484
FXCA62 TJSJ 220843
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
443 AM AST Fri Aug 22 2025

.KEY MESSAGES...

* A Heat Advisory is in effect for 58 municipalities across Puerto
  Rico today. There is limited heat risk extending to Vieques,
  Culebra, and the US Virgin Islands.

* A vigorous tropical wave (Invest 90L) will approach the region
  today, increasing moisture and the potential for showers and
  thunderstorm development across the islands.

* For Puerto Rico, periods of showers and thunderstorms may cause
  localized urban and small stream flooding, 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 later
  today into the weekend, producing hazardous marine conditions,
  dangerous rip currents, and the potential for localized beach
  erosion, especially along north- and northwest-facing beaches.

&&

.SHORT TERM...Today through Sunday...

Overnight conditions were generally fair and quiet. Doppler radar
and satellite imagery detected isolated to scattered showers and mid-
to upper-level clouds across the region. Overnight lows ranged from
8083F across northeastern Puerto Rico, Vieques, Culebra, and the
US Virgin Islands, while higher elevations in the Cordillera Central
cooled to the 60s. With the approach of a tropical wave (Invest
99L), the low-level wind flow shifted from the northeast, although
winds over land remained light to calm and variable.

This morning, a weak mid-level ridge, lingering Saharan dust, and
pockets of drier air will dominate. Attention then turns to Invest
99L, which has a high probability of tropical cyclone formation
according to the 2 AM AST Tropical Weather Outlook. A tropical
depression is likely to form as the system tracks northwest and then
north, well northeast of the local islands. No direct impacts are
expected for Puerto Rico or the US Virgin Islands. However, 99L is
expected to influence local weather through the weekend. By Sunday,
as the system lifts north, a mid-level ridge will strengthen,
introducing drier air and a trade wind inversion.

Today will begin mostly quiet with hazy skies and intervals of
sunshine. As moisture from 90L spreads across the region, shower
and thunderstorm activity will increase. The risk of excessive
rainfall and lightning will rise this morning across the US Virgin
Islands, then expand into Vieques, Culebra, and eastern Puerto
Rico early this afternoon, reaching much of the island later in
the day. With east-northeasterly steering, the most intense
afternoon storms are expected across southern and southwestern
Puerto Rico, where urban and small stream flooding is likely and
localized flash flooding is possible. These hazards will persist
into tonight and Saturday as abundant tropical moisture persists
across the region, while precipitable water values increase to
2.22.3 inches, well above normal for this time of year. The focus
of activity will shift from the USVI and eastern Puerto Rico
tonight and Saturday morning to the interior and northern parts of
Puerto Rico on Saturday afternoon as winds veer east-southeast.
By Saturday night into Sunday, as 99L moves away, generally fair,
warm, and hazy conditions are expected to return.

While rainfall and flooding remain the primary concern through
Saturday, excessive heat will continue to pose risks each day.
Despite expected rain and breezy northeasterly winds, a Heat
Advisory is in effect for 58 municipalities across Puerto Rico
today, primarily in urban and coastal areas, with a limited heat
risk extending to Vieques, Culebra, and the US Virgin Islands.
Lightning hazards will remain a daily threat. By Sunday, expect
stronger easterly trade winds and moderate Saharan dust, reducing
visibility and air quality.

.LONG TERM...Monday through Friday...

Weather conditions will start to improve next week, as moisture
associated with the tropical wave (Invest 90L) monitored by the
National Hurricane Center (NHC) continues to move north and away
from the region. The forecast continues on track with the
positioning of an upper-level trough north of the area by Monday,
however, the islands will not be under the most favorable side of
the trough. At the low levels, a drier air mass will filter into the
area along with a Saharan Air Layer (SAL) with low to moderate
concetrations of Saharan dust. Given the expected scenario, hazy
skies, reduced visibilities, poor air quality and limited showers
are anticipated through at least Tuesday. However, late afternoon
convective activity  with showers and few isolated thunderstorms are
possible across the western municipalities of Puerto Rico each
afternoon.

By mid-week, winds will shift from the south southeast in response
to a strong surface high pressure in the central Atlantic and a long
wave polar trough exiting the eastern coast of the United States.
This will lift moisture associated with a tropical wave (Invest
99L) from the Caribbean Sea. Under this pattern, an increase in
showers are anticipated across Puerto Rico and the U.S. Virgin
Islands through the end of the workweek. The 925 mb temperatures
will remain above-normal climatological values for most of the
period. Highs will continue to reach the upper 80s to low 90s each
day along the coastal and urban areas of the islands. Heat
indices will rise up to the mid to upper 100s, due to the
available moisture and southerly winds. Residents and visitors are
advised to monitor the weather updates and plan accordingly if
having outdoor activities under sun exposure.

&&

.AVIATION...
(06Z TAFs)

VFR conditions will prevail until SHRA/TSRA associated with a
tropical wave develops, briefly reducing conditions to MVFR or IFR.
These impacts are most likely after 22/14Z across USVI terminals,
after 22/16Z at TJSJ, after 22/18Z at TJPS, and after 22/22Z at
TJBQ. Light and variable winds will increase to 1015 kt with
occasional higher gusts after 22/13Z, then diminish to 812 kt after
22/22Z.

&&

.MARINE...

East-northeast to northeast winds will prevail across the islands
today. A strong tropical wave is expected to approach the northeastern
Caribbean today, increasing showers and thunderstorm activity across
the region through early Sunday. A long period northerly swell from
distant Hurricane Erin will arrive as early as this afternoon, resulting
in hazardous seas for the Atlantic waters and passages through early
next week.

&&

.BEACH FORECAST...

A high rip current risk is in effect for the north and east facing
beaches of Puerto Rico through at least Saturday night. However,
as the weekend progresses other beaches of Puerto Rico and the
US Virgin Islands will move from a moderate to a high risk due to
the arrival of a northerly swell associated with distant
Hurricane Erin located over the northern Atlantic. Large breaking
waves of 7 to 10 feet are expected from Saturday into early next
week, resulting in dangerous swimming conditions, life threatening
rip currents, and the potential for localized beach erosion.

Thunderstorms may develop this 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 (Invest
90L) approaches the area from today into late Saturday.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Saturday afternoon for PRZ001-002-
     005-008.

     Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010-011.

VI...None.
AM...None.
&&

$$

ICP/GRS