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374
FXCA62 TJSJ 310719
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
319 AM AST Sat Oct 31 2026

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

.KEY MESSAGES...
Issued at 254 AM AST Sat Oct 31 2026

* Unsettled weather conditions are expected later today and
  tonight, so pack and umbrella if your are planning to go out
  for candy. Be prepared to look for shelter in case thunderstorms
  develop.

* Heat indices above 100 degrees are expected to persist, with
  increasing temperature expected by mid-week. Take plenty of
  breaks when outdoors and stay hydrated.

* Passing showers are expected for the Virgin Islands as well, but
  the risk of flooding is limited to none.

&&

.Short Term(Today through Monday)...
Issued at 254 AM AST Sat Oct 31 2026

The night was quiet across the northeastern Caribbean. A few
showers managed to reach portions of the Virgin Islands and
southeastern Puerto Rico, but accumulations were not significant.
Once again, temperatures cooled down to the low 80s and 70s for
coastal areas, and in the 60s across the Cordillera Central.

An upper level trough axis remains south of Hispaniola. This
feature will pull moisture into the region today, causing
increasing showers in the afternoon and evening hours. The area of
above normal precipitable water is very evident in satellite
imagery. Pack an umbrella if you are planning to collect candy
tonight, and be ready to seek for shelter if you hear thunder. The
first round of showers will be the usual afternoon convection
developing in the interior and west-southwest, were there is a
moderate risk of flooding, like almost every afternoon. Later
tonight, showers will increase along coastal areas of southern and
eastern Puerto Rico, and also in the vicinity of the Virgin
Islands.

To start November on Sunday, this trough will bring plenty of
clouds into the area. The dynamic aloft is somewhat conducive for
showers formation, but there is some dry air intrusion at the
lower levels. Additionally, cloud coverage may reduce the amount
of solar radiation received and hinder local effects. Regardless,
due to the uncertainty that these troughs bring, the probability
of precipitation was left at 60 to 70%, with a moderate risk of
urban and small stream flooding. The reasoning is that even with
strong cloud coverage, the trade winds will still carry showers
and thunderstorms into the local area. There is a risk of frequent
lightning as well with the thunderstorm activity.

The temperature profile still indicate above normal values for
this time of year. For today, the normal high is 88 and normal
low is 76 at the San Juan Area climate site. Elevated heat
indices, in those areas where the sun may peek, will cause issues
for those very sensitive, especially when outdoors and without
effective cooling or hydration.

&&

.Long Term(Tuesday through Saturday)...
Issued at 254 AM AST Sat Oct 31 2026

A generally warm and humid weather pattern is expected across Puerto
Rico during the long term period. Precipitable water values will
remain relatively high, ranging between 1.9 and 2.3 inches, while
low-level relative humidity (850 to 700 mb) will generally remain
between 70% and 85%. Mid-level moisture (700 to 500 mb) also shows
periods of increased humidity, mainly values in the 75th percentile
based on climatology for the month of November. Notably, 925 mb
temperatures are forecast to reach approximately two standard
deviations above normal at the beginning of the period (particularly
Tuesday and Wednesday), supporting warmer than normal conditions. If
widespread cloud cover does not develop, these warm and humid
conditions could promote stronger daytime heating and enhance
afternoon convection. At the surface, light and variable winds are
expected, particularly from Wednesday through Friday, resulting in
very weak steering flow. This will allow locally developed showers
to move slowly, increasing rainfall accumulations and the potential
to observe flooding. Sea-breeze convergence, diurnal heating, and
orographic lifting across the interior and western portions of
Puerto Rico could further enhance shower development. Periods of
heavy rainfall could lead to urban and small stream flooding and
quick river rises.

At the upper levels, no significant changes in the overall pattern
are expected initially, although current global models show some
disagreement regarding the long term evolution. An upper-level low,
also reflected in the surface pattern and associated frontal
boundary, is forecast to swing north of the region across the
Atlantic waters. Toward the end of the week, its influence could
promote a shift in low-level winds from the south, bringing
additional tropical moisture into the region and maintaining a warm
and humid environment. Meanwhile, 500 mb temperatures will remain
near -5 to -6C, with generally moderate lapse rates suggesting some
potential for atmospheric instability. Cloud cover will play an
important role in afternoon convective development, as extensive
cloudiness could limit surface heating and inhibit convection, while
periods of sunshine would favor stronger daytime heating and enhance
shower development. Overall, expect warm and humid conditions,
variable cloud cover, and periods of showers, particularly across
interior and western Puerto Rico. Given the weak steering flow and
potential for slow-moving showers, locally heavy rainfall, increased
rainfall accumulations, and flooding impacts will remain possible.
Continue to monitor the forecast as confidence increases and updated
become available.

&&

.AVIATION...
(06Z TAFS)
Issued at 254 AM AST Sat Oct 31 2026

Mainly VFR conditions are expected to prevail this morning. SHRA
will increase after 17Z for the Cordillera Central, causing
mountain obscuration. Periods of reduced VIS and low ceilings are
expected for TJPS at times. SHRA will increase after 22Z for the
Caribbean Sea and the USVI terminals. These showers could cause
interruptions to operations. Winds are from the SE at 10 kts or
less, shifting slightly from the NE after 15Z at 1-3 kft,
remaining from the SE above 5 kft.

&&

.MARINE...
Issued at 254 AM AST Sat Oct 31 2026

Surface high pressure over the central Atlantic will continue to
promote moderate east to southeasterly winds across the regional
waters during the weekend. Seas will remain between 1 to 3 feet
during the next few days, with ocasional higher seas near the heavy
rainfall and thunderstorms. Expect choppy conditions at times across
portions of the offshore waters and local passage reducing
visibility and generating gusty winds.

&&

.BEACH FORECAST...
Issued at 254 AM AST Sat Oct 31 2026

A moderate risk of rip currents will continue during the next
several days along the northern beaches of Puerto Rico and Culebra,
as well as the US Virgin Islands. This means life-threatening rip
currents are possible in the surf zone. Beachgoers should remain
cautious. Expect periods of afternoon showers and thunderstorms,
particularly across northwestern and northern Puerto Rico. If you
hear thunder, seek shelter immediately inside a sturdy building.


&&

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

$$

SHORT TERM/AVIATION...ERG
LONG TERM....YZR
MARINE/BEACH...GRS

NWS SJU Office Area Forecast Discussion