textproduct: San Juan

KEY MESSAGES

Issued at 355 AM AST Sat Sep 12 2026

- Moderate to high concentrations of Saharan dust will continue today and tonight, bringing hazy skies, reduced visibility and deteriorated air quality.

- A Heat Advisory is in effect for Puerto Rico, Vieques, Culebra and the U.S. Virgin Islands today from 10 AM to 5 PM. Elevated heat risk will continue over the next several days.

- Increasing shower and thunderstorm activity is expected on Sunday and Monday, especially across the interior and western portions of Puerto Rico, with a limited to elevated flood risk.

- Moderate rip current risk is expected through the weekend along northern, eastern and southeastern beaches of Puerto Rico, as well as the beaches of Vieques, Culebra and St. Croix. Rip currents could still occur in low-risk areas.

Short Term(Today through Monday)

Issued at 355 AM AST Sat Sep 12 2026

Hazy skies, deteriorated air quality and reduced visibilities will continue today and tonight as moderate to high concentrations of Saharan dust continue across the region. Individuals in sensitive groups, such as those with respiratory conditions, young children, and older adults, should take appropriate precautions, limit prolonged outside activities and follow the recommendations of health officials, as these conditions will continue throughout the day today into tonight. Warm to hot conditions will also be a concern today as we expect an elevated heat risk. A Heat Advisory is in effect for Puerto Rico, Vieques, Culebra and the U.S. Virgin Islands (USVI) from 10 AM to 5 PM today; please continue to monitor the forecast for any updates, as it could be upgraded to an Extreme Heat Warning for some areas later today. The Saharan Air Layer (SAL) will also maintain unfavorable low-level thermodynamics with a moderate to strong inversion near 850 mb and dry air in the low to mid- levels, despite the presence of an upper-level low just northwest of the local area. Winds will shift a bit more from the east- northeast today. Therefore, expect mainly tranquil conditions with very limited shower activity across the area with some isolated passing showers possible in the USVI and eastern Puerto Rico. In the afternoon, strong diurnal heating combined with the sea breeze and local effects may still result in some isolated convective activity over portions of the western-interior and towards the southwest, but they should be isolated in nature.

The SAL will exit the region later tonight and early morning on Sunday, with low concentrations of Saharan dust expected Sunday and Monday, leaving slightly more favorable conditions for shower activity both days. This will allow for a slight increase in the precipitable water content, around 1.6 inches which is still below normal, with slightly better mid-level moisture as well as a weaker low-level temperature inversion. Steering winds will shift again to more easterly on Sunday, while the upper-level low is still west-northwest of the area with 500 mb temperatures remaining around -7 degrees Celsius, which is favorable for convective activity. Therefore, expect better chances for passing showers across the USVI and eastern portions of Puerto Rico during the overnight and morning hours on Sunday. Then, by Sunday afternoon, expect more scattered showers and isolated thunderstorms across the interior and western portions of Puerto Rico, as well as the potential of shower activity towards portions of the metropolitan area of PR and downwind from the islands. Limited flooding risk will be present in some of these areas, especially over the western-interior and western PR, so prepare for the possibility of ponding of water in roadways and poorly drained areas.

Better moisture filters in by Monday as a low-level perturbation starts to pool more moisture from the southeast, with precipitable water values more in the seasonal range between 1.6-1.8 inches, with winds shifting again to a more east-southeasterly direction. In the meantime, the upper-level low will remain in a favorable position for convective activity and the 500 mb temperatures will remain between -7 and -8 degrees Celsius. Therefore, for Monday expect the potential for passing showers over the USVI and eastern, southeastern Puerto Rico and over the local waters, especially across the Caribbean waters. In the afternoon hours, another round of scattered showers and isolated thunderstorms is expected across the western-interior and western Puerto Rico, extending into the northwest, as well as downwind from El Yunque and the islands. Since the steering winds will be slower this day compared to early in the short term period, there will be higher flooding threat on this day. Hence, we expect a limited to elevated flood threat with the potential for flooding in urban areas and along small streams.

In terms of the heat, elevated risk is expected as more moisture will allow heat indices to increase more than in recent days, especially in areas that don't experience rainfall.

Long Term(Tuesday through Saturday)

Issued at 355 AM AST Sat Sep 12 2026

Some changes were introduced to the long-term forecast, with Tuesday and Thursday having more flooding potential. A surface high pressure across the Central Atlantic will continue to promote easterly- southeasterly breezy winds for most of the period. A transition to a wetter pattern is still expected for the long term period. By Tuesday, moisture associated with a tropical wave combined with the departing upper-level low near Hispaniola will result in PWAT values up to 2.00 inches, near-above climatological normal. Additionally, the vicinity of the upper-level low will likely lead to cooler-than- normal mid-level temperatures (500 mb temperatures between -7 and -8 degrees Celsius) and divergence aloft, allowing cloud growth and enhancing instability. By Wednesday, lingering moisture from these synoptic features, daytime heating, and local effects could enhance convection, especially over western and interior Puerto Rico in the afternoon. However, a shallow drier air slot could be present on the low-to-mid levels (700 - 850 mb), leading to limited shower and thunderstorm activity. By Thursday morning, the latest model guidance continues to suggest a tropical wave approaching the CWA, pooling deep tropical moisture (PWAT values to 2.20 inches). Additionally, the latest model solution suggests enhanced areas of vorticity associated with the wave, increasing the likelihood of an advective weather pattern across windward sections. Lingering moisture on Friday will enhance convection, especially over western and interior portions of Puerto Rico in the afternoon.

With these conditions expected for the long term, both Tuesday and Thursday possess the highest potential for showers and thunderstorms across Puerto Rico and the US Virgin Islands. Rainfall accumulations could lead to urban and small stream flooding, particularly across interior and western Puerto Rico, with a limited flooding risk across the US Virgin Islands and eastern Puerto Rico on Thursday. Meanwhile, on Friday, afternoon convection could lead to ponding of water over roadways and poor drainage areas.

As mentioned in the previous discussion, warmer than normal conditions could still be expected in the long-term forecast, as model guidance suggest seasonal to above normal 925 mb temperatures. Heat indices during peak hours are very likely to exceed 100 degrees Fahrenheit, particularly across urban and low-lying areas. Therefore, an elevated to significant heat threat will prevail throughout the period.

AVIATION

(06Z TAFS) Issued at 355 AM AST Sat Sep 12 2026

Mainly VFR conditions are expected to prevail across all terminals during the next 24 hours. However, trade wind -SHRA could move at times across the USVI terminals. HZ due to Saharan dust will continue across the eastern Caribbean through late tonight, and VSBY may drop to 6SM. East winds expected to increase between 12-18 kt with sea breeze variations and stronger gusts at times aft 12/13z.

MARINE

Issued at 355 AM AST Sat Sep 12 2026

The pressure gradient between a weak surface high north of the region and the Colombian low will maintain moderate to fresh trades through the weekend, mainly across the Caribbean waters and passages. These winds will result in choppy seas of 4 to 5 feet, occasionally up to 6 feet. Small craft should exercise caution. Saharan dust will continue through tonight, limiting shower activity and reducing visibility. Moisture will increase early next week, favoring an increase in shower activity.

BEACH FORECAST

Issued at 355 AM AST Sat Sep 12 2026

Hazy skies, poor air quality, and reduced visibility will continue today due high concentrations of Saharan dust. Heat will also remain an additional concern over the coming days. Beachgoers should stay hydrated and limit prolonged sun exposure.

A moderate risk of rip currents will continue along the northwest to southeastern beaches of Puerto Rico, as well as in Culebra, Vieques, and the U.S. Virgin Islands through the rest of the weekend. Life-threatening rip currents remain possible in these areas, while a low risk is expected along western and more protected beaches. However, rip currents can still occur near groins, jetties, reefs, and piers.

Afternoon thunderstorms are expected to develop mainly across western Puerto Rico from Sunday onward. Beachgoers should remain alert for lightning and rapidly changing weather conditions, and seek shelter if thunderstorms approach.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ001-002.

AM...None.