409
FXCA62 TJSJ 170856
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
456 AM AST Tue Jun 17 2025

.KEY MESSAGES...

* A limited heat threat is expected, with heat index values forecast
  to rise above 100F in urban and coastal areas.

* Hazy skies will persist due to concentrations of a Saharan Air
  Layer (SAL) over the region through Wednesday, gradually
  decreasing by the end of the workweek.

* Breezy to locally windy conditions will continue across the
  region for the next few days, promoting choppy seas and moderate
  risk of rip currents.

&&

.SHORT TERM...Today through Thursday...

Mostly fair conditions prevailed last night, with few passing
showers moving over the Atlantic and Caribbean offshore waters.
Breezy to locally windy conditions continued during the night, with
some gusts above 20 mph recorded at stations in Cayey and Aibonito
in Puerto Rico and St. Croix in the U.S. Virgin Islands (21, 22, and
24 mph respectively). Minimum temperatures were slightly cooler and
ranged from the mid 70s to the lower 80s in urban and lower
elevations, and from low to upper 60s at higher elevations.

No major changes were introduced to the short-term forecast. A
subtropical ridge extending southward into the northern Caribbean
will continue promoting breezy to locally windy conditions under a
E-ESE wind flow for the rest of the forecast period. Based on
satellite-derived products, there`s a drier air mass in the mid-
levels south of the region which, according to the latest model
guidance, should filter into the region later tonight.
Additionally, temperatures in the 500 mb should be warmer than
normal based on climatology (-5 to -5.5 Celsius), not favorable
for deep convection activity. Based on GEOS-5/GMAO, low to
moderate Saharan Air Layer (SAL) concentrations are expected to
linger through at least Wednesday, bringing hazy skies, reduced
visibility, and deteriorating air quality. In the upper levels,
the cut-off lows north of the region and a weak ridge over the CWA
will form an omega block, increasing stability aloft. According
to the GFS and ECMWF deterministic analysis, PWAT values will
gradually decrease to about 1.4 inches, falling into the below
climatological normal range. Nevertheless, local effects, daytime
heating, and sea breeze convergence should trigger shower and
thunderstorm activity each afternoon, particularly over
western/northwestern Puerto Rico. Rainfall accumulations could
enhance flooding potential today, and no flooding risk for the
Wednesday and Thursday. A tropical wave will approach the region
by Thursday night, with the best moisture content remaining south
of the region.

Heat indexes may surpass the 100 degrees Fahrenheit across most
urban and coastal areas today, nevertheless, the heat risk will
remain limited..

&&

.LONG TERM...Friday through Tuesday...

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...

(06Z TAFs)

Mostly VFR conds across all terminals. HZ may reduce VIS due to low
to moderate concentrations of SAL over the CWA through all TAF prd.
Aftn conv over western PR may bring VCTS for JBQ btwn 1716z - 1723z.
Increasing winds from the E-ESE btwn 15 - 19 with gsty winds btwn 25
- 29, slightly reducing btwn 9 - 11 kt after 1723z.

&&

.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, likely leading to an increase in thunderstorm
activity across that region.

&&

.BEACH FORECAST...

Breezy to locally windy conditions continue across the CWA,
maintaining a moderate risk of rip currents for most beaches of
Puerto Rico and the U.S. Virgin Islands. The moderate risk should
persist throughout the workweek into the weekend, beachgoers must
exercise caution due to possible life-threatening rip currents along
these beaches. Showers and thunderstorms that may develop in the
afternoon over western/northwestern Puerto Rico could move over
coastal areas, hence, they should also stay weather alert.

&&

.FIRE WEATHER...

Fire weather conditions are expected to deteriorate today, Tuesday,
as winds increase and a gradual decrease in moisture unfolds across
the region, following the relatively moister conditions observed on
Monday. The southern coastal plains are forecast to experience the
lowest relative humidity values, increasing the risk of fire
development and spread.

The 30-day rainfall percent of normal has declined to between 10-50%
across the southern areas, resulting in increasing areal coverage of
drying soils. With dry fuels already in place, the combination of
strengthening winds and decreasing humidity is likely to promote
more favorable conditions for enhanced wildfire potential,
particularly across wind-prone areas.

Given these evolving conditions,  a Fire Danger Statement is in
effect for the southern coastal plains and hills of Puerto Rico.

Residents and land management officials should continue to exercise
caution and follow any guidance in place. Refer to the latest Fire
Danger Statement (RFDSJU) for more information.

&&

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

$$

MNG/CVB