Urb Altavista Chalets, Puerto Rico Weather Forecast Discussion

079
FXCA62 TJSJ 240839
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
439 AM AST Wed Sep 24 2025
.KEY MESSAGES...
* A robust tropical wave (AL94) will cross the region today into
  Thursday, bringing periods of squally weather with bands of
  heavy showers, thunderstorms, and gusty winds.
* Elevated to significant flooding risks are anticipated from the
  heavy rainfall, with potential for urban and small stream
  flooding, flash flooding, rapid river rises, and landslides in
  steep terrain.
* Hazardous marine conditions are expected as AL94 moves across
  the region, with Small Craft Advisories in effect. Increasing
  winds and seas will create dangerous conditions for mariners.
* An extended wet period will continue into the weekend, as deep
  tropical moisture from AL94 and AL93, moving north of the area,
  sustains rounds of showers and thunderstorms.
* For the U.S. Virgin Islands: Squally weather will persist
  through Thursday with heavy showers, thunderstorms, gusty winds,
  and hazardous marine conditions. Additional rounds of showers
  are expected into the weekend, maintaining a risk of flooding in
  vulnerable areas before gradual improvement by midweek of next
  week.
&&
.SHORT TERM...Today through Friday...
Tranquil conditions prevailed overnight across the region, with only
brief passing showers affecting eastern Puerto Rico and the US
Virgin Islands under light northeasterly winds that gradually
transitioned to more east to southeasterly. A few stronger showers
with isolated thunderstorms developed over the surrounding
waters, occasionally producing lightning south of Saint Croix and
northwest of Aguadilla, though no products were required.
Overnight lows ranged from the upper 70s to the low 80s in coastal
and urban areas, while interior valleys and higher terrain cooled
to the upper 60s to mids, slightly warmer than the previous
nights.
Unlike the last several days, heat will not be the main hazard
today. Instead, attention shifts to a flooding threat as a tropical
wave currently monitored by the National Hurricane Center steadily
approaches our region. Winds will gradually veer to the south,
favoring enhanced moisture transport and heavier rainfall along the
eastern and northern portions of Puerto Rico and the US Virgin
Islands. Model guidance shows above-normal precipitable water values
for this time of year, with mid-level temperatures near -7 degrees
C, both of which support deep convective development.
By late today into Thursday, interaction between the tropical wave
and an upper-level low will maximize instability and rainfall
potential. The most widespread and intense activity is expected
during this window, initially impacting the Virgin Islands before
spreading across Puerto Rico. Therefore, periods of squally weather,
heavy rainfall, and thunderstorms will bring the potential for
significant impacts, including rainfall totals of 2 to 6 inches,
along the northern and eastern half of PR and the US Virgin Islands.
The unsettled and unstable weather pattern will likely persist
through the end of the workweek. However, showers and thunderstorms
are expected to be less widespread after Thursday.
Given already saturated soils across the northern and western half
of Puerto Rico, as well as St. Thomas and western St. Croix, risks
include urban and small stream flooding, flash flooding, rapid river
rises, coastal water surges, and landslides in steep terrain.
Residents and visitors should remain alert for any changes in the
forecast, in order to make informed decisions. For further details,
refer to the hydrology section.
.LONG TERM...Saturday through Wednesday...
For this weekend, soils across the region will likely remain
saturated and streamflows elevated after several days of heavy
rainfall. These antecedent conditions increase the areas
sensitivity to additional rainfall, meaning even moderate showers
could quickly trigger urban and small stream flooding, as well as
isolated landslides in steep terrain.
This weekend, little change is expected in the overall pattern
suggested by previous model cycles. A deep-layer southerly flow,
reinforced by the northward movement of Invest AL94 and AL93, will
continue transporting abundant tropical moisture into the region.
Precipitable water values will remain above normal, generally at
or above 2 inches, maintaining a moist and unstable environment.
In combination with daytime heating and sea breeze convergence,
this will favor frequent rounds of showers and thunderstorms,
particularly during the afternoon. Activity will tend to focus on
southern and eastern Puerto Rico and the smaller islands overnight
and during the morning, then shift toward the interior and
northwestern municipalities in the afternoon. The prevailing
southerly flow will also enhance rainfall potential along the
northern slopes of Puerto Rico.
The ECMWF Extreme Forecast Index now signals the potential for
extreme precipitation, especially across the southern and eastern
portions of the forecast area, although ensemble spread maintains
some uncertainty regarding the exact magnitude and focus.
Regardless, given antecedent soil conditions, the potential for
flooding impacts will remain elevated through the weekend and into
early next week.
Conditions are expected to gradually improve beginning Tuesday as
the moisture plumes shift north and precipitable water values drop
to near or slightly below seasonal levels, around 1.5 to 1.6 inches.
With this drying trend, shower and thunderstorm activity will become
more isolated, and the flood threat should begin to diminish
compared to the weekend. Still, isolated afternoon convection will
remain possible, especially across the western and interior portions
of Puerto Rico.
In addition to rainfall hazards, a persistent southerly flow will
maintain above-normal temperatures and oppressive heat indices
across coastal and urban areas. Extreme Heat Warnings or Advisories
are likely to be needed through much of the period, particularly
over the weekend when moisture levels and heat combine to create
dangerous conditions.
&&
.AVIATION...
(06Z TAFs)
All TAF sites should experience VFR conditions. However,
after 24/14Z, -SHRA/VCTS could affect TIST, TISX, and TJSJ. But
after 24/17Z, TSRA should prompt periods of MVFR conditions
across most TAF sites, with the heaviest TS activity as the
vigorous Tropical Wave being monitored by the National Hurricane
Center moves closer to the region. Winds will remain from the E-SE
and gradually become more S from 24/15-24/22Z at 815 kts and
higher gusts near the heaviest RA and TS associated to the
tropical wave.
&&
.MARINE...
Today, a vigorous tropical wave will move into the islands,
increasing the potential for thunderstorms and leading to localized
hazardous marine conditions. These storms may produce frequent
lightning over exposed waters. In addition, an increase in surface
winds is forecast through at least late tonight, particularly across
the offshore Atlantic waters, Anegada Passage and waters east of
Puerto Rico, around Vieques, Culebra, and St. Thomas, where Small
Craft Advisories are in effect. Seas up to 7 feet, particularly
across the offshore Atlantic waters and Anegada Passage.
&&
.BEACH FORECAST...
Increasing winds and choppy seas as AL94 crosses the region will
maintain hazardous conditions, particularly along the Atlantic-
facing beaches. As a result, a moderate rip current risk will
persist across the northern and eastern beaches of Puerto Rico,
Vieques, Culebra, and the U.S. Virgin Islands. Beachgoers are
urged to exercise caution and avoid swimming in high-risk areas.
In addition, thunderstorms associated with AL94 will pose
significant hazards at the coast. Frequent lightning strikes,
gusty winds, and heavy rainfall may lead to dangerous conditions,
including sudden reductions in visibility and localized flooding
near the shoreline. Remember: if thunder roars, go indoors.
Swimmers and boaters should immediately seek shelter when storms
approach.
&&
.HYDROLOGY...
.PREV DISCUSSION... /issued 315 PM AST Tue Sep 23 2025/
Unsettled weather conditions are expected to persist through much
of the workweek, leading to an elevated flooding threat for both
Puerto Rico and the US Virgin Islands.
The main threats for Puerto Rico are Urban and isolated flash
flooding, landslides, river surges, and some rivers potentially
reaching flood stage. Rainfall accumulations for the event are
expected to range from 4 to 6 inches across the eastern third of
PR, including Vieques and Culebra, and the northwest quadrant,
particularly on Wednesday and Thursday. The best chance for the
flooding threat will be Wednesday through Thursday, when more
vigorous tropical wave activity and an upper-level low will
increase rainfall and thunderstorm coverage.
The primary threats to the US Virgin Islands include water surges
along drain gutters, squally weather with gusty winds, ponding of
water on roads/low-lying areas, urban and isolated flash
flooding, and possible landslides. Expect rainfall amounts of
around 2 to 4 inches, with isolated higher amounts of up to 6
inches. The best chance of flooding is between Wednesday and
Thursday, with stronger showers and thunderstorms.
Thus, we encourage residents and visitors to actively monitor NWS
flood watches and warnings, as additional rainfall on already
saturated soils will heighten vulnerability to flash flooding and
landslides. For further details, see the Hydrologic Outlook
(ESFSJU) issued by WFO-San Juan.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory until 6 AM AST Thursday for AMZ711.
Small Craft Advisory until 6 PM AST this evening for AMZ716-723-
     726.
&&
$$
SHORT TERM/AVIATION...MMC
LONG TERM/MARINE/BEACH FORECAST...CVB
HYDRO...ERG

NWS SJU Office Area Forecast Discussion