Qtas De San Luis 1, Puerto Rico Weather Forecast Discussion

153
FXCA62 TJSJ 161906
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
306 PM AST Tue Sep 16 2025
.KEY MESSAGES...
* A tropical wave combined with an upper-level trough, will increase
  the frequency of showers and thunderstorms particularly late
  tonight and Thursday. As a result, the risk of flooding will
  remain elevated.  Additional hazards include rapid river rises,
  landslides, and frequent lightning.
* The Virgin Islands will also experience an increase in showers and
  thunderstorms, potentially leading to water surges in drainage
  guts, ponding on roadways, and occasional lightning..
* Increasing heat risk is forecast again toward the end of the week
  and into the weekend.
* The National Hurricane Center is currently monitoring two
  tropical waves with low and high formation chances. Invest AL92L
  is expected to move well northeast of the area as a tropical
  cyclone by this weekend. However, this setup could generate
  swells reaching local waters, leading to hazardous marine and
  beach conditions.
&&
.SHORT TERM...Tonight through Thursday...
Showers and thunderstorms earlier today affected northern sections
of Puerto Rico, the US Virgin Islands, and adjacent waters. The
most significant activity was observed across the north-central
interior municipalities of Puerto Rico, where radar estimated 1 to
2 inches of rainfall. With prevailing east-northeast winds, the
El Yunque streamer developed, impacting the eastern interior and
nearby municipalities, while afternoon convection focused over the
southwestern sections of Puerto Rico. Around 9:00 AM, a mass of
dry air filtered into the US Virgin Islands and spread across
Puerto Rico after 11 AM, limiting early afternoon convection.
Temperatures reached the upper 80s to low 90s across coastal and
urban areas, while rural and mountainous areas stayed in the upper
70s to mid-80s.
Tonight, lingering moisture will support showers across windward areas,
with isolated thunderstorms possible over the local waters.
Overnight lows are forecast to range from the upper 70s along the
coasts to the upper 60s across the higher terrain.
On Wednesday, a tropical wave will move across the eastern Caribbean,
bringing a notable increase in moisture. At the same time, an
upper-level trough will enhance instability, creating favorable
conditions for more widespread and organized periods of convection
through Thursday. The combination of deep tropical moisture,
upper-level support, and lighter steering winds will promote slow-
moving showers and thunderstorms capable of producing heavy
rainfall, urban and small stream flooding, flash flooding, and
landslides in steep terrain. Frequent lightning and localized
gusty winds are also expected.
With soils already saturated in parts of Puerto Rico from recent
rains, flood vulnerability remains elevated even for the US Virgin
Islands. Despite occasional cloud cover and rainfall providing
brief relief, hot and humid conditions will persist, but not to
the point of reaching advisory criteria for more than 2 hrs across
coastal and urban areas.
.LONG TERM...Friday through Tuesday...
//from previous discussion//
For Friday and Saturday, the upper-level trough will weaken,
allowing a weak mid-level high pressure to slightly dominate. The
latest model guidance suggests that precipitable water (PWAT)
values will remain within the normal range for this time of year,
ranging from 2.0 inches on Friday and decreasing to 1.5 inches by
Saturday. Expect afternoon showers and isolated thunderstorms
fueled by diurnal heating and local effects. As a result, urban
and small stream flooding is likely to persist each day, and some
rivers may reach flood stage. In the Virgin Islands, periods of
showers and thunderstorms are expected to cause water surges along
guts and drainage channels, ponding on roadways, and isolated
urban flooding. Additional hazards include landslides, frequent
lightning, and gusty winds.
This forecast scenario assumes that the current guidance remains
consistent regarding the tropical wave being monitored by the
National Hurricane Center, which currently has an 80% chance of
formation in the next 48 hours and a 90% chance over the next
seven days. If development continues as expected, the system may
track northwestward toward the northeastern Caribbean. We will
continue to monitor this closely and provide updates throughout
the workweek.
Sunday through Tuesday should be the more stable days of the
forecast period, with 500 mb temperatures warming to above normal,
around -5 to -4 degrees Celsius. Drier air is forecast to move
into the region, with below normal PWAT values persisting through
Tuesday (1.5 inches or lower). Rainfall activity will diminish,
but localized afternoon convection, particularly in the western
sectors of Puerto Rico, will still be possible due to local
effects and diurnal heating. Winds will be very light, so showers
will be very slow moving, and the flood risk will remain mostly
limited. By Tuesday night, another tropical wave is expected to
approach the area, further increasing moisture and bringing better
chances for rain onward.
In terms of temperatures, 925 mb values are forecast to peak at
nearly two standard deviations above normal from Friday into the
weekend. This pattern will elevate the heat risk across the
region. Heat Advisories or even Extreme Heat Warnings may be
issued. Residents and visitors are strongly advised to stay
hydrated, avoid prolonged sun exposure during peak heat hours,
and take extra precautions, particularly those without access to
adequate cooling or hydration.
&&
.AVIATION...
(18Z TAFs)
Mostly VFR conditions should prevail across most TAF sites.
However, brief periods of MVFR condition could affect TJPS from
19Z through 22Z. Overnight, SHRA developing over waters may brush
windward coastal terminals, but moving quickly. On Wednesday,
USVI and eastern PR terminals will see VCSH/VCTS
particularly after 17/12Z. ENE winds at 10-15 kts with sea breeze
variations and higher gusts, bcmg light/VRB aft 23Z, increasing
again 10-15 kts from ENE by 17/13z with stronger gusts near
SHRA/TSRA.
&&
.MARINE...
A surface high-pressure system over the central and eastern Atlantic
will promote moderate to locally fresh easterly winds for the next
few days. Small craft should exercise caution across offshore waters
and local passages. Passing showers and isolated thunderstorms will
continue through the night hours across most waters. By late tonight
through Thursday, stronger thunderstorms are expected across the
regional waters as a stronger tropical wave approaches, enhancing
shower intensity and coverage. Additionally, swells generated by
Invest AL92L as it moves northeast of the region this weekend could
further deteriorate marine conditions, increasing the risk of
hazardous seas.
&&
.BEACH FORECAST...
Rip current risk will remain moderate along northern and eastern
exposed beaches through the period. However, life-threatening rip
currents will still be possible within the surf zone. Beachgoers
should also remain alert for nearby thunderstorms, as lightning
strikes pose a significant hazard near the water. By this weekend,
swells generated by Invest AL92L may further deteriorate coastal
conditions, leading to dangerous surf and increased rip current
risk.
&&
.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...MMC
LONG TERM...YZR
MARINE/BEACH FORECAST...CVB

NWS SJU Office Area Forecast Discussion