763
FXCA62 TJSJ 180840
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
440 AM AST Mon May 18 2026

...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST...

.KEY MESSAGES...
Issued at 440 AM AST Mon May 18 2026

* An upper-level trough northwest of the islands will aid in the
  formation of showers and isolated thunderstorms, particularly
  during the afternoons across the interior and western PR.
  Limited to elevated flooding risk.

* A limited heat risk will continue this week, heat indices
  expected at the mid 90s to low 100s across urban and coastal
  areas.

* Breezy conditions will maintain choppy seas. A Small Craft
  Advisory is in effect for the offshore Atlantic waters through
  at least Tuesday afternoon.

* Localized flooding concerns may gradually increase through the
  holiday weekend, especially in urban and poor drainage areas

* A moderate risk of rip currents will continue this week, mainly
  along the north and east facing beaches of Puerto Rico
  (southern beaches at times), as well as beaches of Vieques,
  Culebra, and the U.S. Virgin Islands.

&&

.Short Term(Today through Wednesday)...
Issued at 440 AM AST Mon May 18 2026

Current satellite derived precipitable water (PWAT) values
indicate a band of moisture over the USVI and eastern PR, moving
westward towards the rest of the region and steered by easterly
winds. PWAT values reach 1.50 inches over the above mentioned
areas gradually decreasing westward and reaching around 1.33
inches over western PR. Fast moving showers continue to be steered
towards windward areas, some being moderate to locally heavy. At
least minimal accumulations have been detected since midnight over
the eastern third of PR, Vieques, Culebra, and the USVI. T-storms
developed earlier tonight mainly west of Mona Island. Patchy fog
was also detected over sectors of interior PR while higher clouds
9from t-storms south of Hispaniola have crossed the Mona Passage
and reached western PR. Lows have been in the low 70s to locally
around 80 over urban and coastal sectors of the islands, and in
the upper 50s to the 60s at higher elevations of PR.

A broad surface high over the Atlantic will promote up to breezy
easterly winds. This easterly flow will continue to steer the
above mentioned band of moisture over the islands during the
morning, with PWAT values reaching normal values for this time of
the year. A band of drier air (with below normal PWAT) will then
be steered towards the islands and reach the eastern region late
morning into the afternoon and then western PR tonight. Current
model guidance suggests that more broad moisture with seasonal
PWAT values will then reach the islands tomorrow morning and
Wednesday under more east to east-southeast breezy flow. PWAT
values can locally reach above normal values during afternoon
convection. An amplifying upper level trough from W-NW of the
region during the period can promote increased instability and
increase the frequency of showers. The presence of patches of
drier air in the mid- levels can help inhibit this activity,
however. Fast moving showers will continue to be steered towards
windward sectors, resulting in periods of variable to showery
weather. This can result in ponding of water over roads and poorly
drained areas with a chance of urban and small stream flooding.
Conditions should improve over windward sectors today as the band
of drier air gradually moves over the area. The upper trough will
also aid in the development of afternoon showers and t-storms, in
addition to diurnal heating, local orographic effects and sea
breeze convergence. These will develop over interior towards the
western half of Puerto Rico, as well as downwind of El Yunque, and
the USVI towards eastern PR. This will result in a limited to
elevated flooding risk and flood advisories might need to be
issued each afternoon. Although this pattern is forecast to
continue tomorrow and Wednesday, the band of drier air approaching
the area can tapper off convection this afternoon as it moves
westward, however current model guidance indicates that it should
still be over eastern PR as afternoon convection develops. Galvez-
Davidson Indices are forecast at up to around 30 each afternoon.

More seasonal, but still warm, highs are expected during the
period, as indicated by 925 mb temperature guidance. Highs will
reach the upper 80s to lower 90s across urban and coastal areas
while heat indices exceed 100 degrees F during the afternoon
hours. This will result in a limited heat risk. Low concentrations
of saharan dust will reach the region from time to time.

&&

.Long Term(Thursday through Monday)...
Issued at 440 AM AST Mon May 18 2026

Model guidance has remained fairly consistent through the long-term
period, supporting a persistent late spring weather pattern across
the local islands and surrounding waters through the Memorial Day
weekend. Broad surface high pressure over the central Atlantic will
maintain moderate to fresh easterly winds, becoming breezy to
locally windy at times late in the week and during the weekend.
Moisture embedded within the trade wind flow will continue to
fluctuate between drier and wetter periods, with some moisture
patches lingering longer from Friday onward and supporting a gradual
increase in shower activity. Minor traces of Saharan dust will
occasionally filter across the area with little impact expected.
Temperatures will remain seasonably warm to hot, while marine and
beach conditions gradually deteriorate through the weekend with
increasing rip current and marine hazards.

At upper levels, the area will remain between troughing over the
western Atlantic and ridging to the southeast, with the trough
gradually shifting closer to the region late in the period. This
pattern will support periods of enhanced upper-level winds and more
favorable conditions for afternoon convection by the weekend and
early next week. Passing showers will continue across windward areas
during the overnight and morning hours, followed by afternoon
showers and isolated thunderstorms mainly across interior and
western Puerto Rico. Localized flooding concerns may gradually
increase through the holiday weekend, especially in urban and poor
drainage areas, along with lightning and gusty winds in the
strongest activity.

&&

.AVIATION...
(06Z TAFS)
Issued at 440 AM AST Mon May 18 2026

Mainly VFR conditions continue. E winds will steer -SHRA/VCSH
towards windward terminals during the period. Btwn 18/16-23Z,
SHRA/+SHRA, with isolated TS, developing across interior to W/NW
PR. This can cause brief MVFR conditions over TJBQ. Lines of -SHRA
/SHRA can also develop from the local islands and El Yunque,
affecting TJSJ. Breezy to Windy E winds up to 15-20 kts, with
higher gusts and sea breeze variations, decreasing after 18/23Z
and gradually becoming more E to ESE.

&&

.MARINE...
Issued at 440 AM AST Mon May 18 2026

A strong surface high pressure over the western Atlantic will
promote moderate to fresh trades through midweek. Winds will turn
more east to southeast thereafter as the surface high moves into the
central Atlantic. These breezy conditions will maintain choppy seas,
particularly across the offshore and exposed Atlantic and Caribbean
waters. A Small Craft Advisory is in effect for the offshore
Atlantic waters through at least late tomorrow. Afternoon and early
evening isolated thunderstorms can be expected each day across the
northwestern waters of Puerto Rico.

&&

.BEACH FORECAST...
Issued at 440 AM AST Mon May 18 2026

A moderate risk of life-threatening rip currents will continue to
prevail across the east, north, and southern beaches of Puerto Rico,
Culebra, Vieques and the U.S. Virgin Islands during the next several
days. Beachgoers should exercise caution when entering these
beaches. This means life-threatening rip currents are possible in
the surf zones, and beachgoers should exercise caution when entering
these beaches.

If you become caught in a rip current, yell for help. Remain calm,
do not exhaust yourself and stay afloat while waiting for help. If
you have to swim out of a rip current, swim parallel to shore and
back toward the beach when possible. Do not attempt to swim directly
against a rip current as you will tire quickly. For additional
information and location-specific rip current details, visit
weather.gov/beach/sju.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory until 6 PM AST Tuesday for AMZ711.

&&

$$

SHORT TERM/AVIATION/KEY MESSAGES...MRR
LONG TERM....ICP
MARINE/BEACH FORECAST...DSR

NWS SJU Office Area Forecast Discussion