774
FXCA62 TJSJ 181806
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
206 PM AST Mon Aug 18 2025

.KEY MESSAGES...

* Showers and isolated thunderstorms from a lingering outer
  rainband from Erin will continue to affect mainly PR and the
  local waters through at least tonight.

* Across the USVI, mostly fair weather conditions are expected for
  most of the week.

* Hazy skies and reduced visibilities are expected through midweek
  due to the arrival of a Saharan Air Layer.

* A tropical wave in the eastern tropical Atlantic with a 60%
  chance of development into a tropical system is expected to
  bring an increase in thunderstorm activity from Friday through
  Saturday.


&&

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

For the rest of today, an outer rainband from Erin will continue
to produce showers and isolated thunderstorms, favoring orographic
lift along the mountain ranges of PR, and across the western
waters of PR. Minor urban and river flooding was observed earlier
in portions of Yauco and Guanica. Rainfall accumulation since
midnight were between 1 and 4 inches along the interior and
southern portions of PR. The higher accumulations were between
Maricao and Orocovis. Runoff from this activity could still cause
streams to increase or remain elevated through this evening.
Therefore, a Flash Flood Watch remains in effect until at least 8
PM AST tonight.

A Saharan Air Layer with moderate to high concentrations of
Saharan dust will gradually filter over the region from Tuesday
through Thursday. This will promote hazy skies, reduced
visibilities, and poor air quality across Puerto Rico and the U.S.
Virgin Islands. Also, shower activity will be limited to localized
afternoon showers over portions of the interior and western PR
each afternoon. Across the USVI and windward areas of PR, brief
passing showers are possible from time to time.

In addition, for Tuesday and Wednesday, the combination of
southeasterly winds with enough shallow moisture content will
cause hot temperatures in general across the islands, and Heat
Advisory conditions are possible for the lower elevations and
urban areas of Puerto Rico. Particularly across the northern and
western sections of the Island.


.LONG TERM...Thursday through Monday...

Variable conditions are still expected over the long-term forecast.
Southeasterly winds are expected to gradually increase from Thursday
onwards as a surface high pressure over the Central Atlantic
migrates westward and lingers northeast of the CWA. PWAT values are
expected to drop to below and seasonal climatological normals (1.3 -
1.5 inches, with the 75th percentile reaching 1.75 inches). From the
latest GEOS-5/GMAO Dust Extinction product, pulses of a Saharan Dust
Layer (SAL) are likely to move over the region through Friday.
Although an upper-level low moving north of the islands is likely to
cool mid-level temperatures (around -7.5 degrees Celsius), deep
convection activity should be limited.

The National Hurricane Center continues monitoring a tropical wave
with a 50% chance of formation in the next seven days. From the
latest Tropical Weather Outlook (TWO), environmental conditions
are looking favorable for the gradual development of the system,
likely to become a tropical depression by the end of the week. As
mentioned in previous discussions, global models show this system
approaching the Caribbean Basin this weekend, although the time of
arrival and trajectory continue to differ. Based on the latest
deterministic guidance, the GFS suggests the system could arrive
by Friday night, while the ECMWF solution shows it approaching
the local area early Saturday. Nevertheless, ensemble models are
now tending to higher PWAT values on Saturday and Sunday (2.1 -
2.3 inches), increasing confidence. Although uncertainty remains
high, we could expect wet conditions by the upcoming weekend.
Resident and visitors should continue to monitor the development
of this system and stay tuned to the latest updates from the NHC.
By Monday, weather conditions should gradually improve across the
islands as a drier air mass may filter into the region.

Under a southeasterly wind flow, warmer temperatures, and available
moisture, heat indexes are likely to exceed 100 degrees Fahrenheit
and even meet Heat Advisory Criteria by the end of the workweek. Due
to the uncertainty regarding the weather conditions of the upcoming
weekend, the heat risk may undergo changes.


&&

.AVIATION...

Mainly VFR conditions expected in general across most terminals
during the next 24 hours. However, tempo MVFR conditions are
possible through 18/23z across the PR terminals due a lingering
rainband from Erin. Southerly winds will prevail btw 10-20 kt,
turning more ESE on Tuesday.


&&

.MARINE...

Swells from Erin will continue to diminish through tonight, and
Small Craft Advisory conditions will no longer be expected for the
rest of the workweek. Moderate to fresh southerly winds will
continue through at least tonight, becoming light to moderate and
from the east to southeast through midweek. A strong tropical
wave is expected to enter the eastern Caribbean on Friday,
increasing thunderstorm activity in general. This wave has a
medium chance 60% of developing into a tropical system by the end
of the week.


&&

.BEACH FORECAST...

The high risk of rip current will persist along the northern coastal
areas of Puerto Rico including the northwestern and northeastern PR.
Improving coastal conditions will be present tonight as the risk
become moderate for the same areas and a low risk for southern
coastal areas, Vieques and St. Croix. Similar coastal conditions
with breaking waves between 3 to 5 feet are forecast until the
latest part of this week when coastal conditions will deteriorate
and the risk will increase again.

For the rest of the day, residents and visitors are urge to stay out
of the northern coastal areas. For more detailed information please
visit www.weather.gov/sju/marine and the Coastal Hazard Message
product (SJUSCFWSJU).


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Flood Watch until 8 PM AST this evening for PRZ001>013.

     High Rip Current Risk until 6 PM AST this evening for PRZ001-002-
     005-008-010.

VI...Flood Watch until 8 PM AST this evening for VIZ001-002.

AM...Small Craft Advisory until midnight AST tonight for AMZ711.

     Small Craft Advisory until 6 PM AST this evening for AMZ712-741-
     742-745.

&&

$$

SHORT TERM...DSR
LONG TERM....MNG
AVIATION...DSR
MARINE...DSR
BEACH...LIS