829
FXCA62 TJSJ 210749
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
349 AM AST Thu Aug 21 2025

.KEY MESSAGES...

* Another hot day is expected for Puerto Rico and the Virgin
  Islands. This level of heat affects those sensitive to heat,
  especially those without effective cooling or adequate
  hydration.

* A strong tropical wave will increase the potential for showers
  and thunderstorms late Friday and on Saturday. Across the Virgin
  Islands, ponding of water in roads is anticipated with isolated
  urban flooding. For Puerto Rico, urban and small stream flooding
  are likely along the interior and northwest, with ponding of
  water elsewhere.

* A northerly swell from distant Hurricane Erin will reach the
  beaches of western and northern Puerto Rico, and the beaches of
  St. Thomas and St. John late Friday and this weekend. Conditions
  will be hazardous for beach goers and for small boats.


&&

.SHORT TERM...Today through Saturday...

Calm weather conditions prevailed overnight across the U.S. Virgin
Islands and Puerto Rico, with just few passing showers over the
Caribbean waters and local passages. Overnight temperatures reached
from the upper 70s to the low 80s across the lower elevations.
Meanwhile, lows remained in the low 70s along the higher elevations.

For today, a Saharan Air Layer (SAL) will continue to bring moderate
to locally high concetrations of Saharan dust into the islands.
Therefore, hazy skies, reduced visibilities and poor air quality
will continue to prevail today. A drier airmass with precipitable
water values (PWAT) below-than-normal (1.05 to 1.30 inches) will
continue to filter into the region today, limiting the development
of showers and thunderstorms through at least Friday.

From Friday onward, we will be transitioning from a dry and stable
environment into a much wetter and humid pattern due to the approach
of a vigorous tropical wave into the region. This tropical wave is
currently being monitored by the National Hurricane Center (NHC).
According to the latest forecast, this wave has a low formation
chance (30%) in the next 48 hours and a high formation chance (70%)
in the next seven days.

The moisture field associated with this wave will reach the islands
by Friday afternoon and it is expected to persist through early
Sunday. The latest model guidances suggest PWAT values up to 2.20
inches on Saturday, enhancing the potential to observe showers and
thunderstorms across the region. By Saturday morning, the U.S.
Virgin Islands will expect showers and isolated thunderstorms across
the area. Later in the afternoon, the activity will be focused over
the interior and norwestern portions of Puerto Rico. At this time,
residents and visitors are advised to continue monitoring the latest
weather updates as we move into the weekend as some of these showers
and thunderstorms could potentially lead to flooding.

.LONG TERM...Sunday through Thursday...

As the tropical disturbance monitored by the National Hurricane
Center lifts well north of the islands, the trade winds will
remain from the southeast. At the upper level, a trough will
approach from the north. Even though the islands will not be in
the most favorable position of the trough, temperatures at mid
levels will cool down, which, along with diurnal heating, should
allow for the development of thunderstorms across the interior and
northwestern Puerto Rico. At the low levels, drier air will filter
in, possibly with some Saharan dust too. This dry air will
maintain mostly fair weather for the Virgin Islands, although a
few passing showers cannot be ruled out.

Early in the workweek, most moisture will be staying close to the
surface, while the upper level trough remains in the vicinity of
the islands. This period will not have too much rain, except for
the usual convection in western Puerto Rico during the afternoon
hours. By mid-week, the surface high over the central Atlantic,
and a long wave polar trough near the eastern coast of the United
States will make the winds to shift from the south-southeast. This
will lift moisture from the Caribbean into the islands. With a
southerly wind flow, we should expected higher than normal
temperatures, and also an increase in showers and thunderstorms.


&&

.AVIATION...
(06Z TAFs)

VFR conds expected across all terminals throughout the fcst period.
Calm wx with hazy skies due to moderate to locally high
concentrations of Saharan dust will prevail today, reducing VIS
across the local terminals. ESE winds will continue to prevail at 10-
15 kt, with higher gusts and sea breeze variations thru 21/23Z.

&&

.MARINE...
Light to moderate southeasterly winds will continue through midweek,
becoming east-northeast from Thursday onward. A strong tropical wave
is expected to approach the eastern Caribbean on Friday, increasing
thunderstorm activity in general through Saturday. A long period
northerly swell from distant Hurricane Erin will arrive late Friday
night and into the weekend, resulting in hazardous seas for the
Atlantic waters and passages.

&&

.BEACH FORECAST...
Moderate rip current risk will persist today and tomorrow across
the islands. Thunderstorms may develop in the afternoon along
western Puerto Rico. This storms could produce dangerous lightning
strikes, so be ready to seek shelter if you hear thunder.

Conditions will deteriorate by the weekend and early next week. A
northerly swell will arrive across the beaches of northern and
western Puerto Rico, and across Culebra, St. Thomas, St. John and
St. Croix as well. This will increase the risk of rip currents to
high, while dangerous breaking waves may develop too.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...GRS
LONG TERM....ERG