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278
FXCA62 TJSJ 170856
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
456 AM AST Mon Nov 17 2025

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

.KEY MESSAGES...
Issued at 428 AM AST Mon Nov 17 2025

* A high risk of rip currents is in effect along the north and
  east facing beaches of Puerto Rico and St. Croix in the U.S.
  Virgin Islands through Wednesday.

* Slow-moving showers and isolated thunderstorms will increase the
  flooding and lightning risk from the interior to western
  portions of Puerto Rico this afternoon.

* More frequent showers and isolated thunderstorms are likely late
  tonight and early Tuesday morning across the U.S. Virgin
  Islands.


&&

.Short Term(Today through Wednesday)...
Issued at 428 AM AST Mon Nov 17 2025

During the overnight hours, mostly calm and stable conditions
prevailed across the region. Skies ranged from mostly clear to
partly cloudy, and minimum temperatures fell into the low to mid-70s
across most coastal areas, with a few locations across the San Juan
metropolitan area and the USVI holding in the upper 70s. Winds
remained very light to calm under a weak surface pressure gradient.

For today, the forecast remains on track. As discussed previously,
the forecast area will be positioned under the subsident side of a
mid- to upper-level trough, while a stationary frontal boundary
lingers well to the northeast of the islands. Global model
climatology comparisons from the 17/00Z runs show a gradual warming
of the 500 mb temperatures as the upper trough continues to shift
eastward, with values rising toward -5 to -6 degrees Celsius.
Although this indicates some erosion of upper-level dynamics, the
temperatures remain cool enough to support moderate instability, an
environment reflected by 700-500 mb lapse rates that remain near or
slightly above climatological mid-range values. A reduction in
moisture is also expected, with precipitable water values between
1.3 and 1.4 inches, which is slightly below normal for mid-November.
These conditions should support a quiet morning with only a few
brief showers moving in from the waters and occasionally brushing
coastal sectors.

By the afternoon, the combination of diurnal heating, sea-breeze
convergence, and local effects will likely generate scattered
showers and possibly a few short-lived thunderstorms across the
interior and western/northwestern portions of Puerto Rico. High-res
models continue to suggest convective development despite the
marginal moisture and weakening instability aloft, and under a
southeasterly steering flow, some showers may also develop downwind
of El Yunque and drift toward the San Juan metropolitan area. Given
the relatively dry mid-levels and the short duration of convection,
the flooding risk remains low for today.

Moisture levels are forecast to recover slightly to near-normal
values on Tuesday, with winds gradually shifting from southeasterly
to more easterly. A surface trough located near the Leeward Islands
may approach the region; however, most associated shower and
thunderstorm activity is expected to remain south across the
Caribbean waters. Even so, afternoon convection driven by local
effects and sea-breeze interactions may still develop over Puerto
Rico, but rainfall accumulations should be limited and the flood
risk low.

By Wednesday, conditions become more favorable for convection as
another polar trough deepens and moves eastward across the western
Atlantic into the northeastern Caribbean. The region is expected to
lie beneath the more favorable, divergent side of this upper-level
feature during the afternoon and evening hours. Model climatology
comparisons show 500 mb temperatures dropping significantly to
around -8 to -9 degrees Celsius, along with steepening 700-500 mb
lapse rates and a notable decrease in 250 mb height fields and 1000-
500 mb thickness values, all indicative of more favorable upper-
level support and increased instability. These changes should
enhance the potential for more widespread showers and isolated
thunderstorms. Northeasterly steering winds may slightly limit
surface heating, but convective development may still be sustained
by stronger upper-level forcing, allowing showers and thunderstorms
to continue or redevelop through the evening and into the overnight
hours across the Atlantic waters, with occasional movement inland.
Under these conditions, the flood risk may increase on Wednesday,
especially in areas that experience repeated showers.

&&

.Long Term(Thursday through Monday)...
Issued at 428 AM AST Mon Nov 17 2025

A gradual improvement in the weather conditions is still anticipated
by the end of the workweek into the upcoming weekend. Thursday
may be the wettest day of the period due to lingering moisture
across the region and the influence of the polar trough. From the
deterministic guidance of the GFS and ECMWF, seasonal Precipitable
Water values (1.5 - 1.6 inches) and colder 500 mb temperatures
(around -8.5 degrees Celsius) are likely. Combined with local
effects and daytime heating , afternoon convection may develop
over the mountain ranges and southwestern Puerto Rico. Although
rainfall accumulations may not reach significant flooding
criteria, ponding of water over roadways, urban areas, and poorly
drained areas may lead to isolated urban and small stream flooding
over the aforementioned areas. By Friday, a surface high pressure
system should build over the western Atlantic, promoting mostly
NE winds. Weather conditions should improve over the weekend as
drier air filters into the region with a mid level ridge
dominating the weather pattern. From the latest model guidance,
PWAT values are likely to decrease and remain below climatological
normal (between 1.2 - 1.4 inches), while 500 mb temperatures will
get warmer than normal by the end of the period. Patches of
moisture will move occasionally across the CWA, bringing light to
moderate passing showers over windward sections of the islands in
the late night into the morning hours. Although afternoon
convection is expected, shower activity will likely be limited
under this weather pattern. Therefore, the flooding and lightning
threat is not expected for the rest of the forecast period.

Model guidance continues to suggest a fall in 925 mb temperatures,
being Thursday cooler than normal. Although the latest GFS
solution is tending to warmer temperatures for Saturday,
temperatures should remain seasonal, presenting no heat threat.
Hence, the heat risk should remain low for the rest of the period.


&&

.AVIATION...
(06Z TAFS)
Issued at 428 AM AST Mon Nov 17 2025


VFR conds will prevail through 17/15Z with SKC to FEW/SCT and
light/VRB winds. Brief -SHRA over local waters may brush coastal
terminals but with no significant impacts. Aft 17/14Z, winds
increase from the ESE-SE at 08-13 kt with sea-breeze influences.
Aftn convection over the interior and wrn PR may bring VCSH/VCTS to
TJSJ and TJBQ, with brief MVFR and mtn obscurations possible. PROB30
TSRA was added for TJBQ/TJSJ during peak heating. Convection
diminishes aft 17/22Z with light winds returning.

&&

.MARINE...
Issued at 428 AM AST Mon Nov 17 2025

A fading pre frontal trough north of the area will keep light to
gentle southeasterly winds today, becoming moderate from the east
late tonight due a surface high pressure over the Central Atlantic.
Pulses of a north northeasterly swell will spread across the Atlantic
waters and local passages through Wednesday. Another approaching pre
frontal trough will promote winds from the northeast later in the
workweek.

&&

.BEACH FORECAST...
Issued at 428 AM AST Mon Nov 17 2025

As mentioned in previous discussions, conditions were expected to
deteriorate due to pulses of north-northeasterly swells arriving
over the region. Current buoy observations show an increase in
period (between 10 - 12 seconds, up to 15 seconds for the San Juan
buoy), which results in breaking waves around 6 feet, promoting
hazardous beach conditions. Hence, a high risk of rip currents is
in effect for the northern and eastern beaches of Puerto Rico,
Culebra, and St. Croix in the U.S. Virgin Islands. Residents and
visitors planning on visiting the beach are urged to exercise
caution, as life- threatening rip currents are present along the
surf zone. Heed the advice of the flag warning system and swim
near a lifeguard. Afternoon convection may produce gusty winds
and lightning, beachgoers should remain weather alert and seek
shelter whenever they hear thunder.

&&

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

VI...High Rip Current Risk through Wednesday afternoon for VIZ002.

AM...None.
&&

$$

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

NWS SJU Office Area Forecast Discussion