662
FXCA62 TJSJ 161801
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
201 PM AST Fri May 16 2025

.KEY MESSAGES...

* A strong upper-level trough will bring an unsettled weather
  pattern, increasing the risk of excessive rainfall impacts across
  Puerto Rico and the U.S. Virgin Islands through Saturday.

* Saharan dust will continue to filter across the region, resulting
  in hazy skies through at least early next week.

* Diurnally induced afternoon showers and thunderstorms are expected
  to develop each day next week across central Puerto Rico.


&&

.SHORT TERM...Today through Sunday...

A deep layered trough west/northwest of the area will continue to
move eastward and over the local area through the rest of the short-
term period. A cut-off low at the upper levels is forecast to
develop and sink further southward just north of Puerto Rico by
Saturday afternoon. This will promote unstable and favorable
conditions for shower and thunderstorm development across portions
of the islands and the local waters. A broad surface high pressure
over the central Atlantic will continue to support moderate to
locally fresh ESE winds through at least this evening. However,
lighter winds with a southeasterly component will prevail during the
weekend as an induced surface trough develops over the northeastern
Caribbean. In terms of moisture content, the precipitable water
content is still expected to range between 1.80-2.00 inches between
this evening and Saturday afternoon. However, a weak Saharan Air
Layer will continue to bring minor concentrations of Saharan dust
into the weekend and hazy conditions will be present across the USVI
and PR. Having said that, we are still expecting thunderstorms with
periods of locally heavy rainfall across the islands during the next
24 hours or so. Therefore, the main weather hazards continue to be
the excessive rainfall that can lead to urban and small stream
flooding, as well as localized flash flooding and mudslides in areas
of steep terrain. On Saturday, due to the proximity of the cut-off
low, the thunderstorms can produce frequent lightning and strong
gusty winds.

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

...From Prev Discussion Issued at 515 AM AST...

Recent model guidance shows no significant changes. A dynamic
interplay between upper-level and surface features will continue
to drive periods of marginal instability across the northeastern
Caribbean. A Tropical Upper Tropospheric Trough (TUTT) Low will
keep lifting northeastward and will likely exit the region by
Monday. A second short-wave upper-level trough will follow,
settling into the region starting Tuesday. This second feature
will help sustain the developing mid-level ridge over the western
Caribbean, reinforcing a marginally unstable weather pattern
across the local area.

At the surface, a weakening induced trough will continue to
disrupt the trade wind flow, keeping winds lighter and more
variable early in the week. A broader surface high over the
central North Atlantic will help trap tropical moisture across the
eastern Caribbean, while a weaker high lifting northeastward over
the western Atlantic will help push most of that moisture south
of the local islands. As this high dissipates, east-southeasterly
winds will likely strengthen and become more established by
midweek, continuing through the latter part of the week. This
pattern shift will not only support a gradual increase in tropical
moisture but may also lead to warmer-than-normal conditions.

The combination of moisture and marginal instability will continue
to favor shower and thunderstorm development, especially in the
afternoons, due to daytime heating and local effects. Starting
Wednesday, conditions will likely become more favorable for deeper
thunderstorm activity as 500 mb temperatures drop to near-normal
levels for this time of year, below -6 C, through the second half
of the week. Light winds, particularly through Tuesday, will
limit storm movement and ventilation, allowing showers and storms
to linger longer over the same areas, increasing the potential for
locally heavy rainfall and flooding.

Meanwhile, Saharan dust will continue over the region, with
concentrations likely peaking on Monday. This could temporarily
suppress rainfall and lower air quality, especially across the
Virgin Islands and eastern Puerto Rico.


&&

.AVIATION...

(18Z TAFs)

SHRA/TSRA is expected to increase through the period. Therefore,
tempo MVFR to brief IFR conditions are possible at times across all
terminals through Saturday. Minor concentrations of Saharan dust
will continue to filter across the local area, but VSBY will remain
P6SM, except in areas of SHRA/TSRA. The 16/12z TJSJ sounding
indicated ESE winds up to 23 kt blo FL030.


&&

.MARINE...

A broad surface high pressure over the central Atlantic will continue
to promote moderate to locally fresh easterly winds through tonight.
As a result, small craft should continue to exercise caution. Over
the weekend and into early next week, a surface trough will gradually
weaken these winds. At the same time, a deep trough to the west of
the region will support an unsettled weather pattern, with a high
chance of thunderstorm development through at least Saturday. These
storms could lead to locally higher winds and seas. In addition, hazy
conditions are likely over the next few days due to the presence of
Saharan dust.


&&

.BEACH FORECAST...

Life-threatening rip currents are possible along most northern and
east-southeast exposed beaches of the islands today and tonight.
The risk will likely diminish by Saturday as winds ease gradually.
However, moderate rip current risk will likely return for Saint
Croix early next week. For more information, please refer to the
latest Coastal Hazard Message (CFWSJU) and Surf Zone Forecast
(SRFSJU).

Shower and thunderstorm activity will likely increase into the
weekend as the region transitions back to a wet and unstable
weather pattern. Reminder: Seek shelter and leave the water or
beach immediately at the first sign of thunder or lightning.


&&

.HYDROLOGY...

Abundant moisture and increasing instability associated with an
approaching upper-level trough will heighten the risk of flooding
across Puerto Rico and the U.S. Virgin Islands through Saturday.
With saturated soils and elevated river levels in some areas, any
additional heavy rainfall could lead to urban and flash flooding,
river flooding, and landslides, even after the heaviest rain has
ended. For more details, please refer to the Hydrologic Outlook
(ESFSJU).


&&

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

$$


DSR/MNG