textproduct: San Juan

KEY MESSAGES

Issued at 352 AM AST Sun Jul 5 2026

* Moderate to high concentrations of suspended Saharan dust will affect the islands through Tuesday and then return Thursday onwards. Expect hazy skies, reduced visibility, and deteriorating air quality. Increased heat risk is anticipated each afternoon.

* Breezy to locally windy conditions will sustain up to a Moderate Risk of Rip Currents and "Small Craft Should Exercise Caution" conditions throughout most of the week.

* A tropical wave can increase the risk of heavy rainfall, thunderstorms, and flooding by Wednesday. Continue to monitor the forecast for updates.

* Elevated fire weather conditions are expected across the southern coastal plains of Puerto Rico today. Relatively dry and breezy conditions will favor the rapid spread of any fires that develop.

Short Term(Today through Tuesday)

Issued at 352 AM AST Sun Jul 5 2026

During the overnight hours, partly to mostly cloudy skies prevailed across the islands while moderate to high concentrations of Saharan Dust filtered into the region. From midnight to 3 AM AST, radar detected no rainfall accumulations. Due to overnight heat trapping by the Saharan Air Layer (SAL), temperatures remained in the upper 70s to low 80s across coastal and lower- elevation sectors, while dropping into the 60s across the interior. Recent satellite-derived precipitable water (PWAT) imagery indicates generally normal values across the area. Today, the continuous influx of moderate to high concentrations of Saharan Dust will promote hazy skies, reduced visibilities, and deteriorated air quality; sensitive groups should follow medical advice. Although PWAT values are expected to stay generally within a normal range despite some fluctuations, a drier mid-level air mass will confine most available moisture below 800 mb, suppressing widespread rainfall and passing showers over windward sectors across Puerto Rico and the U.S. Virgin Islands. Nevertheless, diurnal heating, local effects, and pockets of low- level moisture will support limited afternoon showers across interior and western Puerto Rico, as well as downwind of the USVI, Vieques, Culebra, and El Yunque. Additionally, an upper-level low lingering north of the islands (will linger through most of the period), combined with cool (below normal) 500 mb temperatures and Galvez- Davidson Indices (GDI) reaching 30, can support some lightning strikes mainly over western Puerto Rico.

From tonight through Tuesday, breezy east toeast-southeast winds will continue, steering both Saharan Dust and patches of tropical moisture to maintain normal to locally above- normal PWAT values over the islands. Moderate concentrations of Saharan Dust are forecast to persist tonight through Tuesday, becoming locally high at times, resulting in continued hazy skies and reduced air quality; sensitive groups should follow medical advice. Although 500 mb temperatures and 700 to 500 mb lapse rates will be at more seasonal values, the latest guidance indicates GDI values up to 40 on Monday and 30 on Tuesday. Combined with surges of tropical moisture, this will increase the frequency of passing showers across eastern Puerto Rico and the U.S. Virgin Islands, with afternoon showers and t-storms subsequently developing over the interior and western portions of Puerto Rico where a limited flooding threat is possible, as well as downwind of the USVI, Vieques, Culebra, and El Yunque. Despite lingering dry air in the mid-levels, isolated thunderstorms remain a possibility if daytime heating and local sea-breeze convergence become strong enough to weaken the trade-wind inversion associated with a mid- to-upper- level ridge.

A warm-to-hot weather pattern will encompass the islands during the period due to diurnal heating, particularly from mid-morning into the afternoon hours, and overnight trapping by the Saharan dust layer. With 925 mb temperatures expected to persist at high- end normal values, the issuance of a Heat Advisory may be required later today, depending on the development of the cloud deck that is currently moving over the region during the morning to early afternoon hours.

Long Term(Wednesday through Sunday)

Issued at 352 AM AST Sun Jul 5 2026

The long-term remains on track, though the flooding threat may extend into Thursday. As mentioned in the previous discussion, the wind pattern should remain mostly from the E-ESE, as a surface high pressure builds north of the CWA and maintains breezy to windy conditions across the islands. The latest model guidance continues to suggest a well-defined tropical wave approaching the Caribbean Basin and moving across the CWA on Wednesday, along with a wind surge. This system should pool tropical moisture into the region, with low- and mid-level RH peaking around 80%, well above the climatological normal. Additionally, Grand Ensemble members (including GEFS and ENS) continue to tend to wetter conditions, with PWAT values reaching 2.0 - 2.2 inches. As mentioned in the previous discussion, model guidance suggests cooler than normal mid-level temperatures (500 mb between -8 and -9 degrees Celsius), enhanced vertical motion, and vorticity, indicating favorable conditions for deep convection. In addition, GDI values remain high on Wednesday, particularly over the regional waters and portions of eastern and northwestern Puerto Rico. Hence, the discussed scenario prevails, with showers and isolated thunderstorms moving into windward sections of Puerto Rico and the U.S. Virgin Islands during the morning, followed by afternoon convection over interior and northwestern Puerto Rico. Due to strengthening winds, daytime heating, and local effects, local streamers could develop and move over portions of eastern Puerto Rico, including the San Juan Metropolitan Area. Rainfall accumulations could result in ponding of water over roadways and poorly drained areas, along with urban and small stream flooding over the aforementioned areas, with low flash flood chance over northwestern Puerto Rico. Although the latest NASA/GMAO Dust AOT suggests low concentrations of SAL that may linger across the local area on Wednesday, this could enhance rather than limit lightning activity, particularly during afternoon convection.

In the previous discussion, model guidance suggested a drier air mass filtering into the region by Thursday, though the latest global solutions suggest an abundance of moisture content over the CWA, associated with another tropical wave approaching the Caribbean basin. Although the area of enhanced convection should remain well south of the region, lingering moisture associated to the system combined with daytime heating and local effects may result in afternoon convection, once again, across western/northwestern Puerto Rico, with the San Juan streamer as well. Although periods of heavy rainfall can be expected, theres uncertainty regarding the arrival of the tropical wave and the SAL. For the rest of the forecast period, the dense plume of Saharan Dust should persist and limit shower activity, with isolated showers moving from time to time.

The latest model guidance suggests seasonal 925 mb temperatures, slightly near above climatological normal. Nevertheless, the combination of available moisture and winds with a southerly component will likely trigger heat indices to reach 100 degrees Fahrenheit across urban and coastal areas of the islands. In addition, the dense plume of Saharan Dust will likely inhibit nighttime cooling, with warmer minimum temperatures. Hence, the heat threat will remain elevated throughout the forecast period.

AVIATION

(06Z TAFS) Issued at 352 AM AST Sun Jul 5 2026

VFR conditions across the terminals. ESE winds around 15 to 20 kts with gusts near 24 to 28 kts. HZ will affect the TAF sites, reducing VIS through the period, decreasing this evening. Passing VCSH/-SHRA over windward sectors at times and developing from 05/18 to 05/22 mainly near TJBQ.

MARINE

Issued at 352 AM AST Sun Jul 5 2026

A surface high-pressure system over the central Atlantic will promote a moderate to locally fresh east to east-southeast wind flow through Monday. As a result, seas will remain moderate to locally choppy, especially across the Caribbean waters and local passages. This high pressure will persist through much of the forecast period. By the middle of the week, an approaching tropical wave will increase the potential for showers, thunderstorms, and locally hazardous marine conditions. Meanwhile, a dense Saharan Air Layer (SAL) will continue to spread across the region through early this week, reducing visibility and deteriorating air quality.

BEACH FORECAST

Issued at 352 AM AST Sun Jul 5 2026

A moderate risk of rip currents is expected today along exposed beaches of Puerto Rico, Vieques, Culebra, St. Croix, St. John, and St. Thomas. A low risk is expected across beaches of western Puerto Rico. Even at beaches designated as low-risk, isolated, life- threatening rip currents remain possible, particularly near jetties, groins, reefs, and piers.

Expect hot and humid conditions at the coast, with dangerous heat levels possible during the afternoon. To stay safe, remain well- hydrated, seek shade frequently, wear lightweight clothing, and limit prolonged sun exposure during the hottest parts of the day.

FIRE WEATHER

Issued at 352 AM AST Sun Jul 5 2026

A Fire Danger Statement has been issued for today (from 10 AM to 4 PM AST) across portions of the southern coastal plains of Puerto Rico due to the combination of critically dry fuels, high KBDI values, sustained winds of 15 to 20 mph with higher gusts, and minimum relative humidity values in the low 50s as moderate to high concentrations of Saharan Dust continue moving over the region. Prolonged rainfall deficits, abnormally dry to severe drought conditions, and critically dry fuels also continue. These conditions will favor rapid wildfire ignition and extreme fire behavior, meaning any fires that develop will likely spread quickly and become difficult to contain. Outdoor burning is highly discouraged.

SJU WATCHES/WARNINGS/ADVISORIES

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