Urb San Alfonso Puerto Rico Weather Forecast Discussion

863
FXCA62 TJSJ 120915
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
515 AM AST Sun Oct 12 2025
.KEY MESSAGES...
* Abundant low-level moisture combined with locally effects will
  result in showers across eastern Puerto Rico this morning and
  thunderstorms across western sectors this afternoon. Limited to
  elevated flood risk is in place.
* A Heat Advisory is in effect from 10 AM to 5 PM AST for coastal
  and urban areas of Puerto Rico, Vieques, Culebra, and the U.S.
  Virgin Islands.
* The U.S. Virgin Islands will experience showery weather,
  particularly during early mornings and nights.
* By Thursday into around Saturday, dynamics will be more
  favorable for widespread showers and stronger thunderstorms.
&&
.SHORT TERM...Today through Tuesday...
Persistent showers and thunderstorms developed mainly over the
Caribbean waters and local passages, with some activity moving
across the U.S. Virgin Islands and eastern Puerto Rico, including
Vieques and Culebra. Rainfall estimates were around a quarter of
an inch in these areas. Overnight lows ranged from the upper 60s
in higher elevations to around 80 to 82F along coastal locations,
remaining slightly warmer than normal. Winds were generally light
to calm and variable over land, resulting in limited cooling and
a warm, humid night.
Extreme heat will be the main hazard today, as clearer skies and
east-southeasterly winds boost sunshine and pull in warm, humid
air. A Heat Advisory is in effect for coastal and urban areas of
Puerto Rico and the U.S. Virgin Islands, including Vieques and
Culebra, where feels-like temperatures will likely climb well
above normal, reaching levels that impact most heat-sensitive
individuals, especially those without cooling or proper hydration.
Aloft, a persistent ridge will maintain a trade wind cap and
support drier air aloft, limiting widespread storms. However, with
abundant low-level moisture (PWAT values above 2 inches), intense
heating, and sea-breeze convergence, localized afternoon showers
and thunderstorms are still likely, especially downwind of the
mountains and across the northwest quadrant. Any storm could
produce brief heavy rain, localized flooding, lightning, or gusty
winds. Except for some lingering showers, most hazard risks will
diminish after sunset (after 6:03 PM AST) as daytime heating fades
and instability weakens. Winds will become more easterly later
today as a surface high builds north, but conditions will remain
hot, muggy, and spotty-stormy until early evening.
A strengthening mid- to upper-level ridge will reflect downward
as a surface high, becoming the dominant low-level feature and
maintaining a gentle to moderate easterly breeze tonight, backing
ENE Monday night, then veering ESE and weakening to light winds by
Tuesday night. This high-pressure pattern will lock in
subsidence, a strong inversion, and anomalously warm mid-level
temperatures well above normal, creating an increasingly hostile
environment for widespread convection. Moisture will fluctuate in
patches, with a brief drier period Monday morning, then near-
normal values thereafter, and while heat risks will persist, they
should ease slightly after today as winds turn more E-NE. Rainfall
will remain highly localized and mainly driven by afternoon
heating, with any showers brief, steered by the prevailing flow,
and limited in coverage, though isolated thunder or minor flooding
cannot be ruled out. By late Tuesday night, winds will diminish
in response to an approaching frontal boundary from the northwest,
signaling a potential pattern change heading into the workweek.
.LONG TERM...Wednesday through Sunday...
At the beginning of the long-term period, we will be transitioning
into more favorable conditions for increased shower coverage and
stronger thunderstorms. The mid- to upper-level ridge will begin
to be replaced by a deep-layer trough, while 500 mb temperatures
cool, enhancing thunderstorm development during the afternoon
hours. On Wednesday, surface winds will be light with an easterly
to southeasterly component, then from Thursday onward, winds will
shift to a more southerly direction and remain light. On Wednesday,
afternoon convection is expected to develop over the interior and
move into the western sectors of Puerto Rico. Latest model
guidance indicates that precipitable water (PWAT) values will
remain near average through the period, ranging from 1.9 to 2.0
inches. From Thursday through Saturday, a frontal boundary will
approach and stall north of the area, while southerly flow steers
tropical moisture over the region. Relative humidity values in the
mid to upper levels will increase to above-normal levels.
Additionally, the southerly flow is expected to sustain an
elevated heat risk, with 925 mb temperatures showing values above
two standard deviations. Overall, expect typical afternoon
convection on Wednesday. However, from Thursday through Saturday,
the combination of the mentioned variables, better atmospheric
dynamics and moisture will support stronger afternoon
thunderstorms, increasing the potential for flooding across the
region. This includes flooding of urban areas, roads, small
streams, and washes. By Sunday, ridging is expected to begin
dominating the region once again.
&&
.AVIATION...
(06Z TAFs)
Variable weather conditions will prevail today. SHRA/TSRA are
expected across USVI terminals (TIST/TISX), TJSJ, and TJPS through
12/14Z, with MVFR to brief IFR CIGS/VIS possible in heavier
activity. Additional SHRA/TSRA are likely this afternoon over
interior and NW PR, affecting TJSJ and TJBQ mainly between
12/16-21Z, again producing brief MVFR/IFR restrictions. Most
convection will diminish after sunset. Winds LGT/VRB overnight,
increasing to 1014 kt from the ESE-E after 12/13Z with sea-breeze
influences, then becoming LGT/VRB again after 12/22Z.
&&
.MARINE...
Gentle to moderate east to southeast winds will prevail over the next
few days as the remnants of Jerry shift well north of the area and a
surface high pressure builds across the Central Atlantic. Above normal
moisture content will continue to result in showers and thunderstorms
across the waters this morning, and across the norwestern waters of
Puerto Rico this afternoon. Seas generally below 5 feet throughout
the forecast period.
&&
.BEACH FORECAST...
A low risk of rip currents is present today, however a moderate
risk should return by tonight across beaches along the northern
and eastern coasts of Puerto Rico, Culebra and St. Thomas. That
means life-threatening rip currents are possible in the surf zone
and beachgoers need to exercise caution. Over the coming days,
similar conditions will dominate. For more information, please
refer to the Surf Zone Forecast for Puerto Rico and the U.S.
Virgin Islands (SRFSJU) or visit weather.gov/beach.
Heed the advice of lifeguards, beach patrol flags and signs. 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.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010>013.
VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.
AM...None.
&&
$$
SHORT TERM/AVIATION...ICP
LONG TERM/MARINE/BEACH...YZR

NWS SJU Office Area Forecast Discussion