993
FXCA62 TJSJ 161918
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
318 PM AST Wed Jul 16 2025

.KEY MESSAGES...

* Showers and isolated thunderstorms will continue across the
  region through this evening due to a tropical wave. A Flood
  Advisory is in effect for Cabo Rojo and Lajas until 5:45 PM AST
  today.

* Warmer temperatures and abundant moisture will elevate the heat
  risk for the next few days, with heat indexes likely to meet
  Heat Advisory Criteria.

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

* Increasing winds will lead to choppy seas and a moderate risk of
  rip currents through Thursday night.

&&

.SHORT TERM...Tonight through Friday...

The main feature affecting the islands today, a tropical wave,
alongside diurnal heating and local effects will continue to
promote shower activity over the region, with thunderstorms
currently concentrating over the southwestern quadrant of PR.
Radar estimated rainfall accumulations (as of 3 PM AST) detect 1
to 2 inches over south central to southwestern municipalities.
Most of the region detected at least minimal rainfall estimated
accumulations, with the USVI having up to 0.15 to 0.25 inches.
During the late morning hours, before the rain and cloudiness
became more widespread heat indices around 108 were detected over
western coastal sectors, particularly over the SW quadrant which
then saw relief (in terms of heat) with the above mentioned rain.
High Heat indices around 105 degrees still persisted in areas
without stronger shower activity.

Mid to upper level high pressure continues to affect the region,
limiting stronger shower and t-storm development. However with
the tropical wave, showers will continue to be steered towards
windward sectors of the islands tonight, with current convection
over PR eventually dissipating and/or moving offshore during the
evening. GOES-East detects a large plume of Saharan Dust (with
moderate to high concentrations) behind the tropical wave
currently affecting us. This will result in hazy skies, reduced
visibilities and deteriorated air quality on Thursday and Friday,
with lingering lower concentrations during the weekend (long term
period). However, lingering moisture from the tropical wave will
persist on Thursday, with precipitable water (PWAT) values
reaching 1.8 to 1.9 inches. The seasonal pattern will continue
during the period, although with this increased moisture on
Thursday and with an approaching upper low on Friday. ENE steering
flow will veer to become more southeasterly steering flow by
tomorrow (still breezy) and then back to become easterly to
possibly northeasterly on Friday. Showers will be steered towards
windward locations, followed by afternoon convection (showers and
possibly isolated thunderstorms) over sectors of western Puerto
Rico and downwind from El Yunque and the US Virgin Islands.

A risk to observe ponding of water in roads and poorly drainage
areas, as well as localized flooding in urban areas, roads and
small streams, will continue this evening and tonight and during
the afternoons (mainly over W PR) during the rest of the period.
925 mb temperatures will remain at or above normal, with highs in
the upper 80s to low 90s along the coastal and urban areas with
heat indices above 100 degrees each day (Heat Advisories will
probably be issued). Lows generally in the low 60s over the
interior to mountains to the upper 70s or low 80s over coastal and
urban areas.

.LONG TERM...Saturday through Wednesday...
./FROM PREV DISCUSSION...Issued at 506 AM AST Wed Jul 16 2025/

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...

(18z TAFs)

Mostly VFR conds across all terminals. SHRA moving towards the
islands can promote brief MVFR conditions, especially over TJPS
late in the afternoon (also due to TSRA). Winds from the NE
between 15 to 10 kts with higher gusts through 16/23z, later
gradually decreasing and veering to become more ESE tomorrow. HZ
is forecast for tomorrow, reducing visibilities over the
terminals.

&&

.MARINE...

Showers and isolated thunderstorms due to a tropical wave will
continue moving over the local waters and passages through
Thursday morning. Moderate to fresh easterly winds will promote
choppy seas across the regional waters through at least Thursday
night; small craft should exercise caution. A Saharan Air Layer
moving behind the tropical wave will promote hazy skies and reduce
visibility on Thursday and Friday. Another disturbance is
anticipated at the end of the weekend, potentially increasing
shower activity across the waters.

&&

.BEACH FORECAST...

The moderate risk of rip currents will continue for northern and
eastern beaches of Puerto Rico, Vieques, Culebra, and St. Croix
through Thursday. Beach conditions will gradually improve Friday
into the weekend, as the rip current risk reduces over some areas
except the north central beaches of Puerto Rico and St. Croix.
Winds will increase again by Monday, elevating the risk of rip
currents for most beaches of Puerto Rico and the U.S. Virgin
Islands. Beachgoers should exercise caution when visiting beaches
under moderate risk due to possible life-threatening rip currents
over the surf zone.

&&

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

$$

MRR/MNG