Area Forecast Discussion

KEY MESSAGES

Issued at 208 PM AST Sat Sep 19 2026

- Limited to elevated flooding risk will remain possible each afternoon through early next week with slow-moving showers and thunderstorms, especially across interior and western PR

- Warm to hot conditions are expected through at least midweek, with the possibility of issuing heat-related products.

- A more active weather pattern is expected late in the week as a series of troughs and tropical wave increase the potential for heavy rainfall, frequent lightning, and gusty winds.

- The risk of rip currents is low over the coming days, but toward the end of next week, increasing winds may bring moderate rip current risk along north and east exposed areas.

Short Term(This evening through Monday)

Issued at 208 PM AST Sat Sep 19 2026

A seasonal pattern prevailed during the morning hours, with a few passing showers over the waters and windward coastal areas, but with minimal rainfall accumulations. Heat index values have already reached Heat Advisory criteria in localized areas, and additional locations are expected to experience increasing heat indices through the rest of the afternoon. However, increasing cloud cover and shower activity may locally limit the heat threat. Nevertheless, the Heat Advisory remains in effect through 5 PM AST this afternoon. The 19/12Z TJSJ sounding measured a precipitable water value of 2.15 inches, near the 90th percentile of climatological normal. This above-normal moisture, combined with sufficient instability and daytime heating, will support showers and isolated thunderstorms through the afternoon, particularly across the interior and western/northwestern Puerto Rico, as well as in streamers developing downwind of the smaller islands. As of 1:30 PM AST, isolated showers had developed, including a thunderstorm embedded within a streamer affecting portions of the San Juan metro area.

Tonight into Sunday morning, passing showers will favor the US Virgin Islands and eastern Puerto Rico under the prevailing easterly flow. Low- to mid-level moisture is expected to improve somewhat by Sunday, while 500 mb temperatures and mid-level lapse rates remain near seasonal value. This should support somewhat deeper convection compared with today. At the same time, the low-level steering flow will gradually become more east-southeasterly, shifting the favored area for afternoon convection toward northwestern Puerto Rico. Although widespread organized convection is not anticipated, locally heavy rainfall and isolated thunderstorms will remain possible, with urban and small-stream flooding and isolated flash flooding possible where repeated or persistent rainfall occurs.

By Monday, a somewhat drier layer is expected to return near 700 mb, while precipitable water values decrease slightly. This should once again limit the overall coverage of afternoon convection. However, the 700-500 mb layer is forecast to remain relatively moist, while lapse rates aloft become somewhat more favorable. Therefore, although convection may be more isolated in coverage, any thunderstorms that develop could still produce brief heavy rainfall and isolated thunderstorms. An upper-level trough will also begin moving southeastward toward the northeastern Caribbean late in the period, with its more significant influence expected beyond the short-term forecast period.

The main hazards through the short-term period will continue to be excessive heat and locally heavy rainfall. Very warm low-level temperatures will support another round of elevated heat indices on Sunday and Monday, particularly across coastal and urban areas. Afternoon convection may locally reduce temperatures, but where sufficient heating persists, heat indices could again reach Heat Advisory criteria. The greatest rainfall threat will remain across the interior and western portions of Puerto Rico, with localized flooding also possible across the San Juan metro area and other locations affected by streamers or persistent showers.

Long Term(Tuesday through Saturday)

Issued at 255 AM AST Sat Sep 19 2026

Weather conditions will become increasingly unstable during the long-term period as a mid- to upper-level ridge is replaced by a mid- to upper-level trough later in the week. The ridge will still be located west of the area on Tuesday but will be pushed farther away as the upper-level trough begins to dig into the local area from the north-northeast and remains in place for the remainder of the period. This pattern will continue to support occasional induced surface perturbations, with moisture reaching the area throughout the period and keeping precipitable water values between 1.75 and 2.00 inches, which are seasonal values for this time of year. Additionally, a tropical wave is expected to cross the area between Thursday and Friday, providing additional low-level support for more frequent showers and thunderstorms. Moderate steering winds will prevail through the period, with east to east-southeasterly flow between 10 and 15 knots.

On Tuesday and Wednesday, a more typical pattern is expected, with the usual passing showers across windward areas and afternoon convection across the interior, western, and northwestern portions of Puerto Rico. A more unstable period is expected late in the week (Thursday through Saturday), when the tropical wave crosses the area and the upper-level trough becomes more favorably positioned for convective activity. However, uncertainty remains, as conditions will depend on the exact position of the trough, whose axis is currently forecast to be over the local area. If the current forecast materializes, 500-millibar temperatures during the favorable period are forecast to be around -9 degrees Celsius, with mid-level lapse rates (between 700 and 500 millibars) around 7 degrees Celsius per kilometer. Both are around 2 standard deviations favorable for convective support, suggesting active afternoons with scattered showers and the potential for strong thunderstorms with frequent lightning and locally strong gusty winds, especially across the interior, western, and northwestern sections of Puerto Rico. Additionally, urban and small-stream flooding is expected each afternoon during the second half of the week across these areas, as well as downwind from El Yunque and the U.S. Virgin Islands. On Thursday and Friday, more frequent passing showers are possible across windward areas, including the U.S. Virgin Islands, due to the tropical wave.

An elevated heat risk will continue through the long-term period due to elevated moisture content, which will push heat indices above 100 degrees Fahrenheit each day, especially across lower elevations and urban areas that do not experience rainfall during the warmer part of the day.

AVIATION

(18Z TAFS) Issued at 208 PM AST Sat Sep 19 2026

Mostly VFR conds expected thru the period. SHRA/TSRA will affect mainly TJSJ, TJPS, and TJBQ this aftn, with brief MVFR/IFR conds from reduced VIS/CIGs and possible mtn obscurations. Activity should diminish after 19/22Z, though VCSH will persist overnight at TJSJ/TIST/TISX. Another round of SHRA/TSRA is expected after 20/17Z, again mainly affecting TJSJ/TJPS/TJBQ. Winds E-ESE 10-15 kt with sea breeze variations and occasional gusts near 20 kt, becoming light/VRB overnight.

MARINE

Issued at 208 PM AST Sat Sep 19 2026

A weak surface pressure gradient across the region is supporting gentle to moderate easterly winds across the offshore waters and local Caribbean passages this afternoon through the remainder of the weekend, with light to gentle winds elsewhere. A surface high will briefly build across the northeastern Atlantic early next week, resulting in gentle to moderate east to southeast winds across most waters and moderate chop through at least midweek. Passing showers will remain possible across the regional waters each day, while afternoon thunderstorms may linger into the evening hours across the western waters of Puerto Rico.

BEACH FORECAST

Issued at 208 PM AST Sat Sep 19 2026

The risk of rip currents is expected to remain low across most beaches of Puerto Rico and the U.S. Virgin Islands through at least midweek next week. However, even with a low risk, life- threatening rip currents can occur near groins, jetties, reefs, and piers. Beachgoers should always remain cautious when entering the water. By the end of next week, a moderate risk of rip currents will likely return along north and east exposed beaches.

From the remainder of the weekend into next week, the main beach hazards will be heat and afternoon thunderstorms. Beachgoers should stay hydrated, limit prolonged exposure to the sun, and seek shelter indoors whenever thunder is heard.

SJU WATCHES / WARNINGS / ADVISORIES

PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>013.

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

AM...None.