572
FXCA62 TJSJ 170852
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
452 AM AST Sat May 17 2025

.KEY MESSAGES...

* A strong upper-level trough will maintain an unsettled weather
  pattern, heightening the risk of excessive rainfall across
  Puerto Rico and the U.S. Virgin Islands today and possibly tomorrow.

* Saharan dust will continue to move across the region, resulting
  in hazy skies through at least next Tuesday.

* Afternoon showers and thunderstorms, driven by local effects and
  daytime heating, are expected to develop daily next week across
  the interior and western Puerto Rico.

* Decreasing winds will promote low rip current risk across all
  coastal areas of the islands throughout the weekend.

&&

.SHORT TERM...Today through Monday...

Radar and satellite analysis indicated mostly quiet weather across
Puerto Rico overnight, with areas of fog developing over the
mountainous interior. Since 8 PM AST, scattered showers and isolated
thunderstorms affected eastern Puerto Ricoincluding Vieques and
Culebraas well as the U.S. Virgin Islands, resulting in increased
cloud cover and peak radar-estimated rainfall totals of 1 to 2
inches in Fajardo and Saint John. Winds were generally light, with
measurable speeds below 10 mph mainly observed across windward
coastal areas. Minimum temperatures dropped into the lower 60s in
the higher elevations, while exposed coastal areas of southern and
eastern Puerto Rico, along with the local islands, remained in the
upper 70s.

A mid-to-upper-level cutoff low and its associated surface-induced
trough will remain the dominant weather feature through the weekend.
This pattern will continue to support deep convective development,
especially today and into early Sunday, aided by cooler-than-normal
temperatures aloft and an abundance of tropical moisture. Precipitable
water values today will range between 1.9 and 2.1 inches, near to
above typical levels for this time of year. A gradual drying trend
is anticipated beginning Sunday into early next week, as drier air
filters in behind the upper-level trough. While winds will gradually
diminish as the surface trough settles northeast of the region over
the next few days, breezy conditions may persist today around the
Anegada Passage, just east of the U.S. Virgin Islands.

Despite the presence of a Saharan Air Layer (SAL), which may locally
suppress convective activity, today is still expected to be the most
active day of the short-term period. Scattered to locally numerous
showers and isolated thunderstorms are forecast, with the greatest
impacts likely over eastern Puerto Rico, the Cordillera Central, and
northwestern Puerto Rico, including the San Juan metro area. Rains
from slow-moving showers will support a limited to elevated risk of
excessive rainfall. The strongest thunderstorms may also produce
frequent lightning, strong wind gusts, and the potential for small
hail. Given already saturated soils, urban and small stream
flooding, localized flash flooding, and landslides remain possible.

The overall pattern becomes less favorable for organized convection
by Sunday and Monday. While isolated to scattered showers and
thunderstorms will still be possible, mainly during the afternoon
hours, activity is expected to be less frequent and less intense,
with flooding threats becoming more localized. Temperatures will
remain near seasonal normals throughout the period, though a limited
excessive heat hazard may develop across northern and western Puerto
Rico on Sunday and Monday. Please refer to the Experimental
Graphical Hazardous Weather Outlook for the latest depiction of at-
risk areas.

.LONG TERM...Tuesday through Saturday...

The forecast remains generally on track for the long-term period.
At the surface, a high-pressure system over the Atlantic Basin
will maintain an easterly to southeasterly wind flow throughout
the forecast period. On Tuesday and Wednesday, a weak upper-level
trough will move across the area, while a mid-level ridge builds
over the western Caribbean. Winds will remain light through at
least midweek, then gradually increase from Thursday onward. According
to the latest model guidance, precipitable water values will
fluctuate around seasonal averagesapproximately 1.8 inches on
Tuesday, decreasing to between 1.5 and 1.7 inches on Wednesday and
Thursday, and rising again to around 1.8 inches by Friday
afternoon onward.

Given this pattern, marginally unstable conditions are expected
mainly on Tuesday and Wednesday. The combination of moisture,
daytime heating, and local effects will favor shower development,
particularly during the afternoon hours. However, the likelihood
of thunderstorms will decrease as 500 mb temperatures rise into an
above-average range (around -4 to -5 degrees Celsius), which could
limit convective development. On the other hand, by the end of
the week, temperatures are forecast to return to around -7 degrees
Celsius at 500 mb, increasing the potential for thunderstorm activity
with afternoon convection, but less moisture and instability will
be available. From Thursday through Saturday, the upper- level
trough will shift to our east, placing its convergent side over
the region, while the mid-level ridge shifts eastward and exerts
more influence, promoting more stable atmospheric conditions.
Either way, afternoon convection cannot be ruled out due to the
available moisture (mainly confined in the lower levels of the
atmosphere) and local effects, particularly across the western
portion of Puerto Rico during peak heating hours.

Temperatures are expected to remain warmer than normal throughout the
forecast period, as suggested by 925 mb temperatures, which will
peak on Tuesday and stay above average for the rest of the week.
Also, model are still showing Saharan dust concentrations over the
region at least through Tuesday.

The main takeaway is that there will likely be rainfall activity
each day, mainly during the morning in eastern Puerto Rico and
the U.S. Virgin Islands, and during the afternoon in western
Puerto Rico. Tuesday has the highest probability of afternoon
rainfall in the west, with chances between 60% and 70%, followed
by Wednesday with 50% to 60%. From Thursday through Saturday, more
stable conditions are expected, but available moisture should
still result in afternoon convection, although it is not expected
to be deep or widespread. The flood risk will likely remain limited.

&&

.AVIATION...
(06Z TAFs)

Unsettled conditions will bring MVFR and brief IFR periods at most
TAF sites, continuing across USVI terminals through the period.
SHRA/TSRA will affect Puerto Rico terminals mainly between 17/16-
22Z. Saharan dust concentrations remain minor, with visibility above
6 miles, except near +SHRA/TSRA. Light to calm winds at first,
increasing to 812 mph with strong sea breeze variations after
17/14Z. Gusts possible near strong TSRA.

&&

.MARINE...

A surface trough will influence light to moderate easterly winds. A
deep trough with its axis currently located to the west of the region
will continue to support unsettled weather conditions today, with a
high chance of thunderstorm development mainly across the eastern
local waters. 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...

There is a low rip current risk today across all coastal areas of
the islands as winds are expected to become lighter. However,
life-threatening rip currents often occur near groins, jetties,
reefs, and piers. Similar conditions are expected to prevail over
the weekend and into early next week. A moderate rip current risk
will likely return around midweek. For more information, please
refer to the latest Coastal Hazard Message (CFWSJU) and Surf Zone
Forecast (SRFSJU).

&&

.HYDROLOGY...

Abundant tropical moisture (with heavy moisture levels) and
increased instability due to a nearby cutoff low will keep the
flood risk high across Puerto Rico and the U.S. Virgin Islands
today and tonight. Since soils are already saturated and some
rivers are running high, any additional heavy rain could cause
urban and flash flooding, river flooding, and landslideseven
after the heaviest rain stops. For more details, please see the
Hydrologic Outlook (ESFSJU).

&&

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

$$

SHORT TERM/AVIATION/HYDROLOGY...ICP
LONG TERM/MARINE/BEACH FORECAST...YZR