674
FXCA62 TJSJ 090903
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
503 AM AST Tue Dec 9 2025

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

.KEY MESSAGES...
Issued at 449 AM AST Tue Dec 9 2025

* Marine and beach conditions will deteriorate by the second part
  of the workweek, expect choppy to rough seas and life-
  threatening rip currents.

* Winds will increase late in the week, leading to breezy to windy
  conditions across coastal and elevated areas, lightweight
  outdoor items may be blown around in exposed locations.

* For Puerto Rico, showers are expected to continue during the
  morning hours across the east/southeastern portions. Over
  west/northwest PR, afternoon showers are expected each day.

* For the U.S. Virgin Islands, passing showers will continue
  throughout the morning, becoming frequent later tonight and
  early tomorrow.

&&

.Short Term(Today through Thursday)...
Issued at 449 AM AST Tue Dec 9 2025

Satellite imagery and Doppler radar early tonight showed an area of
higher moisture and increased cloud cover associated with a weak
surface perturbation drifting across the islands. This feature
briefly enhanced shower activity over the U.S. Virgin Islands,
Vieques, Culebra, and the southeastern coastal areas of Puerto Rico.
However, rainfall accumulations were minimal.

For today, the mid-level ridge will continue to influence the
region, shifting slightly eastward as some troughiness approaches
from the northwest. Model guidance indicates that 500 mb
temperatures will be at their lowest of the short-term period. As a
result, 700-500 mb lapse rates will steepen slightly, providing
marginal instability. However, mid-level dryness will continue to
limit vertical development, keeping most showers shallow. The weak
trade wind perturbation will support variable cloudiness and passing
morning showers over windward coastal sectors, followed by localized
afternoon showers across the interior and western/northwestern
Puerto Rico due to diurnal heating and sea breeze convergence.

Wednesday remains slightly wetter. Model guidance indicates
increasing moisture throughout the column, with precipitable water
values nearing 2.0 inches and 700500 mb relative humidity rising to
above-normal levels. This additional moisture, combined with
gradually strengthening easterly to east-southeasterly winds and
marginal instability, will promote more frequent showers. Afternoon
convection over the interior and western portions of Puerto Rico
will be deeper than in recent days, and a few isolated thunderstorms
cannot be ruled out.

Thursday will feature a pattern similar to today, with shallow
moisture embedded in the trade wind flow supporting passing
nighttime and early morning showers over windward areas, followed by
isolated to scattered afternoon showers across the interior and
west/northwest Puerto Rico. However, rainfall accumulations may be
lower than today as breezy to locally windy conditions are expected,
leading to quick-moving showers.

&&

.Long Term(Friday through Tuesday)...
Issued at 449 AM AST Tue Dec 9 2025

Mostly fair conditions are expected to persist this upcoming
weekend, becoming variable early next week. As mentioned in the
previous discussions, a surface high pressure over the Central
Atlantic will strengthen, tightening the pressure gradient, with SE
winds promoting breezy to locally windy conditions on Friday and
Saturday. Additionally, a mid-level ridge lingering northeast of the
CWA should serve as a blocking pattern, as shower activity will
mostly be limited through at least Sunday. Nevertheless,
troughiness in the mid to high levels may weaken the mid-level
ridge, slightly cooling 500 mb temperatures (between -6 and -7
degrees Celsius), and bringing marginal instability. From the latest
model guidance, PWAT values are expected to increase by early
Monday, although model solutions show high variability as ensemble
members show a spread between 1.2 - 1.6 inches. The most likely
scenario is an increase in frequency of showers early Monday over
the windward sections of the islands, with afternoon convection over
western and northwestern PR, including the San Juan streamer. The
latest model solutions also suggest  that the proximity of a col
region north of the region may weaken winds, promoting slow-moving
showers. Taking into account possible marginal instability
conditions, the flooding potential may increase during the
afternoons, with mostly ponding of water over roadways and poorly
drained areas, and a low chance of urban and small stream flooding.
Although widespread lightning risk is not likely, short-lived
isolated thunderstorms cannot be ruled out.

As winds are likely to remain from the southeast, model guidance
continues to suggest warmer-than-normal 925 mb temperatures,
particularly during peak daytime hours. As mentioned in the previous
discussion, the heat index in localized areas over the lower
elevations of the islands may reach 100 degrees Fahrenheit, but no
heat threat is expected during the period.

&&

.AVIATION...
(06Z TAFS)
Issued at 449 AM AST Tue Dec 9 2025

Mainly VFR conds expected across all terminals. Brief -SHRA/VCSH
will continue to move in from the E, mainly affecting USVI terminals
(TIST/TISX) and occasionally TJSJ through 09/14z, with brief
MVFR/IFR VIS/CIGs psbl. Sea-breeze influences aft 14Z may trigger
SHRA/VCSH at TJBQ and interior PR. Winds light/vrb overnight,
becoming ESE 1015 kt with ocnl higher gusts aft 14Z.

&&

.MARINE...
Issued at 449 AM AST Tue Dec 9 2025

A broad surface high pressure over the Central Atlantic will promote
east to southeast gentle to moderate winds today through Wednesday.
The high pressure will strengthen and tighten the pressure gradient
and increase winds, allowing seas to build across the Atlantic waters
and exposed passages. Additionally, a long-period northerly swell
arriving early Thursday will further worsen conditions. Choppy to
rough seas are expected, with Small Craft Advisories likely by the
end of the workweek. A gradual improvement remains possible by the
weekend as winds and swell ease.

&&

.BEACH FORECAST...
Issued at 449 AM AST Tue Dec 9 2025

The moderate risk of rip currents will continue over the north and
east-facing beaches of Puerto Rico and the U.S. Virgin Islands
through at least Wednesday. Beachgoers are encouraged to exercise
caution as life-threatening rip currents are possible along the surf
zone. A long-period northerly swell is expected to arrive early on
Thursday, and spread across the local waters and passages.
Combined with increasing winds, beach conditions are very likely
to deteriorate and become dangerous mainly along north- facing
beaches of the islands. Residents and visitors are urged to check
the beach forecast before going out, and heed the advice of the
flag warning system. As the swell ease and winds weaken, beach
conditions should improve by the latter part of the weekend.

&&

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

$$

SHORT TERM/AVIATION...CVB
LONG TERM/MARINE...MNG

NWS SJU Office Area Forecast Discussion