textproduct: San Juan

KEY MESSAGES

Updated at 226 PM AST Sat Apr 18 2026

* Flooding risk will be highest each afternoon due to showers and thunderstorms, with additional showers overnight. Elevated risk persists through Monday, decreasing to limited from Tuesday onward.

* Thunderstorms will bring frequent lightning and gusty winds. Saturated soils increase the risk of downed trees and power lines. Small hail and brief funnel clouds or waterspouts are possible.

* Choppy, wind-driven seas will continue, with locally dangerous conditions near thunderstorms. A moderate risk of rip currents will persist along north- and east-facing beaches of Puerto Rico and the U.S. Virgin Islands.

* In the U.S. Virgin Islands, periods of showers may cause localized ponding of water, with a limited flooding risk.

Short Term(This evening through Monday)

Issued at 226 PM AST Sat Apr 18 2026

A mid to upper-level trough and a surface high pressure system over the North Atlantic remain the dominant features across the region. Their interaction is promoting a moist, unstable, and breezy pattern. Combined with abundant moisture, diurnal heating, and local effects, this setup is supporting another active afternoon with showers and thunderstorms across interior, western, and northern Puerto Rico, extending into the San Juan Metro Area. Precipitable water values remain above normal, and although gradual drying aloft has begun, sufficient moisture and instability persist to support locally heavy rainfall. As typical, activity will transition from afternoon and early evening convection over land to a more advective pattern overnight, with showers moving across eastern Puerto Rico, the U.S. Virgin Islands, and surrounding waters on easterly winds.

A gradual transition toward slightly drier and more stable conditions is expected as the upper-level trough shifts east, with subsidence developing in its wake while a mid-level ridge builds from the west Sunday into early next week. At the same time, a drier airmass will filter in from the northeast, with moisture becoming increasingly shallow and largely confined below 700800 mb. Despite this, sufficient low-level moisture and daytime heating will support a typical diurnal pattern, with afternoon convection across interior and western Puerto Rico and passing showers overnight and in the morning. Partial morning clearing will promote heating, followed by afternoon convection and increasing cloudiness, with this pattern repeating through the period. Breezy easterly winds will persist, gradually weakening early next week as the surface high shifts and a frontal boundary approaches from the northwest. Across the U.S. Virgin Islands, rainfall will remain more variable, with lower confidence in coverage and intensity.

The primary hazards will continue to be flooding, landslides, lightning, and rapid river rises. Soils remain saturated and rivers elevated, resulting in a low threshold for impacts. Persistent or repeated showers and thunderstorms may quickly lead to urban and small stream flooding, river rises, landslides, and isolated flash flooding. Saturated soils combined with gusty winds may also result in downed trees and power lines. Given ongoing activity this afternoon, impacts may develop rapidly. While conditions may gradually improve by Monday, localized hazards will persist where heavier rainfall occurs. Residents and visitors should remain alert, especially in flood-prone and mountainous areas, and monitor forecasts and warnings.

LONG TERM(Tuesday through Saturday)

Issued at 232 AM AST Sat Apr 18 2026

The current forecast remains consistent with the previous discussion, showing no significant changes. Drier and more stable conditions are anticipated to return as the influence of the short term periods upper-level trough weakens and moves away from the region. A stable air mass will be present aloft due to a building mid-level ridge over the area. At the surface, a dominant high- pressure system over the central Atlantic will maintain light to moderate easterly winds through the middle of the week then becoming northeasterly and southeasterly at the second half of the week. Precipitable water (PWAT) values are expected to drop to normal values with high end normal to above normal values at times.

Local weather will be seasonal with passing showers over windward sectors during the morning and afternoon hours. This will be followed by afternoon convective activity across the interior and W PR on Tuesday with steering wind variations prompting afternoon convection over SW PR on Wednesday and W-NW PR on Thursday and N PR on Friday and Saturday. The combination of available moisture, daytime heating, and local effects could lead to moderate to heavy rainfall and isolated thunderstorms during these afternoons. Temperatures at 500 mb are forecast to remain seasonal (-6 C to - 8 C), isolated thunderstorms are still possible, particularly to end the period due to a nearby mid to upper trough. Given previous rainfall, saturated soil, and elevated river levels, the potential for flooding remains. Temperatures at 925 mb will also stay seasonal throughout most of the period.

AVIATION

(18Z TAFS) Issued at 226 PM AST Sat Apr 18 2026

Mainly VFR conditions will prevail across TAF sites with the exception of SHRA & iso TSRA from 18/18Z thru 18/22Z that could result in intermittent MVFR/IFR at times, particularly in TJBQ. E to ESE winds at around 13 to 15 kts with gusts up to 25 kts and sea breeze variations, becoming lighter overnight and increasing again around 18/13Z onward along with VCSH across USVI TAF sites & TJSJ.

MARINE

Issued at 226 PM AST Sat Apr 18 2026

An unstable weather pattern across the regional waters will persist through Sunday. Periods of strong thunderstorms, particularly across the western waters of Puerto Rico are expected during the afternoon hours. A surface high pressure over the Atlantic will promote mainly moderate to fresh easterly winds across the offshore Atlantic waters and local Caribbean passages, resulting in moderate to locally choppy seas through the weekend. Small craft operators should exercise caution. A small northeasterly swell will also continue to propagate across the Atlantic waters and Caribbean passages through early next week.

BEACH FORECAST

Issued at 226 PM AST Sat Apr 18 2026

There is a a moderate risk of rip currents along the north-facing beaches of Puerto Rico, Culebra, and the U.S. Virgin Islands and this pattern is is expected to prevail over the forecast period. This means that life-threatening rip currents are possible in the surf zones. Beachgoers are encouraged to remain cautious in these exposed areas. Another beach concern is the development of thunderstorms each afternoon, particularly along the northwestern beaches of Puerto Rico. Remain weather-aware due to rapidly changing conditions, and be prepared to seek shelter if you hear thunder.

SJU WATCHES/WARNINGS/ADVISORIES

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