600
FXCA62 TJSJ 100849
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
449 AM AST Wed Sep 10 2025

.KEY MESSAGES...

* A Heat Advisory is in effect for the urban and coastal areas of
  the US Virgin Islands and Puerto Rico, as well as the valleys of
  the eastern interior.

* Strong thunderstorms will develop this afternoon mainly across
  PR`s northwest quadrant, elevating the risk of flooding rain and
  frequent lightning.

* The US Virgin Islands can expect a mixture of sunshine and
  clouds with little or no rain through the rest of the week.

* Warm to hot heat indices will persist across urban and coastal
  locations in the US Virgin Islands and Puerto Rico, worsening
  the second part of the workweek with the arrival of a Saharan
  Air Layer.

* Beachgoers, there is a moderate risk of rip currents for the
  exposed north and east-facing beaches of Puerto Rico and St
  Croix.

&&

.SHORT TERM...Today through Friday...

Calm weather conditions prevailed overnight across the islands, with
few showers observed over the local waters. Minimum temperatures
were seen in the upper 70s to the low 80s in the coastal and urban
areas of the islands, and from the upper 60s to low 70s in the
mountains.

Today, warm temperatures in combination with moisture associated
with a weak tropical wave (former Invest 91L) will result in showers
and isolated thunderstorms across Puerto Rico and the U.S. Virgin
Islands. The latest precipitable water content guidance (PWAT) has
values between 1.80 to 2.05 inches, with the highest values between
mid-morning and late afternoon. The "El Yunque" streamer is expected
to develop too, affecting mostly the San Juan metropolitan area and
adjacent municipalities. Meanwhile, across the U.S. Virgin Islands
mainly fair weather will prevail with the potential to observe
showers from time to time, particularly during the morning hours.

By tonight into Thursday, after the wave departure, slightly drier
conditions will return into the area along with low to moderate
concentrations of Saharan dust particles. However, despite the
presence of the Saharan dust, convective activity is expected each
afternoon over portions of western and northwestern Puerto Rico. As
a result, there is a limited to elevated flooding risk across
portions of Puerto Rico through at least the weekend; whereas the
flooding risk for the U.S. Virgin Islands will remain low.

The 925 mb temperature will remain above-than-normal for the next
few days, which in turn suggest warmer than normal surface
temperatures. Additionally, the available tropical moisture
combined with the warmer trend will sustain an elevated heat
threat for the short- term forecast.

.LONG TERM...Saturday through Wednesday...

Model forecasts indicate that a mid-to-upper-level trough will
remain positioned to the northeast of the US Virgin Islands
(USVI), close to the Leeward Islands. This setup will generate
occasional easterly disturbances that will pass over our region
throughout the weekend. Subsequently, a mid-to-upper-level high-
pressure system is expected to build in from the northeast, likely
between Sunday and Monday. This development will enhance the
trade wind cap and introduce dry air with subsidence aloft.

Late Monday night into Tuesday, a tropical wave is predicted to
move into the Northeast Caribbean, which will increase moisture
levels and stimulate shower and thunderstorm activity.
Additionally, by the middle of next week, a mid-to-upper-level
trough will approach the area, interacting with the lingering
tropical moisture between Wednesday and Thursday.

Under the weather pattern explained above, residents and visitors
in the US Virgin Islands and Puerto Rico can expect excessive heat
that will affect most individuals sensitive to heat, especially
those without effective cooling and/or adequate hydration,
particularly in low-urban and coastal areas, each day through at
least Monday. However, the predominantly wind pattern will bring
occasional surges of moisture arriving on Saturday and early
Sunday. Then, by Sunday and Monday, a stable weather pattern and a
somewhat drier air mass will limit rain activity. Meanwhile,
strong afternoon convection will form each day across the interior
and western sections. The best chance to observe widespread
convective activity is between Tuesday and Thursday, associated
with Tuesday`s tropical wave, followed by the lingering mid to
upper-level trough interacting with abundant tropical moisture on
Wednesday and Thursday.

&&

.AVIATION...
(06Z TAFs)

Mainly VFR conditions are expected to prevail at all TAF during the
fcst period. However, SHRA/TSRA may result in brief periods of MVFR
conds at TJSJ/TJBQ/TJPS aft 10/13Z. Mountain obsc and lower ceilings
are possible, especially with the heaviest rains. VCSH will remain
possible at the TIST/TISX thru 10/23Z. Winds will prevail from the
southeast 10-15 kt with sea breeze variations and higher gusts aft
10/14Z.

&&

.MARINE...

A surface high pressure across the central Atlantic and a front over
the western Atlantic will promote light to moderate east to southeast
winds through the end of the week. A weak tropical wave will move
near the region, enhancing showers and thunderstorms across the
regional waters today. These thunderstorms could bring squally
weather and locally higher seas.

&&

.BEACH FORECAST...

Attention beachgoers: Exercise caution on the exposed north- and
east-facing beaches in Puerto Rico and St. Croix, as there is a
moderate risk of rip currents today and tomorrow. Additionally,
thunderstorms may develop, particularly in the afternoon or
overnight. If you hear thunder, please seek shelter by going
indoors or finding a safe location.

Marine guidance indicates that the risk will decrease to low for
the weekend. Although the risk of rip currents is low, life-
threatening currents often occur near groins, jetties, reefs, and
piersexercise caution. It might become moderate Monday and
Tuesday next week.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010>013.

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

AM...None.
&&

$$

SHORT TERM/AVIATION...GRS
KEY MESSAGE/LONG TERM/MARINE/BEACH FORECAST....CAM