textproduct: San Juan

KEY MESSAGES

Issued at 452 AM AST Sun Jul 19 2026

* Hot conditions will continue through early this week. Heat indices will generally remain below Heat Advisory criteria today, although a few urban and coastal locations could briefly reach advisory thresholds this afternoon. The heat threat is expected to increase on Monday and Tuesday, and Heat Advisories may be needed across portions of the lower elevations.

* A weak tropical wave passing mainly south of the islands this evening will bring a slight increase in passing showers, but widespread rainfall is not expected. Moisture will increase further on Tuesday as a trough approaches, resulting in more numerous showers, isolated thunderstorms, and a limited risk of localized urban and small-stream flooding.

* Low to moderate concentrations of Saharan dust will overspread the islands today through Monday, resulting in hazy skies, reduced air quality, and locally reduced visibility.

* Elevated fire weather conditions will persist today across portions of Puerto Rico, where critically dry vegetation, low relative humidity, and breezy conditions will continue to support rapid fire spread.

Short Term(Today through Tuesday)

Issued at 452 AM AST Sun Jul 19 2026

A few passing showers moved across the local waters and windward sections during the overnight hours, but rainfall accumulations remained light. Temperatures stayed seasonably warm overnight.

A mid-level ridge will remain the dominant weather feature today and Monday, maintaining relatively warm temperatures aloft and below- normal moisture between 700 and 500 mb. Model guidance continues to indicate precipitable water values generally between 1.2 and 1.4 inches through tonight as a weak tropical wave passes mainly south of the local islands. Although the wave will bring a modest increase in low-level moisture, it is embedded within an increasing Saharan Air Layer (SAL), limiting vertical moisture depth and resulting in little overall change to the prevailing weather pattern. Consequently, passing trade wind showers will continue to affect the windward sectors from time to time, while afternoon convection remains confined to portions of western Puerto Rico, where activity should be brief and localized due to persistent mid-level dry air and subsidence.

Saharan dust concentrations are expected to increase today, peaking on Monday, leading to hazy skies, reduced visibilities at times, and deteriorating air quality, especially for sensitive groups. The associated dry air mass will continue to suppress widespread convection despite the modest increase in low-level moisture.

By Tuesday, a tropical upper tropospheric trough (TUTT) low is forecast to sink southward into the northeastern Caribbean while its associated surface-induced inverted trough crosses the local area. This pattern will promote a deeper moisture profile, with precipitable water increasing to around seasonal values, along with higher mid-level humidity and slightly cooler temperatures aloft. As a result, shower coverage is expected to increase across the islands, with scattered afternoon showers and isolated thunderstorms developing, mostly across western Puerto Rico. Locally heavy rainfall may result in ponding of water on roads and localized urban and small-stream flooding, particularly where showers repeatedly affect the same areas.

High temperatures will once again climb into the upper 80s to low 90s across the urban and coastal areas today. However, a slightly more east-northeasterly wind component should provide modest relief compared to recent days, with heat indices generally remaining below Heat Advisory criteria. Nevertheless, a few localized urban and coastal areas could briefly reach Heat Advisory thresholds during the peak afternoon hours. While the overall heat threat is expected to remain limited today, residents and visitors should continue to take precautions to avoid heat-related illnesses, especially those spending extended periods outdoors. By Monday, the combination of warmer overnight temperatures associated with the Saharan Air Layer and persistent hazy conditions may lead to elevated daytime heat indices, and Heat Advisories may become necessary for portions of the urban and coastal areas. On Tuesday, increasing low-level moisture combined with continued warm temperatures may further enhance the heat threat across lower elevations, potentially requiring additional Heat Advisories despite the expected increase in shower and thunderstorm activity later in the day.

Long Term(Wednesday through Sunday)

Issued at 452 AM AST Sun Jul 19 2026

During the beginning of the long-term period, the primary weather feature will continue to be an upper-level low inducing a surface trough northeast of the region. This pattern will weaken the Atlantic high pressure system, promoting lighter winds across the islands. The latest guidance suggests that environmental conditions will be marginally favorable for convective activity. Winds will become lighter from the east-northeast. At 500 mb, temperatures are forecast to cool from above-normal values to near-seasonal levels, around -6C to -7C. In addition, lower 250 mb heights, near- seasonal mid-level relative humidity, and precipitable water (PWAT) values remaining around 1.7 to 1.9 inches will support a more favorable environment for shower and thunderstorm development. A limited flood risk is highlighted across west- central and western Puerto Rico due to the potential for ponding of water on roads and in poorly drained areas, along with a low chance of urban and small- stream flooding.

Additionally, on Wednesday, the combination of light southeasterly flow, seasonal moisture, and 925 mb temperatures around two standard deviations above normal is expected to result in the warmest day of the week. Heat index values will likely reach hazardous levels each afternoon, especially across urban and coastal locations. A significant heat risk is currently highlighted for Wednesday. Heat products will likely need to be issued.

By Thursday and through the remainder of the forecast period, slightly drier air is expected to filter into the region, with PWAT values fluctuating between seasonal and below-normal levels, resulting in more limited shower activity. Winds are forecast to strengthen once again, becoming breezy to locally windy through the weekend. The easterly steering flow will continue to favor passing showers across windward areas during the nighttime and morning hours, followed by afternoon showers over western Puerto Rico driven by local effects and available moisture. The flood risk is expected to remain low, although a few isolated thunderstorms will remain possible each afternoon across western Puerto Rico. In terms of heat, warm to hot conditions are likely to persist most days. An elevated heat risk is currently in place for Thursday, while a limited heat risk is expected on Friday. Continue to monitor the forecast for updates. Residents and visitors are encouraged to continue practicing heat safety measures to reduce the risk of heat- related illnesses.

AVIATION

(06Z TAFS) Issued at 452 AM AST Sun Jul 19 2026

VFR conds will prevail at all TAF sites thru the fcst pd. ENE winds 8-12 kt will increase to 15-20 kt with gusts up to 25-30 kt aft 19/13Z, bcmg lighter (8-12 kt) aft 19/23Z. VCSH psbl at times, with brief SHRA mainly affecting ern terminals during the mrng and wrn PR during the aftn. HZ associated with SAL may reduce vsby to 6 SM at TIST/TISX aft 19/22z. No sig operational impacts expected.

MARINE

Issued at 452 AM AST Sun Jul 19 2026

A surface high pressure over the central Atlantic interacting with the PanamaColombia Low will maintain moderate to locally fresh easterly winds and moderate to locally choppy seas through much of the forecast period. Small Craft should Exercise Caution across most of the local waters today and tonight. A tropical wave will remain well south of Puerto Rico and the U.S. Virgin Islands as it moves westward across the southern Caribbean from late tonight into tomorrow, Monday. Meanwhile, a dense Saharan Air Layer will move over the region from the east tonight through tomorrow, Monday, leading to hazy skies, reduced air quality, and locally reduced visibilities.

BEACH FORECAST

Issued at 452 AM AST Sun Jul 19 2026

Breezy to locally windy conditions will promote a moderate risk of rip currents (rip currents are possible in the surf zone) at most of the local coastline/surf zones. This moderate risk is forecast to persist through at least tomorrow, Monday, and then return by Thursday. Generally weaker winds will result in a low risk of rip currents on Tuesday and Wednesday. However, even if the risk of rip currents is low, life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers. Beachgoers should swim near lifeguards whenever possible, heed beach warning flags, and never swim alone.

A limited to elevated risk of excessive heat is also expected at many local beaches this week, possibly reaching significant levels on Wednesday. Stay well hydrated, seek shade whenever possible, and use sunscreen to reduce the risk of heat-related illness. Saharan Dust concentrations are forecast to increase tonight into tomorrow, Monday.

FIRE WEATHER

Issued at 452 AM AST Sun Jul 19 2026

A Fire Danger Statement (RFD) is in effect for the local fire weather zones. Critically dry fuels and significant soil moisture deficits persist across portions of southern Puerto Rico, with KBDI values remaining above critical thresholds. Although a weak tropical wave approaching from the east will result in a slight increase in low-level moisture later today, the deepest moisture is expected to remain well south of the local islands, with little impact on fire weather conditions through most of the day.

Relative humidity values are still forecast to fall into the upper 40s to lower 50s this afternoon across the southern coastal plains of Puerto Rico and adjacent lower elevations, while easterly to east- northeasterly winds of 15 to 20 mph with higher gusts will continue to support elevated fire weather conditions. Any rainfall associated with the tropical wave is expected to be light and localized, providing little to no meaningful relief to the ongoing dry conditions. Residents and visitors should avoid outdoor burning and exercise caution when using open flames or other potential ignition sources.

SJU WATCHES/WARNINGS/ADVISORIES

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