802
FXCA62 TJSJ 181904
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
304 PM AST Fri Jul 18 2025

.KEY MESSAGES...

* Showers and/or isolated thunderstorms will continue through
  this evening, increasing flooding potential over the
  southwestern quadrant of PR and St. Croix.

* An upper-level and an induced surface trough can boost the
  diurnal shower pattern tomorrow.

* Warmer-than-normal temperatures will continue, with higher
  chances of heat indexes reaching Heat Advisory Criteria, over
  urban and coastal areas.

* There is a moderate risk of life-threatening rip currents across
  the northwestern to northeastern beaches of Puerto Rico through
  this evening. Tonight through most of Saturday, a low risk of
  rip currents will prevail.

&&

.SHORT TERM...Tonight through Saturday


A Saharan Air Layers (SAL) moderate to high concentrations of
Saharan Dust moved from the islands to the Caribbean Sea during
the early morning hours. Only very low to low concentrations of
Saharan Dust will persist this weekend. Diurnal heating, sea
breeze convergence and local orographic effects along with saharan
dust particles filtering in, promoted showers and isolated
thunderstorm formation over the region, mainly affecting interior
to southern/southwestern PR. Showers also developed over and
downwind from south/southwestern St. Croix. Several stations also
reported heat indices in the 100s, isolated to around 110 degrees
fahrenheit in Puerto Rico, over coastal and lower elevation areas
of the islands. Passing showers will be steered towards windward
sectors tonight while showers/thunderstorms currently over the
islands will gradually dissipate and or move offshore. The typical
rain pattern is forecast, boosted on Saturday, with passing
showers in windward locations followed by afternoon convection
across western Puerto Rico and downwind from the U.S. Virgin
Islands. Throughout the forecast period, 925 mb temperatures are
expected to remain at or above normal. Coastal and urban areas
will experience maximum temperatures in the upper 80s to low 90s,
leading to heat indices above 100 degrees daily (heat advisories
will be issued later on). Minimum temperatures will generally
range from the 60s in interior and mountainous regions to the
upper 70s or low 80s in coastal and urban areas. Model guidance
suggests that precipitable water (PWAT) values will remain at 1.5
to 1.9 inches, with local patches of moisture reaching the area
and afternoon convection reaching around 2 inches. ENE steering
flow this evening will veer to become ESE tomorrow and Sunday.
Showers will be steered towards windward locations, followed by
afternoon convection (showers and possibly isolated thunderstorms)
over sectors of northwestern Puerto Rico and downwind from El
Yunque and the US Virgin Islands. A risk to observe ponding of
water in roads and poorly drainage areas, as well as localized
flooding in urban areas, roads and small streams, will mainly
continue in the afternoons during the rest of the period. Despite
observed discrepancies regarding the position of a retrograding
TUTT low and moisture pooling over the islands, Saturday can see a
boost in the diurnal pattern. This TUTT low will induce a surface
trough, which will affect the islands on Saturday, leaving
trailing moisture for Sunday. Therefore, we are currently
forecasting an active Saturday afternoon with isolated to
scattered thunderstorms, primarily across northwestern Puerto
Rico.


.LONG TERM...Monday through Friday...

./FROM PREV DISCUSSION...Issued 501 AM AST Fri Jul 18 2025/

At the beginning of the long-term period, a mid to upper level ridge
building from the central Atlantic into the northeastern Caribbean
will shift further west and displace the prevailing trough pattern.
Winds will shift more from the east. It seems that Monday will be
the day with the least rain and the most stable conditions of the
forecast period. Models suggest precipitable water will drop to
around 1.3 to 1.4 inches, which is below normal values. Mid level
moisture is expected to decrease, and rainfall will likely follow
a more typical diurnal pattern driven by daytime heating and local
effects. Flood risk should range from none to limited at the start
of the week. From Monday night onward, another TUTT low will approach
the region, bringing a wind surge and a series of disturbances with
patches of moisture from Tuesday through Thursday, likely accompanied
by some Saharan dust that could enhance thunderstorms development
during the afternoons. At this time, expect variable weather conditions
during those days and breezy to windy easterly winds, with early
morning convection between the USVI and eastern sections of Puerto
Rico, followed by afternoon convection with isolated thunderstorms
over parts of the interior and western Puerto Rico. Flood risk should
range from limited to elevated each afternoon in those areas. Models
suggest precipitable water will fluctuate around 1.6 to 1.7 inches,
which is considered near seasonal values. Additionally, warm to hot
temperatures will continue to dominate during this period, as indicated
by above- normal 925 mb temperatures. This will likely trigger Heat
Advisory conditions across most coastal and urban areas of Puerto Rico
each day. Residents and visitors are encouraged to stay hydrated, avoid
outdoor activity during peak heat hours, wear light clothing, and check
on vulnerable individuals and pets.

Toward the end of the period, the latest guidance suggests the arrival
of a tropical wave along with well above-normal precipitable water.
Upper-level dynamics will likely support an unstable weather pattern
beginning Friday. Stay tuned over the next few days for forecast
updates and adjustments, as we are still one week out and
confidence remains low.

&&

.AVIATION...
(18Z TAFs)

Mostly VFR conds across all terminals. Brief MVFR conditions
possible over TJBQ (SHRA/VCTS) and TISX (SHRA) during the
afternoon. Winds from the ENE between 15 to 20 kts with higher
gusts through 18/23z, later gradually decreasing to light and
variable and veering to become more E/ESE tomorrow, increasing
again after 19/13Z. SHRA will continue to move towards windward
sectors during the overnight hours.

&&

.MARINE...

A surface high pressure over the Atlantic will mainly promote
moderate easterly winds over several days, becoming moderate to
fresh again around Tuesday next week. Showers and isolated
thunderstorms are expected across the western waters of Puerto
Rico and the Mona passage this afternoon. An induced surface
trough will increase shower activity across the waters during the
weekend.

&&

.BEACH FORECAST...

There is a moderate risk of life-threatening rip currents across
the northwestern to northeastern beaches of Puerto Rico through
this evening. Tonight through most of Saturday, a low risk of rip
currents will prevail across the local beaches. Even if the risk
for rip currents is low, life-threatening rip currents often
occur in the vicinity of groins, jetties, reefs, and piers.
A moderate risk of rip currents will return from Sunday. Heat
index values likely exceeding 100F each day. Stay well hydrated,
seek shade frequently, and avoid outdoor activity during peak sun
hours.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010-011.

VI...None.
AM...None.
&&

$$

MRR/RVT