textproduct: San Juan

KEY MESSAGES

* Hot temperatures, high humidity content, and southeasterly winds will promote an elevated heat risk across the lower elevations and urban areas of Puerto Rico through the end of the week.

* Another, more defined, tropical wave will increase shower and thunderstorm activity tomorrow.

* A plume of moderate to high concentrations of Saharan Dust will reach the islands, leading to hazy skies and deteriorated air quality. This will be particularly noticeable on Thursday and Friday, with lingering concentrations on Saturday.

* Increasing winds will lead to choppy seas and up to a moderate risk of rip currents.

SHORT TERM...Tonight through Thursday

The shower activity continued during the morning, mainly over the local waters moving inland. Based on Doppler Radar rainfall estimates, some areas, including the San Juan Metropolitan Area, received around 0.5 - 0.75 inches. The 12z sounding showed an increase of low to mid relative humidity up to 80 - 84%, with Precipitable Water values of 1.89 inches above climatological normal. Nevertheless, the sounding's 500 mb temperature was warmer than normal, due to the mid-level ridge north of the region. Convection activity already started, with scattered showers moving over the Cordillera Central into northwestern portions of Puerto Rico. Periods of locally heavy rainfall will enhance ponding of water in roadways, urban and poorly drained areas, with a low chance of urban flooding. Weather conditions should gradually improve tonight, with a few passing showers moving across the regional waters and some across windward sections.

A surface high pressure strengthening over the Western Atlantic will promote breezy conditions for the rest of the forecast period. The mid to upper level ridge should linger north of the CWA for the next few days. PWAT values should slightly increase tomorrow as another tropical wave more defined located along 53W south of 20N approaches the local islands, increasing the frequency of showers. 500 mb temperatures should chill to seasonal values (-6 to -7 degrees Celsius), which aids the development of thunderstorm activity. Additionally, the Galvez-Davison Index suggests there's a potential for isolated thunderstorms in the afternoon. Hence, expect showers moving over eastern Puerto Rico and the U.S. Virgin Islands in the morning, and convection activity in the afternoon over the interior and western portions of Puerto Rico. Rainfall accumulations will increase the flooding risk; therefore, urban and small streams flooding is very likely over the aforementioned areas. A dense SAL is anticipated to approach the local islands by Thursday morning, with moderate to high concentrations based on NASA GMAO. These concentrations should limit deep convective activity, bringing hazy skies, reduced visibility, and deteriorated air quality.

Seasonal to above normal temperatures and high moisture content will keep the elevated risk of heat for the rest of the short term, and it's very likely to meet Heat Advisory Criteria.

LONG TERM...Friday through Tuesday

/PREV DISCUSSION... /issued 500 AM AST Tue Jul 15 2025/

Hazy skies, reduced visibility, and poor air quality conditions will continue through Friday due to a lingering Saharan Air Layer. Winds will acquire a northeasterly component as a broad TUTT- induced trough approaches the eastern Caribbean. The combination of the limited moisture content with daytime heating and the sea breeze convergence will trigger afternoon showers with isolated thunderstorms over the southwest quadrant of PR. An advective weather pattern at night will then promote showers across the local waters and over portions of the USVI and northeastern PR.

The TUTT-low and the induced low-level trough are expected to move across the northeastern Caribbean through the weekend. This will bring an increase in shower activity once again across the islands. A tropical wave is expected to move across the region on Monday, followed by another SAL event on Tuesday. In general, these weather features will enhance the early morning convection between the USVI and eastern sections of PR, followed by afternoon thunderstorms over portions of the interior and western PR. Even though the SAL is expected to bring hazy skies and drier mid-level air, the precipitable water content is forecast to range between normal to above normal levels through the end of the long-term period.

Meanwhile, a deep layer ridge building from the central Atlantic into the northeastern Caribbean will push further west and away of the area the trough pattern and promote southeasterly winds by Saturday afternoon. This moist southeast flow will prevail through at least Tuesday, and hot temperatures are expected each day. Likely triggering Heat Advisory conditions across most coastal areas of PR.

AVIATION

(18z TAFs)

Mostly VFR conds across all terminals. -RA/+RA in JQB due SCTD SHRA over NW Puerto Rico, with chance of VCTS. Winds from the NE btwn 16 - 18 kt with gusts up to 26 kt will cont until 15/23z, grdly decreasing btwn 8 - 12 kt. VCSH for all TAF sites is likely after 16/12z.

MARINE

A surface high pressure will strengthen over the western Atlantic early this week. While the current tropical wave will depart late tonight, another is expected to affect local waters tomorrow, Wednesday. Consequently, moderate to locally fresh trade winds and choppy seas will persist across the regional waters through midweek. Increased shower and thunderstorm activity is expected with the passage of these tropical waves. Saharan dust will lead to hazy conditions and reduced visibility on Thursday and Friday, with lingering concentrations on Saturday.

BEACH FORECAST

There is a moderate risk of rip currents for the north and east facing beaches of Puerto Rico as well as for most beaches of Culebra, Vieques, and St. Croix. This moderate risk will spread to the southeastern beaches of Puerto Rico. Up to a moderate risk of rip currents is forecast through the workweek. Although other areas have a low risk of rip currents, these often occur in the vicinity of groins, jetties, reefs, and piers. The potential for isolated thunderstorm development will continue tomorrow across most coastal areas due to another tropical wave. Saharan dust will result in hazy conditions and reduced visibilities on Thursday and Friday, with lingering concentrations on Saturday.

If caught in a rip current, stay calm and yell for help. Conserve your energy by floating until assistance arrives. If you must swim, do so parallel to the shore until you are out of the current, then swim back to the beach. Do not attempt to swim directly against the current, as this will quickly exhaust you.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory until 4 PM AST this afternoon for PRZ001>005-007- 008-010-011.

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