484
FXCA62 TJSJ 171837
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
237 PM AST Mon Aug 17 2026

...New SHORT TERM, AVIATION, MARINE, BEACH FORECAST, FIRE WEATHER...

.KEY MESSAGES...
Issued at 233 PM AST Mon Aug 17 2026

* For the rest of the week, a heat threat will persist due to
  high temperatures and heat indices ranging between 108 and 111
  degrees.

* For the rest of the day into Tuesday, a drier weather pattern
  is expected as a drier air mass dominates the region.

* No significant rainfall accumulations are anticipated; however,
  by Wednesday, an increase in moisture is forecast, which will
  increase the potential for showers and a limited risk of
  flooding, particularly across the western and eastern sections
  of Puerto Rico.

* Unlike yesterday, model guidance suggests a different pattern from
  Thursday into Friday, with much drier air dominating the
  region.

* A low to moderate risk of rip currents is expected across local
  beaches throughout the workweek.

&&

.Short Term(This evening through Wednesday)...
Issued at 233 PM AST Mon Aug 17 2026

Tranquil and generally fair weather conditions will continue across
most of the region through this evening and much of Tuesday as a
relatively dry and stable air mass remains in place. Precipitable
water (PWAT) values will remain near 1.25 to 1.50 inches through
Tuesday, near the lower end of climatological values for this time
of year. This dry air is particularly evident in the mid-levels,
where 700-500 mb relative humidity remains well below normal.
Therefore, shower activity should remain limited despite sufficient
low-level moisture and daytime heating. Nevertheless, local and
diurnal effects could promote isolated to locally scattered showers
across western Puerto Rico each afternoon. Otherwise, mostly fair
weather is expected across the U.S. Virgin Islands and eastern
Puerto Rico. Hot conditions will remain the main concern across
coastal and urban areas, where elevated heat indices are expected.

A transition to wetter and more unsettled weather remains possible
from late Tuesday night into Wednesday as a vigorous tropical wave,
previously designated as Invest 94L, interacts with a deep-layer
trough. The trough is expected to pull the wave and its associated
tropical moisture northward toward the region. However, uncertainty
remains regarding how much of this moisture reaches the local
islands. The GFS has remained consistent in bringing the bulk of the
moisture across the forecast area, while the ECMWF keeps the deepest
moisture north of the region.

If the wetter solution materializes, PWAT values could increase to
around 2.0-2.3 inches, with a substantial increase in moisture
through the mid-levels. This would favor periods of moderate to
locally heavy showers across the U.S. Virgin Islands and eastern
Puerto Rico, followed by additional afternoon activity across
interior and western Puerto Rico. Localized urban and small-stream
flooding could occur where persistent heavy rainfall develops.
Stronger thunderstorms could also produce frequent lightning and
locally strong, gusty winds.

Rainfall associated with this wave would be beneficial given the
worsening drought conditions across portions of the region,
particularly if the wetter GFS solution materializes. Nevertheless,
while this event could provide some temporary relief, it is not
expected to be sufficient to end the ongoing drought conditions.
Continue to monitor subsequent forecasts, as relatively small
changes in the evolution of the wave and trough could significantly
affect Wednesday`s rainfall coverage and amounts.

&&

.Long Term(Thursday through next Sunday)...
Issued at 317 AM AST Mon Aug 17 2026

A generally quiet weather pattern is expected through most of the
period, with no significant rainfall hazards anticipated. Moisture
should gradually decrease Thursday into Friday, favoring brief
passing showers and limited afternoon convection over western Puerto
Rico. Heat will likely remain the main concern, with elevated heat
risk possible where sufficient sunshine occurs.

Forecast confidence decreases toward the end of the period as some
guidance suggests an open tropical wave approaching the northeastern
Caribbean by Sunday or the following weekend. At this range,
however, the signal remains highly uncertain and may partly reflect
climatological influences in the model solutions rather than a well-
defined system. Trends will continue to be monitored.
&&

.AVIATION...
(18Z TAFS)
Issued at 233 PM AST Mon Aug 17 2026

Mainly VFR conds expected across all TAF sites thru the period.
Winds E-ESE at 10-15 kt with sea breeze variations and ocnl gusts up
to 25 kt, becoming light/VRB overnight. SHRA/VCSH expected to
increase aft 18/14Z, mainly across interior and wrn PR. VCTS cannot
be ruled out at TJPS.

&&

.MARINE...
Issued at 233 PM AST Mon Aug 17 2026

A subtropical surface high-pressure ridge centered across the
western Atlantic will remain the dominant feature controlling the
regional pattern. This system will maintain gentle-to-moderate
easterly trade winds across the local waters for the next few days.
Tranquil to moderate marine conditions will prevail through much of
the workweek, with seas generally ranging between 2 and 5 feet
across the Atlantic and Caribbean waters.  Moisture from an
approaching tropical wave is forecast to build by Wednesday,
bringing a higher potential for scattered showers and thunderstorms
over coastal waters, which will elevate the localized lightning
hazard.

&&

.BEACH FORECAST...
Issued at 233 PM AST Mon Aug 17 2026

There is a moderate risk of rip currents across the northern,
eastern, and southern coastal areas of Puerto Rico, Culebra, and
Vieques, as well as the eastern tip of St. Croix. Meanwhile, a low
risk of rip currents is expected along the western sections of the
region. Similar marine conditions, with a low-to-moderate risk of
rip currents, will remain in place for the rest of the workweek. In
addition, an increase in showers and thunderstorms is expected by
Wednesday afternoon. This weather shift will raise the potential
lightning threat across the coastal areas. Beachgoers and residents
should remain cautious and stay informed on changing weather updates
throughout the week.

&&

.FIRE WEATHER...
Issued at 233 PM AST Mon Aug 17 2026

Critical fuel conditions continue across the islands, with extreme
drought affecting portions of eastern Puerto Rico and the southern
coastal plains. Dry conditions have prevailed through much of today,
with low relative humidity, stressed vegetation, and dry fuels
continuing to support elevated fire danger.

The greatest concern remains across southern and southwestern Puerto
Rico, where fuels are exceptionally dry and recent fire activity has
been frequent. Breezy easterly winds with occasional higher gusts
this afternoon could support rapid fire spread and make any ongoing
fires more difficult to control. Elevated fire danger also continues
elsewhere across Puerto Rico, Vieques, Culebra, and the U.S. Virgin
Islands.

Some relief remains possible by Wednesday as a tropical wave could
bring an increase in showers and beneficial rainfall. However,
uncertainty remains regarding rainfall coverage and amounts, and
drought-stressed fuels will likely persist in areas that receive
limited rainfall.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM/AVIATION/FIRE...CVB
LONG TERM...ICP
MARINE/BEACH...LIS