763
FXCA62 TJSJ 181848
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
248 PM AST Wed Jun 18 2025

.KEY MESSAGES...

*Warmer-than-normal low temperatures will persist through at
 least the end of the week.

*Expect breezy to windy easterly winds creating choppy seas and
 moderate risk of rip currents.

*Afternoon convection will develop each day due to diurnal
 heating, orographic effects, and sea breeze.

*Hazy skies will continue through tomorrow, slowly decreasing as
 the concentrations of Saharan Dust particles diminish.

&&

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

Drier-than-normal conditions with haze prevailed throughout the
morning across the islands. Little to no rain activity was
observed, thanks to the Doppler Radar. Maximum temperatures were
in the low to mid-90s across urban and coastal areas, with heat
indices around 107 degrees Fahrenheit. Winds were from the east at
15 to 20 mph, with gusts around 30 mph under the influence of a
sea breeze.

Afternoon convection will be mainly confined to the western
portion of Puerto Rico, with some showers affecting the eastern
site as well as the US Virgin Islands due to streamers that may
form downwind from El Yunque and the islands. Meanwhile, the rest
of the PR and the USVI will generally observe hazy skies with
little to no rain. Once again, the presence of a Saharan Air Layer
will limit the nighttime cooling, promoting warmer-than-normal
minimum temperatures overnight and early tomorrow morning.

A mid-level ridge enhancing the trade wind cap and creating
subsidence aloft will limit the vertical development of any
convection that could form in the next few days. However, a
tropical wave is expected to increase the available humidity
somewhat from Thursday afternoon through Friday, with values
slightly above normal climatological levels. Despite this weak
tropical wave, model guidance suggests that most of the available
moisture is trapped below 850 mb. However, this will lead to an
increase in the frequency and intensity of showers and afternoon
convection from late Thursday night through Friday.

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

The long-term forecast remains on track, with variable conditions
expected to persist this weekend and into the first part of the
workweek. Precipitable Water (PWAT) values are still expected to
gradually decrease to seasonal and near below normal (1.4 - 1.6
inches) this weekend, as a drier air mass in the mid-levels filters
into the region by Saturday. Additionally, the presence of a mid-
level ridge over the CWA and warmer than climatological normal 500
mb temperatures (between -5.8 and -6.0 Celsius) are some of the main
factors that may inhibit deep convection activity. Nevertheless, the
combination of these factors with the available moisture content,
diurnal heating, and local effects is likely to promote shallow
convection in the afternoon, mainly over western/northwestern Puerto
Rico. Rainfall accumulations may enhance the ponding of water in
roadways and poorly drained areas over localized areas under the
aforementioned areas.

From the latest model cycles, global models have continuously
suggested an increase of abundant moisture due to the approach of
a tropical wave. From the latest guidance, both ECMWF and GFS
ensemble members are tending for wetter conditions by Monday
afternoon into early Tuesday, with above-normal PWAT values (1.8 -
2.0 inches). Besides the tropical wave, the mid-level moisture
content will also increase by that period, cooling 500 mb
temperatures (around -7.5 Celsius), which enhances instability and
supports the development of thunderstorms. At the moment, the
most likely scenario is frequent showers and thunderstorms moving
over the local area during Monday evening into early Thursday.
This rainfall activity could elevate the flooding risk mainly over
western and eastern portions of Puerto Rico. Other areas such as
the U.S. Virgin Islands cannot be ruled out. Since this is
expected by days 6 and 7 of the forecast period, uncertainty
remains high. We`ll keep monitoring the next model cycles and
possible outcomes regarding this tropical wave.

For the rest of the period, PWAT values will become seasonal, so
the afternoon convection in the afternoons will be likely once
again over western/northwestern Puerto Rico. A wind surge observed
at the 700 mb layer that may be associated with another suspended
Saharan Air Layer (SAL) will promote once again breezy to locally
windy conditions across the CWA.


&&

.AVIATION...

Expect mostly VFR conditions at all TAF sites during the period,
although hazy skies are expected to persist due to the SAL. VCTS
will develop near JBQ, and some SHRA/+SHRA will impact the eastern
locations of PR and the USVI later this evening or tomorrow
afternoon. Expect winds from the E-ESE 15-20 knots with local sea
breeze accompanied by higher gusts up to 35 kt. Winds are
expected to fluctuate around 11 kt after 18/23Z and return to
breezy winds after 19/13Z tomorrow.

&&

.MARINE...

A persistent subtropical Atlantic ridge extending southward into the
northern Caribbean will continue promoting moderate to locally fresh
easterly winds, resulting in choppy seas, particularly over exposed
areas. Therefore, small craft should exercise caution. A plume of
suspended Saharan dust is expected to linger through the workweek,
reducing visibility. A tropical wave will move across the Caribbean
waters on Thursday and Friday, likely leading to an increase in
thunderstorm activity across that region.

&&

.BEACH FORECAST...

A moderate risk of rip currents will be promoted by breezy to
locally windy easterly winds for most beaches in Puerto Rico and
the US Virgin Islands. Beachgoers must exercise caution due to
life-threatening rip currents along exposed unprotected beaches.

Additionally, please be advised of the possibility of seeing
afternoon thunderstorms, particularly over western and
northwestern Puerto Rico.

&&

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

$$

MORNING CREW...CAM/LIS
AFTERNOON CREW....ICP/YZR