textproduct: San Juan

KEY MESSAGES

Issued at 100 PM AST Mon Jul 20 2026

* A Heat Advisory has been issued for certain coastal and urban areas of Puerto Rico and the U.S. Virgin Islands based on recent observations. This afternoon, heat indices are expected to exceed 107F. The heat threat is projected to intensify further on Tuesday and Wednesday as increased tropical moisture returns. Additional Heat Advisories or even Extreme Heat Warnings may be issued for some lower elevation areas.

* Hazy skies associated with Saharan dust will persist through the rest of today, with conditions gradually improving later this afternoon.

* A shift to a more unsettled weather pattern is anticipated from Tuesday through Wednesday, with additional changes expected later in the weekend and early next week as environmental conditions become more favorable for showers and isolated thunderstorms, especially in the afternoons.

* There is also a slight risk of localized urban and small-stream flooding. Elevated fire weather conditions will persist today, as critically dry vegetation, low relative humidity, and breezy conditions continue to support rapid fire spread. Multiple fire incidents have already been reported across Puerto Rico, highlighting the ongoing fire danger.

* The prevailing easterly winds will maintain a moderate risk of life-threatening rip currents across many beaches of Puerto Rico and the U.S. Virgin Islands throughout much of the week. Beachgoers should swim near lifeguards, heed local beach warnings, and never swim alone.

Short Term(This evening through Wednesday)

Issued at 100 PM AST Mon Jul 20 2026

A warmer-than-normal morning was observed across Puerto Rico and the U.S. Virgin Islands, with minimum temperatures remaining in the upper 70s to lower 80s across most coastal and urban locations. These warmer overnight temperatures were likely enhanced by moderate Saharan dust concentrations over the region, which limited nocturnal cooling. By early afternoon, temperatures had already climbed into the lower 90s at many urban and coastal observing sites. Combined with the lingering low-level moisture associated with yesterday's tropical wave, the increasing temperatures have produced heat indices exceeding advisory criteria, prompting the issuance of a Heat Advisory for portions of the coastal and urban areas of Puerto Rico and the U.S. Virgin Islands. Surface winds were generally from the east at 10 to 20 mph, with occasional higher gusts. Visible satellite imagery and local observations showed mostly sunny skies across much of the region during the morning, while cumulus clouds developed across the Cordillera Central and western Puerto Rico in response to daytime heating, local topographic effects, and sea-breeze convergence. In addition, trade wind streamers developed downwind of Vieques, Culebra, St. Thomas, and St. Croix, producing isolated showers over the local waters and adjacent coastal areas.

Although a drying trend is expected to develop later this afternoon as a pocket of drier air advects into the region on the easterly trade winds, local effects will continue to support afternoon convection across the interior and western sections of Puerto Rico. Daytime heating, sea-breeze convergence, and orographic lifting will promote the development of scattered showers, while trade wind streamers are expected to develop downwind of the U.S. Virgin Islands. Most of these streamers should remain over the local waters; however, those originating from Vieques and Culebra may drift westward and produce passing showers across eastern Puerto Rico.

The Saharan Air Layer will continue to exit the region this evening and will gradually be replaced by an induced surface trough moving westward across the northeastern Caribbean. This feature will increase low-level moisture and enhance shower activity from late tonight through Tuesday, with the greatest coverage expected from mid-morning into the afternoon as local effects become established. While localized ponding of water and isolated urban and small-stream flooding cannot be ruled out under the heaviest showers, the anticipated rainfall will provide much- needed moisture to portions of Puerto Rico and the U.S. Virgin Islands that continue to experience rainfall deficits. According to the latest U.S. Drought Monitor, areas across the islands remain classified under D0 (Abnormally Dry), D1 (Moderate Drought), and D2 (Severe Drought).

A transition to a more unsettled weather pattern is expected on Tuesday as an induced surface trough moves across the northeastern Caribbean. At the same time, the mid-level ridge will weaken, allowing moisture to deepen to near or slightly above climatological levels. This will support more widespread shower activity and isolated thunderstorms, particularly during the afternoon across the interior, western, and southwestern portions of Puerto Rico. Locally heavy rainfall may result in ponding of water and isolated urban and small-stream flooding.

On Wednesday, low-level winds are forecast to veer to the east- southeast, shifting the focus of afternoon convection toward western and northwestern Puerto Rico. This wind shift will also favor warmer temperatures across the northern and western coastal and urban areas. As a result, heat indices may reach Heat Advisory or even Extreme Heat Warning criteria in portions of the lower elevations. At the same time, scattered afternoon showers and isolated thunderstorms continue to develop across the interior and western Puerto Rico.

Long Term(Thursday through next Sunday)

Issued at 441 AM AST Mon Jul 20 2026

For the second half of the workweek, upper level lows will continue to linger northeast and then east-southeast of the region. A surface high over the Atlantic will reestablish itself and once again result in up to breezy conditions, especially Friday and Saturday. Current model guidance has Precipitable water (PWAT) values on a downward trend to start the long term period as moisture from a surface trough moves northeast and away from the region under an east- southeast steering flow. PWAT values should stay within normal values on Thursday, with locally above normal values being observed during afternoon convection over mainly W-NW PR. A drier air mass with below normal PWAT will then quickly move Thursday night into early Friday, before another pulse of moisture reaches the area Friday morning. More humid air masses will then move over the area through the weekend as waves continue moving close to the region, this will maintain a generally normal PWAT over the islands as showers continue to be steered towards windward sectors and afternoon convection (showers and possible t-storms) develop over mainly the western half of Puerto Rico. At 500 mb, temperatures are forecast to cool from above-normal values to near-seasonal levels, around -6C to -7C, late Thursday onwards. Most available moisture should remain below 800mb through at least Saturday while 250 mb heights remain at seasonal values.

By late Sunday through Monday, current guidance suggests that a tropical wave and wind surge will reach the area and elevate PWAT values to above normal values and result in breezy to locally windy E-SE flow. This will increase the flooding risk as quick moving passing showers reach the region and afternoon convection develops over mainly W-PR and downwind of El Yunque and the local islands. As an upper trough moves over the region, available moisture will also reach the upper levels. Warm to hot conditions will persist through at least the weekend, with an elevated heat risk forecast for coastal and lower elevations of the islands. In terms of Saharan Dust, low to moderate concentrations will continue to filter over the islands through the second half of the week and early next week, with guidance suggesting lower concentrations as the above mentioned moisture reaches the islands late Sunday and Monday. Continue to monitor the forecast for updates.

AVIATION

(18Z TAFS) Issued at 100 PM AST Mon Jul 20 2026

VFR conds across all TAF sites. HZ will gradually diminish through 07/23Z. E-NE winds will continue, btwn 15 - 20 kt, gusty winds up to 30 kt, and weakening btwn 5 - 10 kt. Expect VCSH across TIST, TISX, and TJSJ from 21/06Z through 21/13Z.

MARINE

Issued at 100 PM AST Mon Jul 20 2026

A high-pressure system over the central Atlantic is expected to bring moderate to fresh east to east-southeast winds, along with choppy seas, to the region through this evening. An induced surface trough will enhance the likelihood of showers and thunderstorms, as well as increase sea conditions and wind strength near any thunderstorms on Tuesday and Wednesday. Additionally, another tropical wave is forecast to move near the Lesser Antilles by Thursday and approach the region by Friday.

BEACH FORECAST

Issued at 100 PM AST Mon Jul 20 2026

Easterly trade winds will enhance wave-breaking action along many beaches in Puerto Rico (PR) and the U.S. Virgin Islands (USVI) this week, leading to a moderate risk of life-threatening rip currents at north-, east-, and south-facing beaches through the weekend. Beachgoers should swim near lifeguards, obey beach warning flags, and never swim alone.

There is also a risk of extreme heat, especially from late morning to afternoon, due to high temperatures, humidity, and limited shade. To minimize heat-related illnesses, beachgoers should stay hydrated, seek shade, and apply sunscreen regularly.

Starting Tuesday, afternoon showers and isolated thunderstorms are expected, particularly in western Puerto Rico. While most beaches should remain dry, beachgoers should be cautious of thunderstorms, as lightning can strike far from rain. If thunder is heard, leave the beach and seek shelter in a substantial building or hard- topped vehicle.

FIRE WEATHER

Issued at 100 PM AST Mon Jul 20 2026

A Fire Danger Statement remains in effect for the local fire weather zones. Observations continue to support elevated fire weather conditions across the southern half of Puerto Rico and the U.S. Virgin Islands. Critically dry fuels and persistent soil moisture deficits persist across portions of the islands, with Keetch-Byram Drought Index (KBDI) values exceeding critical thresholds. Multiple fire incidents have been reported across Puerto Rico today, highlighting the heightened fire danger. Combined with breezy easterly trade winds, any new fires that develop could spread rapidly through dry vegetation, particularly across Puerto Rico's southern coastal plains. Residents and visitors are urged to avoid outdoor burning and to exercise extreme caution with open flames, discarded cigarettes, and any activity that could generate sparks.

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.