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FXCA62 TJSJ 151738
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
138 PM AST Tue Sep 15 2026

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

.KEY MESSAGES...
Issued at 138 PM AST Tue Sep 15 2026

- The interaction of an upper-level low and tropical moisture to
  the south of the local area will continue to favor the
  development of passing showers and thunderstorms activity across
  Puerto Rico and the U.S Virgin Islands for the rest of the day.

- A drier air mass is expected to reach the islands this evening,
  however, is anticipated to depart the local area by Wednesday
  morning, followed by a vigorous tropical wave.

- The interaction between a tropical wave and the upper-level low
  located westward of the CWA, will enhance widespread convection
  and thunderstorms development particularly over the eastern,
  northern and western Puerto Rico.

- An elevated excessive rainfall risk is anticipated over these
  areas.

- The heat risk will remain limited for the short-term forecast
  period, as cloud cover and rainfall is expected to increase.

- There is a moderate risk of rip currents through Thursday for
  beaches across the US Virgin Islands and Puerto Rico.

&&

.Short Term(This evening through Thursday)...
Issued at 138 PM AST Tue Sep 15 2026

Scattered showers and isolated thunderstorms continued to move
across eastern, northern, and interior Puerto Rico and the U.S.
Virgin Islands throughout the morning. Doppler radar and satellite
imagery show additional activity moving into these areas, though
rainfall accumulations have remained under 1.00 inch. High
temperatures stayed in the mid-to-upper 80s across coastal and
urban areas, while higher mountain elevations experienced much
cooler temperatures in the low-to-mid 70s.

Variable conditions will persist for the rest of today under the
influence of an upper-level low positioned west-northwest of the
region. This system continues to promote instability and cooler
mid- level temperatures (-7C to -8C at 500 mb). Combined with
abundant low-level moisture, this pattern will support strong
shower and thunderstorm development through the afternoon,
particularly over northern, central, and western Puerto Rico.
Consequently, the excessive rainfall risk remains elevated today.
This activity should diminish by sunset, followed by a drier air
mass that will persist through Wednesday morning.

Moisture will increase significantly from Wednesday onward, with
precipitable water (PWAT) values rising above normal to 2.50
inches as a vigorous tropical wave approaches from the east. As
this tropical wave move westward, it will interact with the upper-
level low to enhance thunderstorm activity, further fueled by
daytime heating and local terrain effects. This will maintain the
excessive rainfall risk to elevated levels across eastern,
northern, and western Puerto Rico, as well as the U.S. Virgin
Islands.

Given the anticipated rainfall, urban and small stream flooding
are possible. Isolated flash flooding and landslides cannot be
ruled out in higher terrain, as soils remain saturated from recent
rains, especially in central, western, and northwestern Puerto
Rico. Similar conditions are expected to persist into Thursday as
the tropical wave slowly moves across the area, placing the bulk
of moisture over the area and favoring widespread convection and
localized thunderstorms across eastern, interior, and northwestern
Puerto Rico.

Due to the increased cloud cover and rainfall, the heat risk will
remain limited for the rest of the short-term forecast period.
Residents and visitors are encouraged to monitor local updates as
weather conditions develop throughout the week.

&&

.Long Term(Friday through next Monday)...
Issued at 253 AM AST Tue Sep 15 2026

For the first part of the long-term forecast period, model guidance
suggests that moisture will remain generally near or above
climatological normals. From Friday, residual moisture associated
with the previous tropical wave will linger across the region, with
precipitable water values remaining mainly between 1.7 and 2.0
inches, which is slightly above climatological values. Although the
NASA Goddard Earth Observing System Model V5 suggest some light
concentrations of Saharan Dust particles. Mid-level relative
humidity will also remain relatively high, staying mostly near the
75th percentile and at times approaching +2 standard deviation
threshold, supporting sufficient moisture for shower development.
However, a broad mid-level ridge over the western Atlantic may
enhance relatively warm 500 mb temperatures in the -5 degrees
Celsius range, favoring some mid-level stability and limiting
widespread deep convection. Easterly winds will prevail through
Saturday, with some upper-level cloudiness also expected.
Therefore, passing showers will remain possible across windward
areas, followed by afternoon convection mainly across interior and
western Puerto Rico due to local and diurnal effects.

From Sunday through Tuesday, a surface high pressure building across
the central Atlantic will promote a gradual veering of the winds,
becoming more southeasterly. Model guidance suggests that moisture
will remain available across the region, with precipitable water
generally near or above the 50th percentile and occasionally
approaching the 75th percentile. This moisture will be partly
associated with an induced surface trough over the central Atlantic,
north of the region, which will promote periods of cloudiness and
showers across the area. At the mid-levels, marginally favorable
conditions will persist, with 500 mb temperatures near -6 degrees
Celsius potentially supporting a few thunderstorms where sufficient
moisture and instability are present. In general, the combination of
sufficient moisture, moderate southeasterly winds, and local effects
will maintain the potential for passing showers and afternoon
convection through early next week. Additionally, the available low-
level moisture and southeasterly winds will favor warmer
temperatures, particularly along northern coastal and urban areas.
Therefore, a limited to locally significant heat threat can be
expected during this portion of the period. Residents are urged to
stay hydrated and follow recommendations from local health
officials.

&&

.AVIATION...
(18Z TAFS)
Issued at 138 PM AST Tue Sep 15 2026

Mainly VFR conds are expected to prevail across all terminals
during the next 24 hours. -SHRA/TSRA will continue to develop over
northern, central and western PR leading to TEMPO MVFR conds at
TJSJ/TJBQ/TJPS btw 15/18-23Z. VCSH possible at TIST/TISX thru
15/23Z. E winds 12-16 kt with gusts up to 26 kt through early this
evening, becoming light and VRB aft 15/23Z. Winds are expected to
increase again to 12-18 kt with gusts and sea breeze variations
aft 16/13-14Z.

&&

.MARINE...
Issued at 138 PM AST Tue Sep 15 2026

Moderate easterly winds will prevail across the region, as a
broad surface high is over the western Atlantic alongside a few
induced surface troughs. A tropical wave approaching the Anegada
Passage tomorrow, Wednesday, morning and gradually moving over the
rest of the local waters through Thursday is expected to bring
heightened shower and thunderstorm activity. Seas will continue
under 5 feet with winds up to 15 knots, with locally higher gusts
and seas near showers and thunderstorms. As the tropical wave
shifts west of the area on Friday and leaves behind lingering
moisture, conditions will gradually improve, while moderate
easterly trade winds persist into the weekend.

&&

.BEACH FORECAST...
Issued at 138 PM AST Tue Sep 15 2026

A moderate risk of rip currents (life-threatening rip currents
are possible in the surf zone) will continue through the evening
along most of the north-facing coast of Puerto Rico. The moderate
risk will become more localized tonight through Thursday with a
low risk of rip currents dominating most coastal areas. Remember,
even if the risk for rip currents is low, life-threatening rip
currents often occur in the vicinity of groins, jetties, reefs,
and piers. A tropical wave will increase showers and thunderstorms
mainly Wednesday into Thursday. Beachgoers should seek shelter
immediately if thunder is heard. A more broad moderate risk is
forecast on Friday for the north-facing beaches of Puerto Rico
followed by a generally low risk of rip currents throughout the
weekend.

&&

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

$$

KEY MESSAGES...JCP
SHORT TERM/AVIATION...GRS
LONG TERM....LIS
MARINE/BEACH FORECAST...MRR

NWS SJU Office Area Forecast Discussion