textproduct: San Juan

KEY MESSAGES

Issued at 420 AM AST Tue Oct 6 2026

- Hot and humid conditions are expected today before afternoon convection develops. A Heat Advisory is in effect from 10 AM to 4 PM AST across Puerto Rico and the U.S. Virgin Islands. The heat risk will be higher along the coasts, in lowlands, interior valleys, and urban areas.

- Afternoon showers and thunderstorms will develop today, mainly across interior into western Puerto Rico, with a limited to elevated flood risk.

- An approaching tropical wave will bring a wetter and more active weather pattern from late tonight into Thursday, maintaining mainly an elevated flood risk.

- A moderate risk of rip currents is expected today through Thursday along portions of the north and east-facing beaches of the islands, with a lower risk returning Friday into the weekend.

Short Term(Today through Thursday)

Issued at 420 AM AST Tue Oct 6 2026

Passing showers developed overnight across the offshore waters, the US Virgin Islands and eastern Puerto Rico leaving minimal accumulation. A deep convective cell developed but remained offshore to the northwest of Puerto Rico. Heat indices ranged in the mid-to-upper 80s. A calm to locally breezy steering flow remained from the ESE bringing warm and humid air into the region.

A high pressure system over the central Atlantic ocean will continue to promote the ESE wind flow with speed varying from calm conditions on Tuesday, to breezy from Wednesday into Thursday. A couple of mid- level ridges will enhance warming and lead to the weakening of the lapse rates at these levels. This feature will act to limit the development of deep convective cells. Nonetheless, local effects and humid conditions throughout the short term period should provide the sufficient instability for the development of convective activity each day.

Today will be characterized by below seasonal precipitable water values and dry mid-levels during the morning hours across the CWA. Conditions will gradually become more favorable for the development of convection into the afternoon as PWAT values increase and the atmospheric column moistens across the western interior and northwestern Puerto Rico. Coupled to local effects, this will favor the development of showers and localized thunderstorms. While the focus of the activity for tomorrow will be in the aforementioned region, the US Virgin Islands and eastern Puerto Rico may also receive rainfall associated to advective showers during the morning and night time, as well as from streamers from El Yunque and the smaller islands during the afternoon. Thus, there is a moderate flooding risk expected across this region. This means that flooding in urban areas, roads, small streams and washes as well as a low chance of isolated floods is possible.

Wednesday will be the most active weather day in the short term forecast as a tropical wave filters into the region on Wednesday morning. Model guidance suggest that in contrast to today, this system will bring above average PWAT values and will moisten the atmospheric column increasing the potential for widespread convection and thunderstorm activity. Where deep convective cells develop individuals may experience downpours and stronger wind gusts. Recurrent showers will increase the flooding potential. Thus, a moderate flooding risk will be present on Wednesday across the areas expected to be impacted by heavy rainfall. Citizens and visitors should remain updated with the latest forecasts.

A typical pattern for the season is expected by Thursday. Lingering moisture from the tropical wave should remain over the region further enhancing the development of showers and thunderstorms. The focus of the activity will be on the western interior and northwestern sectors of Puerto Rico during the afternoon where the bulk of the associated moisture will remain from the wave passage.

The available moisture, 925mb temperatures 2 standard deviations above average, and weak surface winds, the heat risk for today is expected to be elevated. Thus a heat advisory is in effect from 10 am to 4 pm today. The enhanced convective activity expected for the remainder of the short term forecast will lead to a limited heat risk on Wednesday and is expected to increase to elevated on Thursday. These levels of heat may affect individuals sensitive to heat, without effective cooling and adequate hydration.

Long Term(Friday through Tuesday)

Issued at 420 AM AST Tue Oct 6 2026

A weak mid-to-upper-level ridge is expected to build just north of the region on Friday. This ridge will promote mid-level warming, suppress lapse rates, and introduce drier air aloft, significantly capping widespread convective development. On Friday, the 500 mb temperatures are forecast to increase to near -4C, roughly two standard deviations above normal for early October, while precipitable water values temporarily drop to around 1.50 inches. Despite this overall drying trend, intense daytime heating, along with local sea breeze convergence, will still trigger localized afternoon showers across the western interior of PR. A similar pattern will continue on Saturday, with a slight influx of low-level moisture supporting brief, passing trade wind showers over windward areas during the overnight and early in the morning, followed by typical afternoon shower development over the interior and western PR.

By Sunday and into early next week, global model guidance suggests a more unstable atmosphere, as an upper-level trough or cut-off low digs near or just southeast of the islands. This will promote anomalously low geopotential heights and cold temperatures aloft, with 500 mb temperatures dropping sharply to near -8C by Sunday and Monday, and steeper mid-level lapse rates. Concurrently, broad-scale velocity potential guidance indicates a highly favorable environment for upper-level divergence over the Caribbean basin, strongly supporting deep, organized convection across the islands. Alongside these upper-level dynamics, a surge of deep tropical moisture will pool across the area, driving seasonal precipitable water values of near 2.00 inches.

Given the highly unstable environment, saturated atmospheric column, and excellent upper-level venting, widespread showers and strong thunderstorms are anticipated across the forecast area. The primary impacts will be an elevated risk of urban and small-stream flooding, localized flash flooding, and frequent lightning, particularly over the mountain ranges of PR during the afternoon hours. However, the USVI could also see periods of heavy rainfall. Light low-level steering flow will keep showers slow-moving through Monday, supporting the increasing flood threat. By Tuesday, trade winds are expected to strengthen as a surface high pressure builds north of the region, increasing low-level winds around 20 knots; however, they will turn more southeast, and 925 mb temps could rise well above seasonal levels and increase the heat threat once again across the islands.

AVIATION

(06Z TAFS) Issued at 420 AM AST Tue Oct 6 2026

VFR conds expected through most of the forecast period. Passing VCSH across TJSJ and the USVI sites early today. Aft 06/17Z, -SHRA/-TSRA are expected to develop across the interior into W PR, producing effects at TJBQ/TJPS, and streamers may also briefly affect TJSJ. As a result, intermittent/brief MVFR conds due to vis reductions, lower cigs, and mountain obsc. ESE winds will increase to 10-12 kt, with typical sea breeze variations. VCSH/-SHRA are forecast to increase tonight across USVI TAF sites and TJSJ due to an approaching tropical wave.

MARINE

Issued at 420 AM AST Tue Oct 6 2026

The surface high over the western Atlantic continues to move east, as a low (frontal boundary) makes its way into the western Atlantic, easing the pressure gradient and resulting in light to moderate east to east- southeast winds. A small, distant, northerly swell will reach the Atlantic waters tonight through late Thursday. Winds will increase somewhat Tuesday night into Wednesday as another surface high builds over the western Atlantic, and a tropical wave approaches the local area, slightly tightening the pressure gradient. The tropical wave will cross the local area early Wednesday into Thursday, increasing the shower and thunderstorm activity.

BEACH FORECAST

Issued at 420 AM AST Tue Oct 6 2026

Small north-northeasterly swell will result in a moderate rip current risk through Thursday along the north and east-facing beaches of Puerto Rico, Vieques, Culebra and the U.S. Virgin Islands. However, life-threatening rip currents can still occur near groins, jetties, reefs, and piers. The low risk is expected to return across all beaches Friday and into the weekend. Keep in mind that the daily afternoon lightning risk will also affect local beaches, primarily across northern and western Puerto Rico. Seek shelter immediately if thunder is heard.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon for VIZ001-002.

AM...None.