textproduct: San Juan

KEY MESSAGES

Updated at 241 PM AST Thu Jul 23 2026

* A heat advisory remains in effect across all coastal areas of Puerto Rico, Culebra, and Vieques, including the vicinity of Caguas and the U.S. Virgin Islands.

* A weak tropical wave will move across the islands by tomorrow morning, increasing the frequency of shower activity in some areas.

* A more vigorous tropical wave is forecast from late Sunday into Monday, increasing the potential for showers and flooding on Monday.

* There is a moderate risk of rip currents across locally exposed beaches for the rest of the workweek into the weekend.

Short Term(This evening through Saturday)

Issued at 241 PM AST Thu Jul 23 2026

A Saharan Air Layer (SAL) will remain the dominant weather feature through the short-term period. The associated dry air intrusion will continue strengthening tonight, with precipitable water values falling to around 1.0 inch, nearly two standard deviations below normal, while pronounced drying between 700 and 500 mb limits cloud development and rainfall potential. Although a lingering upper-level trough will maintain modest instability aloft, current radar trends indicate that showers have struggled to persist as they encounter the increasingly dry environment. Therefore, only isolated afternoon convection is expected across portions of northwestern Puerto Rico before activity quickly diminishes this evening. Hazy skies, reduced air quality, and above-normal temperatures will continue across the islands, supporting elevated heat indices in the lower elevations.

On Friday, a weak tropical wave will briefly increase low-level moisture, allowing precipitable water values to recover to near seasonal levels and supporting a more typical pattern of passing morning showers followed by scattered afternoon convection across the interior and western portions of Puerto Rico. However, the moisture increase is expected to be short-lived, as the surrounding SAL airmass remains in place. While locally heavy downpours cannot be ruled out given the persistent upper-level trough and cooler temperatures aloft, widespread or long-duration rainfall is not anticipated.

By Friday evening into Saturday, Saharan dust concentrations are forecast to increase once again to moderate levels as the transient moisture plume exits the region. Although moisture should remain near climatological values, the return of drier mid-level air will once again limit convective coverage, resulting in a more localized pattern of afternoon showers and isolated thunderstorms over western Puerto Rico. Hazy skies and warm temperatures will persist through the remainder of the short-term period.

Long Term(Sunday through next Wednesday)

Issued at 241 PM AST Thu Jul 23 2026

Current model guidance suggests an evolving wetter pattern to begin the long-term period. A tropical wave-driven moisture surge will affect the region, with PWAT values following an increasing trend to above-normal levels from Sunday through early Wednesday. Aloft, an upper-level low positioned over the islands through Monday will maintain lower 250 mb heights and cooler 500 mb temperatures, with both remaining near or below seasonal norms. Moisture will also increase in the mid and upper levels, creating a more unstable pattern across the islands. As a result, Sunday and Monday are expected to have the most favorable dynamics for the development of showers and thunderstorms. Additionally, winds are forecast to remain very light on those days, resulting in slow-moving showers and thunderstorms with the potential for higher rainfall accumulations.

By Tuesday and Wednesday, the upper-level low will shift slightly west of the area, allowing for increasing stability aloft, as reflected by rising heights. As Atlantic high pressure gradually strengthens, easterly winds are expected to increase, becoming breezy to locally windy. However, moisture content will remain above average and, when combined with local effects and diurnal heating, will likely result in windward showers during the night and morning, followed by afternoon convection across the interior and western portions of Puerto Rico.

The flood threat remains elevated on Sunday, with the potential for flooding in urban areas, roads, small streams, and washes. From Monday through Tuesday, the flood threat is currently limited, with ponding of water on roads and in poorly drained areas being the primary concern. This forecast will be refined with each update as confidence continues to increase.

Drier air will begin filtering into the area Wednesday night through Thursday, accompanied by probably some Saharan dust. As PWAT values decrease to near seasonal levels, isolated trade wind showers will continue, but shower coverage is expected to be more limited than during the previous days. Throughout most of the forecast period, warm to hot conditions will persist, bringing a limited to elevated heat risk across coastal and urban areas of the islands.

AVIATION

(18Z TAFS) Issued at 241 PM AST Thu Jul 23 2026

Mainly VFR conds will prevail at all TAF sites through the fcst period. Limited SHRA/TSRA development is expected this aftn due to dry air associated with a SAL, although brief MVFR/MTN OBSC cannot be ruled out near TJBQ. HZ will persist across the local islands. Sfc winds ESE 10-15 kt with higher gusts and sea breeze variations through early evening, becoming lighter overnight and increasing again aft 24/13Z.

MARINE

Issued at 241 PM AST Thu Jul 23 2026

No changes were made to the marine forecast. At the surface, broad high pressure will maintain moderate to locally fresh winds for the rest of the day into Saturday. Winds will remain from the east-southeast today, becoming more easterly by Saturday. Saharan dust particles will remain in place, slightly limiting visibility but not affecting marine operations. A weak tropical wave is forecast to move across the region on Friday, but widespread shower activity is not anticipated. A more vigorous tropical wave is expected from late Sunday into Monday, increasing available moisture and thunderstorm activity over the local Caribbean waters, which will result in localized hazardous marine conditions.

BEACH FORECAST

Issued at 241 PM AST Thu Jul 23 2026

There is a low to moderate risk of rip currents across all locally exposed beaches of Puerto Rico and the U.S. Virgin Islands. Life- threatening rip currents are possible, and beachgoers should exercise caution and swim near a lifeguard whenever possible. Hazy conditions are expected into Friday due to Saharan dust. Isolated thunderstorms may also develop at times, posing a lightning risk at area beaches. This activity is associated with a weak tropical wave moving across the area tomorrow, followed by a more vigorous tropical wave expected Sunday into Monday.

FIRE WEATHER

Issued at 241 PM AST Thu Jul 23 2026

An interesting pattern is forecast for tomorrow's fire danger threat. A much drier airmass will dominate the region by early morning; however, the arrival of a weak tropical wave will quickly increase relative humidity (RH) across the area. Meanwhile, surface winds will remain below threshold values, mostly ranging between 10-15 mph. Given the expected conditions, there will be a moderate fire danger risk during the day.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007- 008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.