689
FXCA62 TJSJ 211855
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
255 PM AST Mon Jul 21 2025

.KEY MESSAGES...

* An increase in showers is forecast from this evening into the
  early morning hours of tomorrow, mainly affecting eastern and
  northeastern Puerto Rico and the U.S. Virgin Islands.

* An increase in winds is forecast for Wednesday, resulting in
  wind- driven seas across offshore Atlantic and Caribbean waters,
  as well as the local passages.

* The bulk of the moisture associated with the tropical wave being
  monitored by the National Hurricane Center will stay well to our
  south over the Caribbean Waters.

 * Another tropical wave with a better shape and plenty of
   moisture will move into the islands late Sunday into Monday,
   increasing the potential for widespread shower activity.

&&

.SHORT TERM...Tonight through Wednesday....

A relatively calm and stable weather pattern prevailed during the
morning hours as a slightly drier airmass moved into the region
with Precipitable Water (PWAT) of 1.3 inches. Therefore, residents
experienced mostly clear to partly cloudy skies. At noon, some
brief passing showers were observed by the Doppler Radar near St.
Thomas. Daytime temperatures were in the lower 90s along the
southern coastal plains and eastern sections and in the high 80s
across the rest of the coast and urban areas. Heat indices ranged
between 100 and 105 degrees along the north-central coastal and
some spots across southern coastal areas. Winds remained from the
east-southeast up to 15 mph with gusty winds up to 25 mph.

From this evening into the early hours of tomorrow, an easterly
perturbation with PWAT of 1.83 inches is expected, as shown by the
GOES satellite-derived imagery. Therefore, the forecast calls for
an increase in showers along most of eastern Puerto Rico and the
U.S. Virgin Islands, moving eastward during the rest of the night,
affecting the local waters as well. A pattern change is expected
early Tuesday morning when moisture retreats from the region and a
slightly drier air mass with Saharan dust filters in. According to
the latest run of the NASA Goddard Earth Observing System Model V5
at 12Z today, hazy skies are expected as early as 6 AM across the
Virgin Islands, then shifting towards Puerto Rico. Concentrations
are expected to be light to moderate, sufficient to result in a
day of hazy skies and causing respiratory problems for vulnerable
communities. A similar pattern is expected for Wednesday during
the day with the presence of dust and some moisture in the region.
This combination would result in warm conditions with an elevated
risk of observing heat indices affecting coastal and urban
sectors of the island.

.LONG TERM...Thursday through Monday... from previous
discussion...

At the beginning of the long-term period, breezy to locally windy
conditions will persist as a subtropical surface high-pressure
system over the Atlantic Ocean maintains a tightened pressure
gradient. Aloft, the mid-level high-pressure system will shift to
the western Atlantic. Moderate concentrations of Saharan dust
particles are expected to remain in the area, resulting in hazy
skies and reduced air quality, particularly affecting individuals
with sensitive health conditions.

The latest model guidance continues to indicate a transition to a
wetter pattern beginning Thursday afternoon or Friday, due to an
approaching tropical wave. This tropical wave, designated Invest
94L, is currently located about 900 miles east-southeast of the
Lesser Antilles. It is being monitored by the National Hurricane
Center and has a low chance, 20 percent, of development over both
the next 2 and 7 days. While environmental conditions are generally
favorable for some development in the short term, they are expected
to become unfavorable for further development by midweek. Regardless
of development, if the guidance holds, we can expect an increase in
the potential for showers and thunderstorms, starting over the
Virgin Islands and spreading into Puerto Rico as early as Thursday
or early Friday. Flood risk are expected to be elevated, keep in
mind that flooding in urban areas, roads, small streams, and washes
are possible and possible isolated flash floods. Residents and
visitors are encouraged to stay informed through official sources as
we continue to monitor and update the forecast.

Trailing moisture will likely maintain a typical summer pattern
across the U.S. Virgin Islands and Puerto Rico over the weekend.
Beyond that, models are indicating the approach of another tropical
wave, currently located near 29 degrees west longitude and south of
17 degrees north latitude, which could increase the potential for
showers and thunderstorms early next week. Stay tuned over the
coming days for forecast updates and adjustments, as confidence
remains low at this time. Meanwhile, warm to hot temperatures will
persist. Residents and visitors are advised to stay hydrated, limit
outdoor activities during peak heat hours, wear light clothing, and
check on vulnerable individuals and pets.

&&

.AVIATION...
(18Z)

Mostly VFR conditions will persist for the rest of the
afternoon hours across all TAF sites. Briefly MVFR conditions are
possible from 2206-2210Z across TIST and TJSJ due to an increase
in SHRA and winds, resulting in lowers VIS and lower ceilings.
Winds will continue from the E-SE, becoming more variables as the
SFC feature passes the region. Winds will peak tomorrow at 22/15Z
up to 15 knots with gusty winds.

&&

.MARINE...

A surface high pressure over the Atlantic will mainly promote
moderate easterly winds over several days, becoming moderate to fresh
again around Tuesday next week. A weak easterly perturbation will
move across the US Virgin Islands and Puerto Rico around Tuesday,
followed by a more robust tropical wave after midweek. Despite the
expected systems expected this week, afternoon convective activity
will promote showers and thunderstorms each day over portions of
western Puerto Rico and surrounding waters.

&&

.BEACH FORECAST...

The risk of rip current will become moderate tonight across
northern coastal areas from Aguadilla to Carolina, and will
persist moderate for Culebra and St. Croix. The increase in winds
from Tuesday into Wednesday will turn moderate the rip current
risk across eastern side of Puerto Rico, spreading to the rest of
the areas during the week. Resident and visitors are urge to
exercise caution along the not designated swimmers coastal areas.

&&

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

$$

SHORT TERM & AVIATION...Morning Crew
LONG TERM....Midnight crew