Marine Weather and Tide Forecast for Isabela, PR

.Short Term(This afternoon through Thursday)...
Issued at 1154 AM AST Tue Oct 13 2026

A lingering upper-level trough to the west will maintain favorable convective instability across the local islands today. However, a building upper-level ridge to the east will gradually increase 250 mb heights and mid-tropospheric thicknesses through midweek. Deep- layer atmospheric moisture remains abundant through early Wednesday, with precipitable water (PWAT) values fluctuating near or slightly above 2.00 inches, which is between the 50th and 75th percentiles of climatology. Low-level steering winds will prevail from the east- northeast at 10 to 15 knots, steering passing showers across windward coastal areas of PR and the U.S. Virgin Islands during the late night and early morning hours. By early this afternoon, the available moisture combined with daytime heating, the local sea breeze convergence, and topography will trigger scattered to numerous showers and isolated thunderstorms across the interior and western sections of PR. This activity could produce localized urban and small-stream flooding, as well as isolated mudslides in steep terrain.

The atmospheric profile dries and stabilizes late Wednesday into Thursday as the ridge strengthens and holds over the northeastern Caribbean. PWAT drops steadily toward 1.65 to 1.75 inches, falling near or below the seasonal normals. At the same time, mid-level moisture collapses as 700–500 mb relative humidity values plummet from near 60 percent down to 30 to 40 percent. This mid-level drying is accompanied by pronounced subsidence warming, with 500 mb temperatures rising from around -7.5C up to -4.0C and lapse rates dropping into a more stable 5.3 to 5.8C/km range. This thermal cap aloft will suppress vertical development, limiting afternoon activity to shallow, localized showers over the interior and southwestern PR. Elsewhere across PR and the USVI, sunny to partly cloudy skies and predominantly dry weather will prevail.

In terms of the heat risk through the short-term period, the 925 mb temperatures are expected to range between 21-23C, yielding daytime highs in the upper 80s to low 90s across the coastal plains and urban areas. With lingering low-level humidity, afternoon heat indices are expected to climb to 102-107F in general, posing a limited heat risk for outdoor workers and vulnerable populations.

.Long Term(Friday through next Tuesday)...
Issued at 1154 AM AST Tue Oct 13 2026

Some changes were introduced to the long term forecast, as the stable weather conditions may linger in the weekend. As mentioned in the previous discussion, winds on Friday should continue under a E- ENE pattern due a frontal boundary north of the CWA As a surface high builds over the Western Atlantic, winds may veer and become from the E-SE by Saturday, becoming easterly on Sunday and Monday.
In the previous discussion, Friday was forecast as the less "active" day due to a mid-level ridge dominating the weather pattern through Saturday, becoming unstable by Sunday due to an upper-level trough approaching the Caribbean. Nevertheless, the latest model solutions are tending to more stable conditions as the upper-level trough may linger over the Central Atlantic while the ridge gets closer to the local area. With the presence of the ridge, warmer than usual mid level temperatures (above -6 C) are very likely, along with subsidence, indicating stability aloft. Although moisture content remains near seasonal values (PWATs up to 1.75 inches), a dry slot into the mid levels (below 50%) filtering on Friday will also limit deep afternoon convection. Additionally, the Galvez-Davison Index (GDI) suggest that the best chance of observing convection activity should be in the afternoons over interior and western Puerto Rico, but mainly shallow. In general, daytime heating, local effects, and seabreeze convergence are likely the main drivers of the afternoon convection, though the weather conditions won't necessarily support long-lasting thunderstorms, lowering the flood threat. Over the weekend, these rainfall accumulations may lead to ponding of water over roadways and poorly drained areas over the aforementioned areas. The latest guidance suggest a warmer conditions this weekend, with 925 mb temperatures warmer for this time of the year. During peak hours of the day, particularly in urban and low-lying areas, heat indices could exceed 100 F, affecting mainly sensitive groups. Therefore, the heat threat will remain limited to elevated throughout the forecast period.

AVIATION
(18Z TAFS)
Issued at 1154 AM AST Tue Oct 13 2026

Mainly VFR conditions are expected to prevail across all terminals during the next 24 hours. However, tempo MVFR to brief IFR conds are possible at TJPS thru 13/22z due to locally induced +SHRA/TSRA over the interior. Mtn tops obscd expected along the Cordillera. Across the rest of the PR terminals, mostly VCTS expected thru the same period, while VCSH prevails across the USVI terminals. The 13/12z TJSJ sounding indicated ENE winds up to 17 kt blo FL100. Light winds with a northeasterly component expected overnight, increasing 8-12 kt aft 14/13z.

MARINE
Issued at 1154 AM AST Tue Oct 13 2026

The lingering upper-level low will maintain unsettled conditions across the regional waters through midweek, supporting showers and isolated thunderstorms, particularly during the afternoon and extending into the evening hours as daytime heating increases instability. Lightning remains the primary hazard, with locally gusty winds and reduced visibility also possible. Otherwise, moderate east to east-southeast winds and generally tranquil seas will prevail. A persistent northeasterly swell will affect the Atlantic waters, with seas gradually building across the northern and offshore waters by the weekend. Mariners should remain alert for rapidly changing conditions near thunderstorms.

BEACH FORECAST
Issued at 1154 AM AST Tue Oct 13 2026

A moderate risk of rip currents along the north and east facing beaches of Puerto Rico, including Culebra should remain through the workweek and expand to additional beaches, including portions of the U.S. Virgin Islands, by Saturday. Elsewhere, low rip current risk will prevail. Afternoon thunderstorms will also pose a lightning threat to beachgoers, particularly along the coast of Puerto Rico. Additionally, heat concerns are expected to return toward the end of the workweek and continue into the weekend, especially during the late morning and afternoon hours.

SJU WATCHES/WARNINGS/ADVISORIES
PR...None.
VI...None.
AM...None.