textproduct: San Juan

KEY MESSAGES

Issued at 245 AM AST Thu Aug 20 2026

- A heat threat will be present from today through the rest of the workweek as warm temperatures and heat indices between 108 and 111 degrees persist across the islands.

- A drier airmass with Saharan dust particles will dominate the local region today, limiting cloudiness and shower development across the area. Although showers are not anticipated across most of PR, some afternoon showers resulting from diurnal patterns are forecast across western PR.

- For today, fire danger threats are expected as a drier airmass results in RH values in the mid to upper 40s and sustained winds up to 15 mph.

- There is a low to moderate risk of rip currents along all coastal areas from today into the upcoming weekend.

Short Term(Today through Saturday)

Issued at 245 AM AST Thu Aug 20 2026

Tranquil weather conditions prevailed in the night across the region as the thunderstorms and convective showers associated with the latest tropical wave moved farther away from the local CWA, allowing a drier and more stable air mass to filter into the region. In its wake, this atmospheric drying suppressed widespread cloud formation, leaving mostly clear skies across the islands overnight. Despite the settled conditions, overnight temperatures remained warm, particularly along coastal and urban areas where temperatures reached the low 80s across the San Juan Metro area. Meanwhile, the higher elevations experienced slightly cooler nighttime lows in the mid to upper 70s.

Today, a significantly drier air mass will filter across the region, with Precipitable Water (PWAT) values dropping well below two standard deviations from normal. Embedded within this dry air mass, moderate concentrations of Saharan dust will prevail, producing hazy skies and heavily suppressing widespread shower development throughout the area. At 250 mb, an upper-level trough lies directly over the region, with its most active divergent zone positioned just east of the local area. At the surface, a dominant high-pressure system will maintain a moderate east-to- southeast wind flow, which will continue to drive unseasonably warm air across the island. Given these exceptionally hot conditions, a Heat Advisory is in effect today from 10:00 AM to 5:00 PM AST, as heat indices are expected to peak between 108 and 111 degrees Fahrenheit. Although rainfall chances remain very low across most of the area due to this dry regime, the breezy trade winds may still transport a few localized, fast-moving showers across parts of western Puerto Rico during the afternoon hours.

On Saturday, upper-level conditions will remain similar as a cut- off low lingers just north of the region, maintaining a southwesterly flow aloft. At mid-levels, high pressure extending across the area will promote atmospheric stability and limit widespread deep convection. However, a transition will take place closer to the surface as trade winds shift more from the east, allowing pockets of moisture to filter into the local islands. Precipitable water (PWAT) values are forecast to range between 1.5 and 1.7 inches as these moisture patches move across the region. Under this pattern, brief passing showers will periodically move across windward and eastern sections of the islands. By late afternoon, the combination of daytime heating, sea breeze convergence, and local effects will trigger localized convective development across western Puerto Rico, bringing a limited flood threat to that area on Saturday.

Long Term(Sunday through Thursday)

Issued at 245 AM AST Thu Aug 20 2026

Moisture is expected to increase Sunday into Monday as a tropical wave approaches the region. The latest model guidance suggests precipitable water and lower to mid level relative humidity increasing to near or above normal values for this time of year, particularly by Monday. This should favor an increase in passing showers and localized afternoon convection, with locally heavy rainfall possible. As a result, the main hazards Sunday into Monday will be lightning and localized flooding, especially where repeated or heavier showers and thunderstorms occur.

A drying trend is then expected behind the wave Tuesday into midweek. Although low-level moisture may linger, mid-level relative humidity decreases considerably through Wednesday as heights rise and temperatures aloft gradually warm. This should increasingly limit the vertical depth and coverage of convection and reduce the potential for significant rainfall. With fewer showers and greater periods of sunshine, heat will become the primary hazard from Tuesday into midweek, with elevated to locally significant heat risk possible during the afternoon hours.

Overall, the primary hazards transition from localized flooding and lightning Sunday into Monday to increasing heat concerns Tuesday through midweek.

AVIATION

(06Z TAFS) Issued at 245 AM AST Thu Aug 20 2026

VFR conditions will generally prevail across all terminals throughout the forecast period, though localized reductions in VIS down to 6SM are expected due to widespread haze (HZ). Surface winds will increase after 20/14Z to around 15 knots from the E-SE, with occasional higher gusts. Afternoon convection may trigger VCSH at TJBQ around 20/20Z diminishing at 20/23Z.

MARINE

Issued at 245 AM AST Thu Aug 20 2026

Moderate to locally fresh easterly winds will continue today, maintaining locally choppy conditions across portions of the offshore waters and local passages. Winds will gradually relax Friday into the Weekend, though occasionally breezy conditions may persist across the more exposed waters. marine conditions are expected to deteriorate again Sunday into early next week as winds and seas increases.

BEACH FORECAST

Issued at 245 AM AST Thu Aug 20 2026

A moderate risk of rip currents will continue today along the exposed beaches of northern and eastern Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands, with generally lower risk elsewhere. Life-threatening rip currents remain possible in the surf zone particularly near jetties, reefs, groins, and piers. Beachgoers should continue to exercise caution and pay attention to local beach conditions.

FIRE WEATHER

Issued at 245 AM AST Thu Aug 20 2026

An elevated fire danger threat is expected today due to a combination of dry atmospheric conditions and breezy winds. Relative humidity (RH) values are forecast to drop into the mid and even lower 40s during the peak heating hours of the afternoon. Combined with sustained east-southeast winds between 10 and 15 mph and dry fuels, soil, and environmental conditions will be favorable for rapid fire ignition and spread. Therefore, a a high Fire Danger Statement (RFD) will be issued for the area.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001>005-007-008-010>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ001-002.

AM...None.