574
FXCA62 TJSJ 160906
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
506 AM AST Wed Jul 16 2025

.KEY MESSAGES...

* A tropical wave will approach the islands today, increasing
  moisture and the potential to observe showers and thunderstorms.

* If prolonged period of heavy rain do not happen across the lower
  elevations and urban areas of Puerto Rico and the US Virgin
  Islands during the peak heating hours, we can observe an
  elevated heat risk. Stay tuned for any forecast update.

* Moderate to high concentrations of Saharan dust particles will
  promote hazy skies, reduced visibilities and poor air quality
  across Puerto Rico and the U.S. Virgin Islands tomorrow and
  Friday.

* Increasing winds will lead to choppy seas and moderate risk of
  rip currents.

&&

.SHORT TERM...Today through Friday...

Overnight minimum temperatures ranged from the low 60s in the
mountains to the upper 70s or low 80s in coastal urban areas. The
Doppler Radar detected scattered to numerous showers moving inland
over the USVI and the windward locations of PR overnight. Winds were
mainly from the east at 10 to 15 mph, with locally higher gusts near
showers. This activity is associated with the leading edge of
today`s tropical wave.

Showers will increase by mid-morning and continue into the afternoon
as the tropical wave approaches the islands. While the risk of rain
is elevated for Puerto Rico and slight for the US Virgin Islands, we
cannot dismiss the possibility of urban flooding in the Virgin
Islands due to periods of moderate to heavy rainfall associated with
this tropical wave, especially during the afternoon and evening. The
subsidence and dry air aloft associated with the presence of a mid-
to upper-level high pressure is hurting the potential to observe
scattered to widespread thunderstorms. However, for this reason, we
only include isolated thunderstorms in the forecast.

Additionally, the available moisture, combined with intense diurnal
heating, poses a risk to the most vulnerable communities, especially
if the regions experience rain activity that is not prolonged enough
to mitigate this heating. We have decided to hold off on issuing the
Heat Advisory until mid-morning (around the peak heating period
between 10 AM and 2 PM AST) to assess whether the expected rainfall
will have a cooling effect on temperatures. If rainfall sufficiently
cools the area, the advisory may not be necessary. However, if the
anticipated showers do not develop, dangerously high heat indices
could still occur. Therefore, please stay tuned for updates from our
office, should they become necessary.

Behind today`s wave, we noticed another Saharan Air Layer, which
will bring another round of hazy skies and deteriorating air
quality, due to suspended dust particles, from Thursday into Friday.
However, the lingering moisture will result in the typical weather
pattern observed during the summer, characterized by passing showers
across the windward locations, followed by afternoon convection
across the western portions of PR and downwind from the US Virgin
Islands.

Seasonal to above-anormal temperatures and high moisture content
will keep an elevated to high risk of heat for the rest of the short
term, and it`s very likely to meet Heat Advisory or even Extreme
Heat Warning Criteria.

.LONG TERM...Saturday through Wednesday...

During the long-term forecast period, the latest model guidance
suggests that precipitable water content will generally range
from near seasonal values to below average, with the exception of
Sunday, which is showing above normal values of up to 1.9 inches.
Generally, a TUTT-low and an induced low-level trough are expected
to move across the northeastern Caribbean through the weekend,
bringing an increase in shower activity. A disturbance is also
expected to move across the region from Sunday into early next
week. In general, these weather features will enhance early
morning convection between the USVI and eastern sections of Puerto
Rico, followed by afternoon thunderstorms over portions of the
interior and western Puerto Rico. Early next week through
Wednesday, a deep layer ridge building from the central Atlantic
into the northeastern Caribbean will shift further west and
displace the trough pattern. Mid-level moisture is expected to
decrease, and rainfall will likely follow a more typical diurnal
pattern driven by daytime heating and local effects. At this time,
the flood risk during these days is expected to range from none
to limited. Additionally, warm to hot temperatures are anticipated
each day, likely triggering Heat Advisory conditions across most
coastal and urban areas of Puerto Rico. Residents and visitors are
encouraged to stay hydrated, avoid outdoor activity during peak
heat hours, wear light clothing, and check on vulnerable
individuals and pets.

&&

.AVIATION...
(06Z TAFs)

A tropical wave will bring occasional SHRA/+SHRA across the local
terminals. This activity could result in brief MVFR or even IFR
conditions, especially between 16/14 and 18z near IST/ISX/JSJ, and
between 16/16 and 23z at JSJ/JPS/JBQ. TSRA may also develop along
the Cordillera Central into the W/SW part of PR. Rain activity will
return once again overnight across E-PR/USVI after 16/23z. Expect
winds between 10-15 kt through 16/13z, then E/ENE winds 15-20 kt
with higher gusts near 25-30kt and sea breeze variations.

&&

.MARINE...

A tropical wave will bring increasing winds, showers and isolated
thunderstorms from today into Thursday morning. Moderate to fresh
easterly winds and choppy seas are expected across the regional
waters at least through Thursday. A Saharan Air Layer moving
behind the tropical wave will promote hazy skies on Thursday and
Friday. Another disturbance is anticipated at the end of the
weekend potentially increasing showers activity across the waters.

&&

.BEACH FORECAST...

A moderate risk of life-threatening rip currents will likely persist
today into the end of the week across the northern, eastern, and
southeastern beaches of Puerto Rico, Vieques, Culebra, and the USVI.
A low risk is expected elsewhere; however, life-threatening rip
currents may still occur, especially near groins, jetties, reefs,
and piers. Always swim near a lifeguard and follow the advice of
local beach patrols and flag warning systems.

&&

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

$$

SHORT TERM/AVIATION...CAM
LONG TERM/MARINE/BEACH FORECAST...YZR