textproduct: San Juan

KEY MESSAGES

Issued at 113 PM AST Sat Oct 31 2026

- Shower and thunderstorm activity will continue this afternoon into night hours, bringing periods of heavy rainfall, gusty winds, and lightning. Seek shelter whenever you hear thunder, and don't forget the umbrella while trick-or-treating.

- For the U.S. Virgin Islands, passing showers tonight may lead to ponding of water over roadways and poor drainage areas, thought the flooding risk remains low.

- Heat indices exceeding 100 F are very likely each day, with warmer temperatures by midweek, across urban and low-lying areas of the islands.

Short Term(This evening through Monday)

Issued at 113 PM AST Sat Oct 31 2026

A surface high pressure will continue to build over the western Atlantic through the short term, promoting moderate trades across the region. The 31/12z TJSJ upper-air sounding indicated a moistening boundary layer with precipitable water (PWAT) at 1.78 inches, alongside moderate instability (MUCAPE at 2561 J/kg) and low level ESE winds. However, a dry air layer lingers between 700 and 500 mb. That said, the combination of the available moisture, diurnal heating, sea breeze convergence, and orographic forcing along the Cordillera Central will trigger scattered to locally numerous showers with isolated thunderstorms across the mountain ranges of Puerto Rico. With steering winds out of the east-southeast at 10 to 15 knots, activity should drift toward the west and northwestern coasts through the late afternoon. Ensemble guidance shows areas of 1.50 to 2.50 inches, mainly over the western interior, which can lead to urban and small-stream flooding. Across the U.S. Virgin Islands and eastern Puerto Rico, partly sunny skies will prevail with isolated trade wind showers.

From late tonight into Sunday, an upper-level trough over Hispaniola will continue to pull moisture over the region and aid in the development of advective showers across the USVI and eastern PR. PWAT content is expected to peak around 2.00 inches with modest moistening in the 700-500 mb layer through most of the day. This will support another round of convection by midday across the interior and western PR, with rain rates sufficiently heavy to cause localized minor flooding. Meanwhile, the U.S. Virgin Islands will experience somewhat breezy conditions with passing light to moderate trade wind showers.

By Monday, a drying trend begins as drier mid-level air filters into the region. Model soundings show PWAT values dropping into the 1.65 to 1.75 inch range and mid-level RH declining. Although the cap strengthens slightly, sufficient low-level moisture and daytime heating will still promote scattered afternoon convection across northwestern Puerto Rico. Elsewhere across the islands, including the USVI, mostly fair weather is expected during the day, however, another pulse of low-level moisture content from the east is expected during the night, enhancing nighttime convection between the USVI and eastern PR.

Long Term(Tuesday through Saturday)

Issued at 254 AM AST Sat Oct 31 2026

A generally warm and humid weather pattern is expected across Puerto Rico during the long term period. Precipitable water values will remain relatively high, ranging between 1.9 and 2.3 inches, while low-level relative humidity (850 to 700 mb) will generally remain between 70% and 85%. Mid-level moisture (700 to 500 mb) also shows periods of increased humidity, mainly values in the 75th percentile based on climatology for the month of November. Notably, 925 mb temperatures are forecast to reach approximately two standard deviations above normal at the beginning of the period (particularly Tuesday and Wednesday), supporting warmer than normal conditions. If widespread cloud cover does not develop, these warm and humid conditions could promote stronger daytime heating and enhance afternoon convection. At the surface, light and variable winds are expected, particularly from Wednesday through Friday, resulting in very weak steering flow. This will allow locally developed showers to move slowly, increasing rainfall accumulations and the potential to observe flooding. Sea-breeze convergence, diurnal heating, and orographic lifting across the interior and western portions of Puerto Rico could further enhance shower development. Periods of heavy rainfall could lead to urban and small stream flooding and quick river rises.

At the upper levels, no significant changes in the overall pattern are expected initially, although current global models show some disagreement regarding the long term evolution. An upper-level low, also reflected in the surface pattern and associated frontal boundary, is forecast to swing north of the region across the Atlantic waters. Toward the end of the week, its influence could promote a shift in low-level winds from the south, bringing additional tropical moisture into the region and maintaining a warm and humid environment. Meanwhile, 500 mb temperatures will remain near -5 to -6C, with generally moderate lapse rates suggesting some potential for atmospheric instability. Cloud cover will play an important role in afternoon convective development, as extensive cloudiness could limit surface heating and inhibit convection, while periods of sunshine would favor stronger daytime heating and enhance shower development. Overall, expect warm and humid conditions, variable cloud cover, and periods of showers, particularly across interior and western Puerto Rico. Given the weak steering flow and potential for slow-moving showers, locally heavy rainfall, increased rainfall accumulations, and flooding impacts will remain possible. Continue to monitor the forecast as confidence increases and updated become available.

AVIATION

(18Z TAFS) Issued at 113 PM AST Sat Oct 31 2026

VFR conditions will generally prevail across all terminals through the period. However, SHRA/TSRA development over the interior of PR may cause tempo MVFR conds across the PR terminals through 31/22Z. Also, VCTS could prevail across all terminals including TIST/TISX thru 31/21z due to streamers developing downwind of the islands Surface winds will remain from the E to ESE at 10 to 15 kt with sea breeze variations, becoming light and variable after 31/23Z.

MARINE

Issued at 113 PM AST Sat Oct 31 2026

A surface high pressure over the central Atlantic will continue to promote light to moderate east to southeasterly winds across the regional waters for the rest of the weekend. Seas between will continue for the rest of the weekend, with ocasional higher seas, gusty winds, and slight visibility reduction near showers and thunderstorms. Monday onwards, building seas between 4 and 6 feet could lead to choppy conditions at times across the Atlantic and Caribbean offshore waters and local passages.

BEACH FORECAST

Issued at 113 PM AST Sat Oct 31 2026

The beach forecast remains on track. The latest nearshore buoys continue to report seas between 2 and 3 feet, resulting in breaking wave around 2 and 4 feet. Therefore, the moderate risk of rip currents continue for northern and eastern exposed beaches of Puerto Rico, including Culebra, and St. Croix in the U.S. Virgin Islands; low elsewhere. With the increment in winds, St. Thomas and St. John may also be upgraded to moderate tomorrow, Sunday. Beachgoers must exercise caution, as life-threatening rip currents are possible along beaches under moderate risk. The risk should gradually lower by the end of the next workweek as winds weaken, while no significant swell event is expected over the next few days.

SJU WATCHES/WARNINGS/ADVISORIES

PR...None. VI...None. AM...None.