textproduct: San Juan

KEY MESSAGES

* Showers and/or isolated thunderstorms will continue through this evening, increasing flooding potential over the southwestern quadrant of PR and St. Croix. * An upper-level and an induced surface trough can boost the diurnal shower pattern tomorrow.

* Warmer-than-normal temperatures will continue, with higher chances of heat indexes reaching Heat Advisory Criteria, over urban and coastal areas.

* There is a moderate risk of life-threatening rip currents across the northwestern to northeastern beaches of Puerto Rico through this evening. Tonight through most of Saturday, a low risk of rip currents will prevail.

/FROM PREV DISCUSSION

Issued 501 AM AST Fri Jul 18 2025/

At the beginning of the long-term period, a mid to upper level ridge building from the central Atlantic into the northeastern Caribbean will shift further west and displace the prevailing trough pattern. Winds will shift more from the east. It seems that Monday will be the day with the least rain and the most stable conditions of the forecast period. Models suggest precipitable water will drop to around 1.3 to 1.4 inches, which is below normal values. Mid level moisture is expected to decrease, and rainfall will likely follow a more typical diurnal pattern driven by daytime heating and local effects. Flood risk should range from none to limited at the start of the week. From Monday night onward, another TUTT low will approach the region, bringing a wind surge and a series of disturbances with patches of moisture from Tuesday through Thursday, likely accompanied by some Saharan dust that could enhance thunderstorms development during the afternoons. At this time, expect variable weather conditions during those days and breezy to windy easterly winds, with early morning convection between the USVI and eastern sections of Puerto Rico, followed by afternoon convection with isolated thunderstorms over parts of the interior and western Puerto Rico. Flood risk should range from limited to elevated each afternoon in those areas. Models suggest precipitable water will fluctuate around 1.6 to 1.7 inches, which is considered near seasonal values. Additionally, warm to hot temperatures will continue to dominate during this period, as indicated by above- normal 925 mb temperatures. This will likely trigger Heat Advisory conditions across most coastal and urban areas of Puerto Rico each day. Residents and visitors are encouraged to stay hydrated, avoid outdoor activity during peak heat hours, wear light clothing, and check on vulnerable individuals and pets.

Toward the end of the period, the latest guidance suggests the arrival of a tropical wave along with well above-normal precipitable water. Upper-level dynamics will likely support an unstable weather pattern beginning Friday. Stay tuned over the next few days for forecast updates and adjustments, as we are still one week out and confidence remains low.

AVIATION

(18Z TAFs)

Mostly VFR conds across all terminals. Brief MVFR conditions possible over TJBQ (SHRA/VCTS) and TISX (SHRA) during the afternoon. Winds from the ENE between 15 to 20 kts with higher gusts through 18/23z, later gradually decreasing to light and variable and veering to become more E/ESE tomorrow, increasing again after 19/13Z. SHRA will continue to move towards windward sectors during the overnight hours.

MARINE

A surface high pressure over the Atlantic will mainly promote moderate easterly winds over several days, becoming moderate to fresh again around Tuesday next week. Showers and isolated thunderstorms are expected across the western waters of Puerto Rico and the Mona passage this afternoon. An induced surface trough will increase shower activity across the waters during the weekend.

BEACH FORECAST

There is a moderate risk of life-threatening rip currents across the northwestern to northeastern beaches of Puerto Rico through this evening. Tonight through most of Saturday, a low risk of rip currents will prevail across the local beaches. Even if the risk for rip currents is low, life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers. A moderate risk of rip currents will return from Sunday. Heat index values likely exceeding 100F each day. Stay well hydrated, seek shade frequently, and avoid outdoor activity during peak sun hours.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007- 008-010-011.

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