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380
FXCA62 TJSJ 310833
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
433 AM AST Wed Dec 31 2025

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

.KEY MESSAGES...
Issued at 424 AM AST Wed Dec 31 2025

* The forecast for today " New Year`s Eve" into Thursday calls for
  a more unsettled pattern as an upper- level trough and
  associated frontal boundary approach from the west.

* A more unsettled pattern late in the week, with increasing
  moisture and instability aloft for tomorrow

* For today,life-threatening rip currents will continue to prevail
  across the north-facing beaches of Puerto Rico, Culebra, and the
  northern U.S. Virgin Islands. Improving conditions are forecast
  from tomorrow.

* For the U.S. Virgin Islands, some showers are forecast, however
  the well developed showers will remain west of the islands.

&&

.Short Term(Today through Friday)...
Issued at 424 AM AST Wed Dec 31 2025

Today through Friday...

The short-term forecast remains on track, beginning with relatively
stable and pleasant conditions today, followed by a gradual
transition toward a more unsettled pattern late Wednesday night
through Friday as a deepening polar trough crosses the northeastern
Caribbean. For today, mostly fair weather is expected across much of
the forecast area. The low-level flow will remain from the east-
southeast, resulting in mostly light to moderate winds. Only
isolated brief showers are expected, mainly across portions of
western Puerto Rico during the afternoon due to local and diurnal
effects.

From tonight onwards, a deepening polar trough will advance eastward
across the region, leading to increasingly favorable upper-level
dynamics. Model guidance shows strengthening ventilation aloft
associated with a 60-80 kt jet at 250 mb moving over the area. In
addition, mid-level cold air advection will lower 500 mb
temperatures to around -9 to -10 C, particularly on Thursday (New
Years Day), resulting in steeper lapse rates and enhanced
instability aloft.

At the low to mid levels, tropical moisture continues to be drawn
northward, with GOES-19 PWAT imagery already showing higher moisture
values west of Puerto Rico. Precipitable water values are forecast
to increase to near or slightly above climatological normals, while
700-500 mb relative humidity rises to well above two standard
deviations above normal. Despite this, the latest model guidance and
ECMWF Extreme Forecast Index indicate that the highest moisture
content and most organized convective activity will remain primarily
west and north of the forecast area; over Hispaniola, the Mona
Passage, and the Atlantic waters. Puerto Rico and the U.S. Virgin
Islands are expected to remain on the eastern and southeastern
periphery of the most active region.

Additionally, persistent dry air intrusions in the 850-700 mb layer
noted in several model cycles may somewhat inhibit convective
development locally. As a result, widespread thunderstorm activity
is not anticipated. However, sufficiently low-level moisture
combined with increasing instability aloft and favorable upper-level
dynamics could still lead to localized shower activity and isolated
thunderstorms, particularly when enhanced by local island effects.

From Thursday into Friday, shower coverage is expected to increase
somewhat, particularly during the afternoon and evening hours across
western and interior sections of Puerto Rico, driven by diurnal
heating, sea breeze convergence, and local effects. Isolated showers
and thunderstorms may also develop during the overnight and early
morning hours over the surrounding waters, occasionally brushing
coastal areas, especially along the Mona Passage and Atlantic
waters. Activity across the U.S. Virgin Islands is expected to
remain more limited and mainly in the form of passing showers.

Overall, the short-term period will feature a transition from
todays relatively stable conditions toward a more unsettled pattern
late in the week, with increasing moisture and instability aloft.
While the greatest impacts are expected to remain over areas west
and north of the forecast region, localized heavy rainfall remains
possible. Any area experiencing prolonged rainfall could see limited
to moderate flooding, including urban and small stream flooding,
which may create hazardous travel conditions during the holiday
period.

&&

.Long Term(Saturday through Wednesday)...
Issued at 424 AM AST Wed Dec 31 2025

From Saturday into Sunday, weather conditions will remain
dependent on a high-pressure system building over the central
Atlantic. On Saturday, surface winds will remain from the east,
veering on Sunday more from the southeast to south, and eventually
from the west with the approach of a pre-frontal boundary late.
At the upper levels, conditions will rapidly improve as the
subsidence side of the upper-level trough moves eastward, allowing
a more stable pattern to dominate. At 500 mb, a ridge will move
in, allowing warmer temperatures of 6 to -5 degrees. Although
stable conditions will persist at the mid to upper levels, at the
surface, as surface winds vary, shallow patches of moisture will
support some morning showers across the eastern sections, with
afternoon showers developing across the interior and western
Puerto Rico. According to the global model guidance, precipitable
water values will remain within the climatological normals,
ranging from 1.5 to 1.7 inches. These surface conditions and the
local mountain effect might aid some very short-lived showers at
the peak of the afternoon on Saturday and Sunday.

Conditions will rapidly deteriorate from late Monday into Tuesday
as a deep mid- to upper-level trough moves across the forecast
area. This mid- to upper-level feature will enhance very unstable
weather conditions across the islands, with colder mid-level
temperatures ranging from 9 to 10 degrees. In addition to these
very favorable conditions for thunderstorm activity, the GDI index
also shows an increase in the potential for scattered showers
late Monday into Tuesday. Furthermore, 850500 mb relative
humidity values will spike well above climatological norms,
surpassing two standard deviations by Tuesday. Although changes
are highly likely, residents and visitors are urged to stay tuned
for further updates, especially given the Three Kings Day
celebration on Tuesday.

&&

.AVIATION...
(06Z TAFS)
Issued at 424 AM AST Wed Dec 31 2025

Mainly VFR conds exp across all TAF sites thru the fcst pd.
SHRA/VCSH psbl at times, mainly across western PR terminals. VCTS
included at TJBQ late tonight as convective activity over the
western waters may approach the area. Widespread TSRA not
anticipated. Sfc winds light and vrb overnight, becoming E-SE 812
kt aft 1315Z with higher gusts and sea breeze variations.

&&

.MARINE...
Issued at 424 AM AST Wed Dec 31 2025

A weakening northerly swell will continue to result in brief
hazardous marine conditions, especially across offshore waters
during the day, improving in the night hours. A surface high
pressure system moving eastward from the western into the central
Atlantic, interacting with the remnants of an old frontal
boundary, will maintain a light east-southeasterly wind flow
across the region. As the frontal boundary approaches, increased
moisture will increase the potential for showers and isolated
thunderstorms, as well as localized hazardous marine conditions.
During the period, pulses of long-period northerly swell will
continue to propagate across the Atlantic waters and Caribbean
passages through the middle of the week. Small craft operators are
urged to exercise caution, particularly across the exposed
Atlantic waters and Caribbean passages.

&&

.BEACH FORECAST...
Issued at 424 AM AST Wed Dec 31 2025

For the rest of the day, energy from the weakening northerly
swell will continue to pose a high risk of rip currents across
northern coastal areas of Puerto Rico and Culebra until 6 PM this
evening. For the southern coastal areas and the western side of
Puerto Rico, including Vieques, and the U.S. Virgin Islands, there
will be a moderate rip current. Although conditions are forecast
to slightly improve, continuous pulses of energy from further
swells will remain near the coast, resulting in hazardous coastal
conditions.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through this afternoon for PRZ001-002-005-
     008-012.

VI...High Rip Current Risk until 6 AM AST early this morning for
     VIZ001.

AM...None.
&&

$$

SHORT TERM & AVIATION...CVB
LONG TERM, BEACH FORECAST & MARINE....LIS

NWS SJU Office Area Forecast Discussion