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518
FXCA62 TJSJ 080729
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
329 AM AST Tue Sep 8 2026

...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST, FIRE WEATHER...

.KEY MESSAGES...
Issued at 326 AM AST Tue Sep 8 2026

- A Heat Advisory will be in effect today from 10 AM to 5 PM AST
  for Puerto Rico and the U.S. Virgin Islands.

- A drier air mass filtering over the region will persist today, however
  locally induced afternoon showers and t-storms are still
  forecast under a breezy ESE flow.

- Tomorrow and Thursday, tropical waves is forecast to move over the
  islands, increasing the potential for showers and a limited
  flooding threat.

- Moderate Saharan Dust concentrations will arrive from time to time
  before a dense Saharan Air Layer (SAL) with high concentrations
  arrives Thursday night.

- A moderate risk of rip currents is expected today along the northern
  and eastern beaches of Puerto Rico, Culebra, Vieques, and the
  U.S. Virgin Islands.

&&

.Short Term(Today through Thursday)...
Issued at 326 AM AST Tue Sep 8 2026

Since midnight, radar estimated accumulations were detected
mainly over St. Croix and eastern to southeastern Puerto Rico. The
highest accumulations were observed over the offshore Atlantic
Waters where isolated t-storms managed to develop. Stations have
reported lows in the upper 70s to low 80s, across urban and lower-
elevation areas of northeastern, eastern, southern and
southwestern Puerto Rico, in the mid to upper 70s across urban and
lower-elevation areas of north-central to western Puerto Rico, in
the low 80s across Vieques, Culebra and the USVI, and in the 60s
to low 70s across higher elevations of Puerto Rico. A surface high
pressure strengthening over the central Atlantic will continue
tightening the local pressure gradient and result in breezy,
locally windy, ESE flow, especially along coastal areas throughout
the short term period. Drier air has been filtering over the
islands during the overnight hours and will continue to filter in
today and tonight. This drier air mass will continue to displace
the moisture field and related disturbance to our north.
Precipitable water (PWAT) values will decrease to between 1.45 to
1.8 inches, below normal to normal values, today and this evening.
Available moisture will also be limited to mainly below 850 mb,
as ridging establishes itself in the mid to upper levels. Low to
locally moderate Saharan Dust concentrations will also move over
the islands, particularly during the early morning hours. This
will all promote a drier and more stable weather pattern compared
to the rest of the short term period with 700 to 500 mb lapse
rates decreasing to below-seasonal values and 500 mb temperatures
warming up to above seasonal values today. Local patches of
moisture moving over the region and afternoon convection over
western PR are still forecast to result in locally higher and
generally seasonal PWAT. Passing showers will continue to reach
windward sectors under an ESE flow. Diurnal heating, sea breeze
convergence and local effects will still result in afternoon
showers and t-storms, over mainly interior to the NW quadrant of
PR, with afternoon convection also possible downwind of the local
islands and El Yunque. Even drier air is then forecast to arrive
late tonight into the early overnight hours tomorrow, with PWAT
values ranging from 1 to 1.4 inches. An elevated heat risk will be
present today with heat indices reaching advisory criteria mainly
from 10 AM to 5 PM AST. Although the above mentioned showers and
t-storms can provide some relief this afternoon, a heat advisory
will be in effect for the islands during that time.

A couple of tropical waves will increase moisture and shower
activity across the islands tomorrow, Wednesday, and Thursday. The
first tropical wave and its associated moisture field will move
over the region early Wednesday. Although low to moderate saharan
dust concentrations will also arrive at this time, these will be
relatively short lived. The moisture surge will drive PWAT values
to between 1.8 and around 2.0 inches across the area. However,
current guidance indicates pockets of drier air will also filter
in during the afternoon. Available moisture will somewhat expand
back into the mid-levels, while 500 mb temperatures are expected
to drop to around -6 to -7C. A favorable wind pattern will
persist across the upper levels, supported by high pressure near
250 mb staying to the north of the area. These conditions will
favor advective showers and thunderstorms across windward sectors
driven by breezy to locally windy east-southeast flow. Convection
is also anticipated during the afternoon over interior and west-
northwest Puerto Rico, as well as downwind of El Yunque and the
neighboring islands. Although a limited flooding potential
remains, stronger wind speeds may act to restrict localized
rainfall totals.

The moisture field from another tropical wave will then reach the
area on Thursday, maintaining PWAT values up to around 2 inches
once again. With 500 mb remaining similar and breezy to locally
windy ESE flow continuing, a similar pattern is forecast with
passing showers across the eastern region during the early morning
hours. By the afternoon, convection is forecast to shift over
mainly the western interior and northwestern Puerto Rico, and
possibly downwind of the local islands and El Yunque. These
regions are currently under a limited flood threat. Moderate
concentrations of Saharan Dust will then filter in late Thursday
afternoon and current guidance is suggesting that high
concentrations due to a dense Saharan Air Layer will arrive
Thursday night into the long term period. This will promote
deteriorated air quality due to haze, reduced visibilities and a
general drying pattern to end the short term period. A limited
heat risk is forecast for Wednesday and Thursday as more seasonal
925 mb temperatures are forecast; please continue to monitor the
forecast for any updates.

&&

.Long Term(Friday through Tuesday)...
Issued at 326 AM AST Tue Sep 8 2026

A surface high-pressure system over the central Atlantic is
expected to strengthen its pressure gradient, promoting breezy to
locally windy southeasterly flow through Friday before shifting
easterly from Saturday onward. On Friday, lingering moisture
associated with a departing tropical wave will bring passing
morning showers across the islands. However, a drier air mass will
quickly move over the region, dropping precipitable water values
to 1.30-1.40 inches, nearly two standard deviations below normal,
and bringing stable conditions to the area. Stability will also
build aloft as a mid- level ridge moves in. Moderate to locally
high concentrations of Saharan dust will accompany the dry air
mass, bringing hazy skies, reduced visibility, and poor air
quality. Residents and visitors are urged to follow medical
recommendations and closely monitor weather forecasts for updates.

Over the weekend, fast-moving morning showers are expected across
portions of eastern Puerto Rico and the U.S. Virgin Islands.
Afternoon showers and isolated thunderstorms fueled by daytime
heating and local effects will follow, though impacts will remain
limited with no flooding concerns. From Sunday through Tuesday, an
upper-level low moving toward the northeastern Caribbean will
gradually increase instability and bring moisture back up to
seasonal levels across the region.

&&

.AVIATION...
(06Z TAFS)
Issued at 326 AM AST Tue Sep 8 2026

Mainly VFR conditions are expected across the terminals. Passing
- SHRA/VCSH possible over the eastern terminals. HZ (Saharan Dust)
gradually decreasing during the early morning hours. Locally
induced SHRA/-TSRA can result in brief MVFR mainly over TJBQ btwn
08/17-21z, streamers can also form towards TJSJ at this time.
Winds light and VRB to calm through 08/13z, then becoming E-ESE at
up to around 15 kt, with sea breeze variations and stronger
gusts.

&&

.MARINE...
Issued at 326 AM AST Tue Sep 8 2026

Surface high pressure strengthening across the central Atlantic
will promote moderate to fresh winds, with higher gusts, today and
over the next few days. This will result in choppy to locally
rough marine conditions. Seas will generally build to 4 to 6 feet
across the offshore waters and local passages, with occasional
seas up to 8 feet possible by tonight and through around next
Thursday. Passing showers will continue across portions of the
regional waters today, with locally higher winds and seas possible
in and near thunderstorms. A tropical wave arriving by midweek
will bring additional periods of showers and thunderstorms
Wednesday and Thursday.

&&

.BEACH FORECAST...
Issued at 326 AM AST Tue Sep 8 2026

Today, there is a moderate risk of rip currents along the northern
and eastern beaches of Puerto Rico, Culebra, Vieques, and the U.S.
Virgin Islands, due to increasing winds and building seas. A
moderate risk means life-threatening rip currents are possible and
beachgoers are urged to exercise caution. By midweek through next
weekend, the moderate risk is expected to persist and extend to most
local beaches.

Another beach hazard will be the heat. Beachgoers should remain
hydrated and limit prolonged sun exposure. Additionally, gusty winds
and lightning due to afternoon showers and isolated thunderstorms,
particularly across the western beaches of Puerto Rico. Beachgoers
should seek shelter whenever they hear thunder.

&&

.FIRE WEATHER...
Issued at 326 AM AST Tue Sep 8 2026

Elevated KBDI values signal persistent dry conditions in deeper
soil and fuel layers across southwestern Puerto Rico, most notably
around Cabo Rojo. Breezy ESE winds combined with incoming drier
air maintain a moderate fire danger risk today, with locally
elevated threat levels possible in Cabo Rojo and neighboring
areas. Weather pattern updates and fuel moisture trends will be
monitored closely for potential increases in fire risk.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>013.

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

AM...None.
&&

$$

SHORT TERM/AVIATION...MRR
LONG TERM....GRS
MARINE/BEACH...YZR

NWS SJU Office Area Forecast Discussion