693
FXCA62 TJSJ 211909
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
309 PM AST Thu Aug 21 2025

.KEY MESSAGES...

* The leading edge of a vigorous tropical wave (AL90) will start
  to approach the region late Friday into Saturday and is likely
  to lift north-northeast of the islands this weekend, with some
  potential to develop into a low-pressure system or tropical
  depression. Regardless of development, unsettled weather is
  expected.

* For Puerto Rico, periods of showers and thunderstorms may cause
  localized urban and small stream flooding. Flash flooding and
  localized landslides in steep terrain cannot be ruled out,
  particularly across northwestern, north central, and eastern
  Puerto Rico.

* For the U.S. Virgin Islands, passing showers and thunderstorms may
  result in ponding of water on roads and isolated urban flooding.

* A northerly swell from distant Hurricane Erin will arrive late
  Friday into the weekend, producing hazardous marine conditions,
  dangerous rip currents, and the potential for localized beach
  erosion, especially along north- and northwest-facing beaches.
  These conditions could be extended into early next week.

&&

.SHORT TERM...Tonight through Saturday...

Hot and hazy conditions prevailed today, with generally light
shower activity limited to downwind of the USVI (and around St.
Croix), Vieques and Culebra under east-northeast steering wind
flow. Highs were in the low to mid 80s at higher elevations and up
to the low to mid 90s in lower elevations. A Heat Advisory
remains in effect for coastal and urban areas of Puerto Rico, as
well as for Vieques, Culebra, St. Thomas, St. John and St. Croix,
until 5 PM. Heat indices across several of these sectors have
reached above 105 degrees, with sectors of northern, western and
southwestern PR reporting values around 110 degrees. Shower
activity remains posible over interior to WSW Puerto Rico. The
highest precipitable water (PWAT) values were observed over the
local offshore Caribbean Waters and around St. Croix at 1.6 to
1.75 in. A Saharan Air Layer (SAL) will continue to filter
moderate to locally high concetrations of Saharan dust into the
islands for the rest of today and tonight. Hazy skies, reduced
visibilities and deteriorated air quality.

A transition from a dry and stable pattern towards a much wetter
and humid pattern is expected on Friday due to the approach of a
vigorous tropical wave. This tropical wave (AL90) currently has a
medium (50%) formation chance in the next 48 hours and a high
(70%) formation chance in the next 7 days. Current model guidance
has PWAT values reaching 2 inches over the USVI and eastern PR
tomorrow morning, with afternoon convection possible over interior
to WSW PR, as well downwind of the local islands. Regardless of
if a Tropical Cyclone develops, an increase in shower and
thunderstorm activity is expected across the local islands as the
leading edge of the wave approaches Friday night into Saturday as
this tropical wave, currently a few miles east of the Leeward
Islands, moves north- northeast of Puerto Rico and the U.S. Virgin
Islands. PWAT values over the region are forecast to be at around
2 inches. Latest model guidance continues to suggest values
reaching up to 2.20 inches on Saturday, increasing the potential
to observe showers and thunderstorms across the region. By Friday
night to saturday morning, showers and isolated thunderstorms are
forecast across the USVI and eastern PR. Later in the afternoon,
the activity will be focused over the interior, northwestern, and
north-central areas of Puerto Rico by late morning to afternoon on
Saturday. The risk of flooding is elevated, particularly for
northwestern and eastern sections of Puerto Rico and the U.S.
Virgin Islands. Urban and small stream flooding, localized flash
flooding, and mudslides are possible. For the U.S. Virgin Islands,
passing showers and t-storms may result in ponding of water on
roads and isolated urban flooding. Residents should continue to
monitor the latest updates from WFO San Juan and the National
Hurricane Center (NHC) regarding AL90.

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

.PREV DISCUSSION... /issued 349 AM AST Thu Aug 21 2025/

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...
(18Z TAFs)

Fair weather conditions with hazy skies due to suspended Saharan
Dust will continue late today and tonight. Winds will prevail
from the ENE at 12-17 kt with gusts up to 25 kt and sea breeze
variations. Winds decreasing after 21/23z and increasing again
from the ENE after 22/13z at 12-18 kt with higher gusts and sea
breeze variations. An increase in SHRA/VCSH is possible for the
eastern terminals by that time as moisture from a tropical wave
starts to reach the islands.

&&

.MARINE...

Winds will be from the east tonight, shifting to the northeast on
Friday as the tropical wave (AL90) approaches the region, and
then turning southeast as the system tracks north-northeast of the
islands this weekend. The wave will also bring an increase in
showers and thunderstorms across the local waters from Friday
through the weekend.

A long-period northerly swell from distant Hurricane Erin will
arrive late Friday into the weekend, resulting in hazardous seas of
6 to 8 feet, with occasional seas up to 10 feet across the Atlantic
waters and passages.

&&

.BEACH FORECAST...

A moderate rip current risk will persist through Friday across the
islands. Thunderstorms may develop Friday afternoon along
southwestern Puerto Rico, producing dangerous lightning strikes; be
prepared to seek shelter if thunder is heard. The frequency of
showers and thunderstorms will increase as a vigorous tropical wave
(AL90) approaches the area late Friday night into Saturday.

By the weekend, beach conditions will deteriorate significantly as
northerly swells from distant Hurricane Erin reach the beaches of
northern and northwestern Puerto Rico, as well as Culebra, Vieques,
and the U.S. Virgin Islands. Large breaking waves of 7 to 10 feet
are expected from Saturday into early next week, resulting in
dangerous swimming conditions, a high risk of rip currents, and the
potential for localized beach erosion.

&&



.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

CVB/MRR