325
FXCA62 TJSJ 080829
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
429 AM AST Mon Dec 8 2025

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

.KEY MESSAGES...
Issued at 357 AM AST Mon Dec 8 2025

* Marine and beach conditions will deteriorate by the second part
  of the workweek, bringing 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 by midweek.
  Lightweight outdoor items may be blown around in exposed
  locations.

* Mostly fair weather conditions are expected for the first part
  of the workweek across the islands, with passing showers
  promoting ponding of water across roadways and poorly drained
  areas.

* For the U.S. Virgin Islands, few passing showers are very likely
  during the night through the early morning hours.

&&

.Short Term(Today through Wednesday)...
Issued at 357 AM AST Mon Dec 8 2025

Overnight cloud cover was variable across Puerto Rico and the U.S.
Virgin Islands, with only isolated and brief showers developing
over the local waters and occasionally brushing windward coastal
areas. Winds remained light to moderate from the east, with
typical land breezes dominating through the night.

Satellite imagery early this morning shows a pronounced dry slot
advancing toward the islands. This feature is expected to drop
precipitable water values below 1.0 inch, which is roughly two
standard deviations below normal for this time of year. As this dry
air moves overhead, mostly fair weather conditions should prevail
today, with limited shower activity.

For the remainder of the short-term period, a strong mid to upper-
level ridge northeast of the region will continue to dominate the
overall pattern. The ridge will gradually shift eastward, allowing
some troughiness to approach from the northwest Tuesday through
Wednesday. Although the area will remain mainly under ridge
influence, model guidance indicates a slight increase in mid-level
instability. Lapse rates at 500 mb are forecast to dip to near-
normal values between -6 and -7 degrees Celsius, with slightly
steeper lapse rates between 700 and 500 mb. This subtle change aloft
could enhance shallow convection. At lower levels, patches of
moisture embedded in the trade wind flow will move across the region
from time to time. These will increase shower frequency,
particularly during the nighttime and early morning hours along
windward coastal sectors. During the afternoons, convection is
expected to develop across interior and western/northwestern Puerto
Rico. While most activity should remain shallow, a few short-lived
thunderstorms cannot be ruled out, mostly on Wednesday.

At the surface, the high pressure over the central Atlantic will
shift slightly eastward, loosening the pressure gradient over the
region. Lighter steering winds will allow any showers that form to
move more slowly. Given this setup, a limited flooding threat has
been introduced for both Tuesday and Wednesday. Impacts should
remain minor, consisting mainly of ponding of water on roadways and
in poorly drained areas, with a low chance of urban and small-stream
flooding.

&&

.Long Term(Thursday through Monday)...
Issued at 357 AM AST Mon Dec 8 2025

No major changes were introduced to the forecast, as a broad mid-
level ridge will dominate and promote mostly fair conditions and
limited rainfall across the CWA. Moisture content will vary at
times, as a broad surface high pressure moving over the Central
Atlantic will bring patches of moisture, increasing the frequency of
showers across the islands. Based on the deterministic guidance of
the GFS and ECMWF, PWAT values will likely remain below normal
(between 1.2 and 1.4 inches), with a low chance of near above
climatological normal (up to 1.6 inches). Additionally, mid-level
moisture RH is expected to plummet well below normal (down to 10%)
while low to mid-level lapse rates shall remain seasonal to below
normal (3 - 5 degrees Celsius per kilometer). In terms of
instability, deep convection activity may be limited as 500 mb
temperatures will likely be warmer than normal (between -5 and -6
degrees Celsius), as the presence of the mid-level ridge should
promote stability aloft. A seasonal weather pattern is very likely
over the period, with passing showers moving over windward
sections and the U.S. Virgin Island streamers by noon, while
daytime heating combined with local effects and available moisture
should be enough for afternoon convection each day, particularly
over the western and northwestern portions of Puerto Rico. Due to
increasing winds, likely to promote breezy to locally windy
conditions, showers should become more progressive and less
stationary, reducing the chance of flooding. Although a lightning
risk is not expected through the period, short-lived isolated
thunderstorms cannot be ruled out, particularly by the end of the
period.

Under a southeasterly wind flow, warmer-than-normal temperatures are
expected throughout the period, particularly during the day.
Although isolated areas may reach heat indexes of 100 degrees
Fahrenheit at low elevations on the islands, no heat threat is
expected in the long-term forecast.

&&

.AVIATION...
(06Z TAFS)
Issued at 357 AM AST Mon Dec 8 2025

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. However, trade wind showers could move at
times across the USVI and east terminals of PR late in the forecast
period. ESE winds are expected to increase btw 10-15 kt aft 14z.

&&

.MARINE...
Issued at 357 AM AST Mon Dec 8 2025

A broad surface high pressure over the Central Atlantic will promote
east to southeast moderate to locally fresh winds today, weakening
and becoming moderate tonight. By midweek, strengthening surface
high pressure north of the region will 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 late Thursday night 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 357 AM AST Mon Dec 8 2025

The beach forecast remains on track. 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. Winds will likely
increase by Wednesday, with breaking waves increasing between 5 to
6 feet. Additionally, a long- period northerly swell is expected
to arrive by late Thursday night and spread across the local
waters and passages, deteriorating beach conditions particularly
along north- facing beaches of the islands. Hence, the chance of
life- threatening rip currents will increase to high over the
aforementioned areas. 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 diminishes, beach conditions
should improve by the upcoming weekend.

&&

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

$$

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