textproduct: San Juan

KEY MESSAGES

Issued at 245 PM AST Fri Jul 31 2026

- Showers and thunderstorm will continue to affect the western half of Puerto Rico due to the passage of an easterly disturbance. Expect ponding of water over roads.

- A tropical wave will move across the region on Saturday, increasing the chance of showers and thunderstorms through the weekend, which may result in localized flooding and disruptions to outdoor activities.

- Low concentrations of Saharan dust will persist through Sunday night before increasing to moderate levels on Monday, which may reduce air quality and visibility while aggravating respiratory conditions for sensitive individuals.

- Hot conditions will return by Sunday across Puerto Rico and the U.S. Virgin Islands, increasing the risk of heat-related illnesses, especially for vulnerable populations, outdoor workers, and anyone spending extended periods outdoors.

Short Term(This evening through Sunday)

Issued at 245 PM AST Fri Jul 31 2026

Variable conditions prevailed this morning and through the early afternoon as a wind perturbation increased moisture from the surface to at least 700 mb. Combined with the influence of an upper-level low west of the region, these conditions have produced passing showers across windward areas and a greater coverage of afternoon convection across north-central and western Puerto Rico. Based on radar estimates, maximum rainfall amounts of 0.30 to 0.50 inches fell earlier today. Around 1 PM AST, a few thunderstorms developed, producing localized rainfall totals of 1 to 2 inches across north-central and western municipalities. These estimates are based on data available through 130 PM AST, with additional rainfall still occurring. Daytime high temperatures ranged from the mid 80s to the low 90s, with heat indices reaching between 100 and 110F across localized, typically warmer areas. The latest NASA GEOS Dust Aerosol Optical Depth (AOD) analysis suggests that only traces of Saharan dust are present today, and similar conditions are expected to persist through the forecast period, providing a temporary break from more significant dust concentrations.

At the surface, high pressure over the central Atlantic is maintaining an easterly wind flow. For the remainder of the afternoon, scattered to numerous showers and isolated thunderstorms are expected to continue, particularly across the western interior and western Puerto Rico. A limited to elevated flood risk remains in effect across western Puerto Rico due to the potential for ponding of water on roads and in poorly drained areas, as well as localized urban and small stream flooding. Tonight through early Saturday, the latest model guidance suggests a drying trend before the arrival of the next tropical wave.

The likelihood of rainfall will increase over the weekend, with the greatest potential expected on Saturday and possibly extending into Sunday. By late Saturday morning, moisture associated with a tropical wave is forecast to move into the region, accompanied by lighter easterly to southeasterly winds. Precipitable water (PWAT) values are expected to rise above normal, exceeding 2 inches on Saturday, with deeper moisture extending into the mid and upper levels of the atmosphere, according to the latest GFS cross-section analysis. At the same time, 500-mb temperatures are forecast to cool to around -7C, creating a more favorable environment for thunderstorm development. A limited to elevated flood risk will remain in place, especially since the lighter steering flow may allow showers and thunderstorms to move more slowly, resulting in higher rainfall accumulations. Modest moisture will persist, while the upper-level low lingers west of Puerto Rico on Sunday. As a result, afternoon showers and thunderstorms are expected to continue, with the greatest flooding threat focused across the interior, western, and northwestern portions of Puerto Rico, as well as possibly in areas downwind of El Yunque.

In addition, 925-mb temperatures are forecast to remain near to slightly above normal on Sunday, which may increase the likelihood of heat-related products being issued. Continue to monitor the forecast for updates as confidence increases and forecast uncertainty decreases.

Long Term(Monday through Friday)

Issued at 405 AM AST Fri Jul 31 2026

Mainly drier and warm-to-hot conditions are expected in the long- term forecast, with the best chance of flooding on Monday and Wednesday. The CWA should continue under an ESE-SE wind pattern due to surface high pressure over the Central Atlantic. As mentioned in the previous discussion, Monday should have been a transition day due to lingering moisture from a tropical wave and an upper-level low, with conditions becoming drier than normal during the morning hours as a dense plume of SAL with moderate concentrations reaches the local area, bringing hazy skies, reduced visibility, and deteriorated air quality. From the latest model guidance, PWAT values should plummet to below climatological normal (most likely between 1.4 and 1.6 inches), with a dry slot in the mid levels (below 40 %). Additionally, the Grand Ensemble shows members tending to drier conditions on both Monday and Tuesday, with low variability (PWAT difference of 0.2 - 0.3 inches). Nevertheless, Saharan dust concentrations may lower and low-level moisture content may increase on Wednesday due to the approach of an induced trough that should remain north of the CWA. From the latest model guidance, PWAT values may increase to seasonal values (between 1.5 and 1.7 inches), though theres variability between ensemble models, with ENS tending to wetter conditions compared to GEFS members. In terms of instability, some lingering troughiness may aid deep convection near the CWA, with cool mid-level temperatures (500 mb temperatures around -6 degrees Celsius). Additionally, the Galvez-Davison Index (GDI) suggests the potential of isolated thunderstorms due to afternoon convection, particularly over western portions of Puerto Rico. Taking into consideration well-above-normal temperatures (925 mb temperatures above the 75th percentile), combined with local effects and sea breeze convergence, afternoon convection could still be expected over western Puerto Rico, though they may become short- lived with low precipitation efficiency. Rainfall accumulations may lead to ponding of water over roadways and poorly drained areas, particularly on Monday and Wednesday. Drier- than-normal conditions should return on Thursday, with limited shower activity and no flooding or lightning threat.

As mentioned in previous discussions, model guidance continues to suggest warmer-than-normal conditions across the islands, particularly over urban and low-lying areas. Taking into account the available moisture and the influence of SAL, maximum temperatures could reach the low and mid 90s while heat indices will likely be between the mid 100s and low 110s, affecting anyone without effective cooling and/or exposed to prolonged sun exposure, including sensitive groups. Therefore, the heat threat will remain elevated to significant throughout the next workweek.

AVIATION

(18Z TAFS) Issued at 245 PM AST Fri Jul 31 2026

Mainly VFR conditions are present. TJBQ have the best chance to observe TSRA/VCSH thru 31/20-21Z. Visibilities should improve as just traces of Saharan dust will be present. The arrival of moisture from a tropical wave will increase the chance of showers from 01/14Z onward. MVFR conds will be more likely. Easterly winds, with gusts near 23 to 28 kts thru 31/23Z, then around 10 kts or less overnight. Winds increasing again after 01/13-14Z.

MARINE

Issued at 245 PM AST Fri Jul 31 2026

Moderate to fresh trades will prevail through next week as a surface high pressure continues to build over the central Atlantic. Choppy seas up to 6 feet will persist through Saturday across most local waters and passages. A tropical wave will move across the regional waters Saturday into Sunday, increasing shower and thunderstorm activity. Saharan dust concentrations will remain low through Monday.

BEACH FORECAST

Issued at 245 PM AST Fri Jul 31 2026

A moderate risk of rip currents will persist across the northwest to southeast beaches of Puerto Rico, Vieques, Culebra, and St. Croix through Saturday. A low risk is expected across the western to southern beaches of PR, and most beaches of St. Thomas and St. John through the weekend. Nevertheless, beachgoers should be aware that life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers even when the risk is low. By Tuesday, the risk will become low across all coastal areas.

FIRE WEATHER

Issued at 245 PM AST Fri Jul 31 2026

Minimum relative humidity values have remained near or above critical thresholds today. Influx of moisture during the weekend could aid in improving overall fire weather conditions. However, critically dry fuels persist across southern Puerto Rico and St. Croix, with Keetch-Byram Drought Index (KBDI) values remaining well above the critical Threshold. East to east-southeast winds are expected to remain above or near the critical levels through at least today, slightly which could promote the rapid spread of any fire that develops. Residents and visitors should avoid outdoor burning and exercise caution with activities that could generate sparks or open flames, especially across areas with critically dry vegetation.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory until 4 PM AST this afternoon for PRZ001>005-007- 008-010>013.

VI...Heat Advisory until 4 PM AST this afternoon for VIZ001-002.

AM...None.