical wave is expected to increase the
frequency of showers and thundershowers on Friday into Saturday.
Based on GFS and ECMWF deterministic analysis, ensemble members
continue suggesting an increase in moisture content, but there is
quite a bit of variability regarding PWAT values (1.5 - 2.0 inches).
Nevertheless, showers and thunderstorms should develop mainly on
Friday afternoon into early Saturday, particularly over
western/northwestern Puerto Rico moving over the local waters.
Then, drier air mass in the mid- levels should filter into the
region, with relative humidities dropping to 15 - 20%. This can
be seen in the temperatures at 500 mb where they again warm
(around -6 Celsius), which may inhibit deep convection activity.
Model solutions continue suggesting the arrival of another
tropical wave for Monday, raising PWAT values between 1.8 to 2.0
inches (near above-normal). The most likely scenario is to see
increased shower and thunderstorm coverage across the local area,
elevating the flooding potential. Nevertheless, uncertainty
remains elevated due to variability in previous model cycles.
Winds may increase by Monday into Tuesday, promoting windy
conditions across the CWA.

&&

.AVIATION...

Expect mostly VFR conditions at all TAF sites during the period,
although hazy skies are expected to persist due to the SAL. VCTS
will develop near JBQ, and some SHRA/+SHRA will impact the eastern
locations of PR and the USVI later this evening or tomorrow
morning. Expect winds from the E-ENE 15-25 knots with local sea
breezes accompanied by higher gusts up to 35 kt. Winds are
expected to fluctuate around 11 kt after 17/23Z and return to
similar wind conditions as today, after 18/13Z tomorrow.

&&

.MARINE...

A persistent Atlantic subtropical ridge extending southward into the
northern Caribbean will continue to strengthen throughout the
period. This will lead to increasing moderate to locally fresh
easterly winds, resulting in choppy seas, particularly over exposed
areas. Therefore, small craft should exercise caution. A plume of
suspended Saharan dust is expected to linger through the workweek,
reducing visibility. A tropical wave will move across the Caribbean
waters on Thursday and Friday, likely leading to an increase in
thunderstorm activity across that region.

&&

.BEACH FORECAST...

A moderate risk of rip currents will be promoted by breezy to
locally windy easterly winds for most beaches in Puerto Rico and
the US Virgin Islands. Beachgoers must exercise caution due to
life-threatening rip currents along exposed unprotected beaches.
Additionally, please be advised of the possibility of seeing
afternoon thunderstorms, particularly over western and
northwestern Puerto Rico.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

MORNING CREW...CAM/LIS
EVENING CREW...ICP/YZR