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850
FXCA62 TJSJ 170715
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
315 AM AST Thu Sep 17 2026

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

.KEY MESSAGES...
Issued at 315 AM AST Thu Sep 17 2026

- The tropical wave to the west of Puerto Rico will continue to drive
  rainfall and thunderstorm activity over the US Virgin Islands
  and Puerto Rico throughout Thursday.

- Lingering moisture from the wave and a low levels of Saharan
  Dust are expected by Friday. With rainfall and thunderstorms
  are expected over the western side of Puerto Rico during the
  afternoon hours.

- Extreme heat risk is expected to gradually increase across the CWA,
  with risk levels expected to be limited today, moderate on
  Friday and significant by Saturday.

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

&&

.Short Term(Today through Saturday)...
Issued at 315 AM AST Thu Sep 17 2026

The past 24 hours have been active in terms of rainfall activity for
the US Virgin Islands and Puerto Rico due to the passage of a
tropical wave over the forecast area. Radar estimates show some
municipalities across the eastern (e.g., Canovanas, Rio Grande,
Gurabo, Juncos, San Lorenzo, Caguas, Cidra, Cayey, Las Piedras and
Humacao), northwestern (e.g., Aguada, Anasco, Moca, San Sebastian
and Las Marias), and interior (e.g., Adjuntas) regions of Puerto
Rico receiving from 2-4 inches of rain. While the rain associated to
the passage of the system does not put an end to the ongoing
drought, it does provide some respite to to the Carraizo and La
Plata Basins which are currently under observation and adjustment
levels respectively. The streamflow of the rivers over the eastern
side of Puerto Rico for today is much above normal, we might see
some of the effects from the rainfall to the water levels in the
days that follow as the river runoffs make their way to the dams.

The rainfall and thunderstorm activity is forecasted to continue
today as the wave axis is currently to the east of Puerto Rico and
continues to bring an influx of moisture and instability to the
region. Precipitable water (PWAT) values continue to be from
seasonal to above seasonal levels which is favorable for the
development of rainfall activity. Breezy conditions along with the
ESE windflow may also lead to the development of advective showers
over eastern Puerto Rico during the morning hours. Mid-tropospheric
temperatures will be warmer than normal, which may act as a limiting
factor for the development of deep convection. Nonetheless, local
effects (i.e., daytime heating, orographic effects, and sea breeze
convergence) may couple with the instability brought by the wave to
produce active periods of widespread rainfall and localized
thunderstorm activity over the US Virgin Islands and Puerto Rico
throughout the day. The areas that will see the highest rainfall
activity are eastern, interior and northwestern portions of Puerto
Rico. Nonetheless rainfall activity is expected across the entire
CWA. The elevated risk for excessive rainfall means that flooding in
urban areas, roads, small streams and washes along with a low change
of isolated flash floods is possible. Given the enhanced rainfall
received over the past 72 hours, we cannot rule out the possibility
of landslides with increased risk over the regions of high terrain,
as well as regions of the east, northwest and interior of Puerto
Rico where soil remains saturated from recent rainfall activity.

While the highest PWAT values will be seen today lingering moisture
from the tropical wave will persist into Friday. This will be
accompanied by low Saharan Dust concentrations. An upper-level ridge
will also make its way to the south of the US Virgin Islands and
Puerto Rico. Warmer than average mid-tropospheric temperatures will
also persist into Friday. These atmospheric conditions will act as
limiting factors for the development of rainfall activity as they
will lead to more stable mid-levels. Nonetheless we expect that the
lingering moisture from the wave coupled to local effects will lead
to passing showers particularly across northwestern Puerto Rico in
the afternoon as well as advective showers over the east during the
morning. The risk for excessive rainfall for Friday is also
elevated. Conditions are expected to become more seasonal by
Saturday. With a less saturated low-to-mid atmospheric column,
rainfall is primarily forecasted for the west and interior of Puerto
Rico during the afternoon hours driven by local effects and
seasonal PWAT values.

The heat risk for today is limited and will gradually increase to
moderate on Friday and Significant on Saturday. The significant
level of heat risk affects anyone without effective cooling and/or
adequate hydration. Impacts likely in some health systems, heat
sensitive industries and infrastructure. We recommend that
individuals be cautions when pursuing outdoor activities and to
remain well hydrated as they go into the weekend.

&&

.Long Term(Sunday through Thursday)...
Issued at 315 AM AST Thu Sep 17 2026

To start the next week, lingering tropical moisture will continue
over the islands under an east-southeast steering flow. Precipitable
water (PWAT) values will start at around 2 to 2.25 inches, high-end
normal to above normal values for this time of the year, before
becoming more spotty as patches of drier air filter in from Thursday
onwards. Throughout the period, areas of low pressure and surface
high pressure will move into the western Atlantic, sustaining a
generally light to moderate east-southeast steering flow.  An upper-
level low to the northeast will be present to start the week and
will continue to move southward and approach the islands. Current
model guidance suggests that this system will be east and northeast
of the islands by early next workweek and last through the long term
period. As of the latest guidance, 500 mb temperatures are forecast
at around -6 degrees Celsius while available moisture reaches the
upper levels through at least midweek, before retreating to the mid
levels. Uncertainty remains regarding this and the available
moisture reaching the islands midweek onwards. Please continue to
monitor the forecast for future updates. Light to moderate east-
southeasterly winds will steer ongoing showers toward windward
eastern areas, primarily during nighttime and morning hours. The
positioning of an upper-level low to the east and southeast brings
the potential for thunderstorm activity alongside these showers. In
the afternoons,  shower and thunderstorm development is forecast
across interior and western Puerto Rico, as well as downwind of El
Yunque, Vieques, Culebra, and the US Virgin Islands. Light to
moderate steering flow will allow rainfall to linger, also
maintaining a risk of flooding, particularly during the afternoon
hours across interior to western Puerto Rico. Additionally,
persistent moisture combined with the east-southeast wind flow may
result in limited to elevated heat risks.

&&

.AVIATION...
(06Z TAFS)
Issued at 315 AM AST Thu Sep 17 2026

06Z TAFs

Mainly VFR conds early. SHRA/TSRA will increase again as the day
progresses, leading to TEMPO MVFR conds across all terminals at
times. Light and variable winds increasing to 15-20 kt with higher
and becoming more easterly through 17/23Z. Light and VRB aft 17/23Z.

&&

.MARINE...
Issued at 315 AM AST Thu Sep 17 2026

A departing tropical wave will continue to promote showers and
thunderstorms today into tonight, producing locally hazardous marine
conditions, including wind gusts, frequent lightning, reduced
visibility, and higher seas. A surface high pressure system over the
western Atlantic will then dominate the regional weather pattern,
yielding moderate to locally fresh easterly winds. Mariners should
remain alert, as conditions could rapidly deteriorate in and around
stronger thunderstorms.

&&

.BEACH FORECAST...
Issued at 315 AM AST Thu Sep 17 2026

A moderate risk of rip currents is expected along the northern and
eastern beaches of Puerto Rico, Vieques, Culebra, and St Croix. As
winds weaken by Friday into early next week, the rip current risk
will drop to low across all local beaches. Meanwhile, a departing
tropical wave will continue to promote showers and thunderstorms
along and near coastal areas today. Beachgoers should remain weather
aware and seek shelter if thunderstorms develop nearby.

&&

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

$$

DAY... MRR/JCP/MNG/MMC
EVE... CAM/LIS/KML
MID... ICP/RVM

NWS SJU Office Area Forecast Discussion