textproduct: San Juan

KEY MESSAGES

Issued at 440 AM AST Sat Oct 10 2026

- A surface induced trough and ENE steering flow will promote more variable weather across windward sectors today.

- Afternoon showers and isolated t-storms are also forecast to develop over mainly interior to southwestern PR today. Streamers from the local islands can also develop and affect the eastern region.

- An increase in instability is expected as an upper-level low approaches and moves over the region late tonight into early tomorrow, increasing the flood and lightning threat through at least mid-week.

- A low rip current risk will persist through at least early Tuesday. However, life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers.

Short Term(Today through Monday)

Issued at 440 AM AST Sat Oct 10 2026

Current satellite derived precipitable water (PWAT) imagery indicates an induced surface trough and its moisture field (with values around 2 inches) moving in from the ENE. This has resulted in fast moving showers (some having a mid to high chance of becoming isolated t-storms) over the region under an ENE steering flow. Since midnight, radar estimated at least minimal accumulations over northern, eastern and southeastern Puerto Rico, as well as over Vieques, Culebra and the USVI. Lows have ranged in the low 60s to low 70s across interior Puerto Rico, and in the mid 70s to low 80s across lower elevations of the islands.

As the surface induced trough moves over the region and ENE steering flow dominates, fast moving showers (some having a mid to high chance of becoming isolated t-storms) will continue to reach windward sectors of the islands, particularly to start the day. PWAT values will continue at around 2 inches through at least the early afternoon, decreasing slightly but gradually after. A mid to upper level low will also gradually reach the region from the east along with an upper level jet that will increase divergence and ventilation, particularly later in the afternoon and tonight. 500 mb temperatures will also gradually cool throughout the day to more seasonal values. With the ENE steering flow, diurnal heating and local effects, afternoon showers and isolated t-storms are forecast to develop over mainly the interior to the southwestern quadrant of Puerto Rico. Streamers from the local islands can also develop and affect the eastern region. A limited to elevated flooding risk is forecast for today, with the elevated flooding threat likely remaining over interior to southwestern Puerto Rico. As the mid to upper level low moves closer to the region tonight, frequent passing showers and possible t-storms are forecast to affect windward sectors tonight under a more E and then ESE steering flow.

As ESE steering flow returns between tonight and early tomorrow, Sunday, columnar moisture is forecast to decrease. PWAT values should stay at seasonal to low-end seasonal values on Sunday. However, the mid to upper level low will continue to move over and south of the islands. This will continue to rapidly cool 500 mb temperatures to around -7C while increasing lapse rates. An upper level jet over the region will continue to increase divergence and ventilation. This will continue to support passing showers, and isolated t-storms, across windward sectors of the islands, followed by afternoon convection over interior to mainly the northwestern quadrant of Puerto Rico. Streamers from the local islands and El Yunque are also forecast to affect the eastern region. Available moisture, diurnal heating, and local orographic effects will also aid in this afternoon convection (showers and isolated t-storms), and result in a limited lightning risk and a limited to elevated flooding risk. The mid to upper level low will continue to meander, albeit moving northward, on Monday. Although the upper level jet will not be as strong, deeper columnar moisture will move over the islands from the east, increasing PWAT to high end seasonal values once again. With ESE steering flow persisting, deeper moisture, 500 mb temperatures staying around -7C. A similar overall pattern is forecast for Monday, albeit perhaps not as localized. This includes passing showers, and isolated t-storms, across windward sectors of the islands mainly during the morning and night. During the afternoon, afternoon showers and t-storms are forecast to develop over the interior to mainly the northwestern quadrant of Puerto Rico while streamers from the local islands and El Yunque possibly reach the eastern region. A limited heat risk will persist throughout the weekend, possibly increasing to elevated to start the next workweek.

Long Term(Tuesday through Friday)

Issued at 440 AM AST Sat Oct 10 2026

A strong upper-level low will be the dominant feature for most of the long term period. The upper-level low will meander south of the islands, with associated instability aloft dropping 500 mb temperatures below -7 degrees Celsius on Tuesday and Wednesday, causing steep mid-level lapse rates. Furthermore, associated to the upper-level low, a surface-induced trough will favor low-level convergence and seasonal to above normal PWAT values around 1.85 to 2.20 inches, which, combined with daytime heating and local effects, could support deep, organized convection each day. Although the upper-level low is forecast to exit the area from Thursday onwards, lingering moisture could still promote shower and thunderstorm activity.

On Tuesday and Wednesday, a gentle-to-moderate east-southeasterly flow will steer slow-moving showers and thunderstorms, primarily focusing activity over the interior to northwestern sections of Puerto Rico. For the remainder of the period, model guidance suggests a frontal boundary exiting the eastern CONUS, inducing a pre-frontal trough north of the CWA. This interaction will result in winds shifting to a more east-northeasterly flow, focusing the convection onto the interior and southwestern Puerto Rico. Regardless, passing shower and thunderstorm activity cannot be ruled out across eastern Puerto Rico and the USVI each day.

These conditions will maintain a limited to elevated risk of flooding each day, with urban and small stream flooding and flash flooding possible particularly on Tuesday and Wednesday, when lighter winds will favor slow-moving showers with higher rainfall accumulations. In addition, localized strong wind gusts and frequent lightning are expected with the strongest thunderstorms.

AVIATION

(06Z TAFS) Issued at 440 AM AST Sat Oct 10 2026

Mainly VFR conds are forecast. Passing VCSH/-SHRA (possibly isolated VCTS) across most terminals to start the period. Aftn SHRA/TSRA to develop across interior to W-SW PR, affecting mostly TJPS thru 10/23Z. However, streamers can also develop from the islands and affect the E terminals at this time. ENE flow to persist. Winds increasing to around 13 to 18 kts higher gusts and sea breeze variations after 10/13Z, decreasing after 10/23Z as flow becomes more E and then ESE. Passing VCSH/-SHRA (possibly isolated VCTS) under this flow after around 10/23Z.

MARINE

Issued at 440 AM AST Sat Oct 10 2026

A surface-induced trough moving across the region will promote increased shower and thunderstorm activity across the local waters today and tonight. Thunderstorms may produce locally hazardous conditions, including frequent lightning, gusty winds, reduced visibility, and locally higher seas. Moderate to fresh easterly trade winds will persist, with choppy seas expected across portions of the offshore and windward waters. Small craft should exercise caution where choppy conditions occur. A surface high building across the western Atlantic will help maintain moderate easterly trades through the remainder of the weekend.

BEACH FORECAST

Issued at 440 AM AST Sat Oct 10 2026

The low rip current risk is forecast to persist this weekend and early next week. The latest nearshore buoys continue to report seas between 2 and 3 feet. Nearshore waters should range between 1 to 3 feet. Nevertheless, beachgoers are encouraged to visit beaches with lifeguards, as life-threatening rip currents can often occur near groins, jetties, piers, and reefs. The risk may upgrade by mid next week, with a moderate risk across northern and eastern beaches of the islands. In addition to rip currents, beachgoers should pay attention to the weather forecast this weekend and early next week, as upcoming wetter and unstable conditions will bring showers and thunderstorms across the islands. Expect periods of heavy showers, gusty winds, and lightning.

SJU WATCHES/WARNINGS/ADVISORIES

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