textproduct: San Juan

KEY MESSAGES

Issued at 300 PM AST Thu Aug 20 2026

- A drier airmass with Saharan dust particles will dominate the local region the rest of today and Friday, with limited afternoon showers expected across western Puerto Rico. During the weekend, patches of moisture may increase slightly the frequency of passing showers and afternoon convection across western Puerto Rico.

- A Heat Advisory remains in effect until this afternoon across urban and coastal areas of the islands. The heat will likely continue to be a threat in the coming days.

- A moderate risk of rip currents will return tonight into Friday along the beaches of northern and eastern Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands.

- A Fire Danger Statement (RFD) remains in effect until 5 PM AST this afternoon, particularly across the southern plains and hills of Puerto Rico due to minimum relative humidity values in the mid 50s and easterly winds near 15 mph and higher gusts.

Short Term(This evening through Saturday)

Issued at 300 PM AST Thu Aug 20 2026

A generally hot, dry and hazy day was experienced across the islands, as a dry air mass with below normal precipitable water (PWAT) values and moderate to high Saharan Dust concentrations have moved over the region. Although an upper-level low is lingering north of the islands and a tropical wave is moving well south, mid- level ridging and the above mentioned dry air with Saharan Dust has served to inhibit shower activity and limit available moisture to the lower levels. The latest local sounding has served to further evidence this as it reported a strong 800 to 850mb inversion along with PWAT values at 1.31 inches. Current Satellite derived PWAT values also indicate values ranging from around 1 to 1.33 inches, with the exception of NW PR where afternoon convection has resulted in up to 1.7 in of PWAT. These convective showers and isolated t- storms have developed due to the up to breezy ESE flow, orographic effects and sea breeze convergence.

Breezy steering flow during the next few days will be generally easterly to east-southeasterly during the period and gradually decrease on Saturday. This flow could steer passing showers over windward sectors tonight as afternoon convection sharply dissipates this afternoon. The upper level low will gradually move north and away from the region as the week continues. Patches of moisture advecting over the region will become more frequent as the short term period progresses. This will sustain more seasonal PWAT values, passing showers during the overnight and morning, and afternoon convective showers and isolated t-storms over mainly western PR. Saharan Dust concentrations will gradually decrease as the period continues with current guidance suggesting low to locally moderate concentrations on Saturday. This activity should be more notable on Saturday as available moisture, decreased dust concentrations, diurnal heating, sea breeze convergence and local effects can result in a limited flooding risk over western Puerto Rico. Dry mid levels with Saharan dust concentrations will continue to limit shower activity and keep available moisture in the low levels. Temperatures have reached the 90s while heat indices have reached the mid to upper 100s to the low 110s across urban and coastal sectors of the islands. A Heat Advisory will continue for these areas through 5 PM AST; heat risks are expected each late morning to afternoon during the period. Residents and visitors should take necessary precautions and monitor for additional heat products during the period as both low and high temperatures remain seasonal to above seasonal.

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

(18Z TAFS) Issued at 300 PM AST Thu Aug 20 2026

VFR conditions will prevail, with a Saharan Air Layer resulting in local reductions in visibilities and HZ. VCSH/very brief VCTS possible near TJBQ. Passing VCSH can move over the eastern region late tonight. E to ESE flow decreasing to 5 to 10 kts after 21/23Z, and increasing again to 21/13z to around 14 to 20 kts with higher gusts.

MARINE

Issued at 300 PM AST Thu Aug 20 2026

Moderate to locally fresh easterly winds will continue the rest of today into tonight, maintaining locally choppy conditions across portions of the offshore waters and local passages, where Small Craft should exercise caution. Seas 4 to 5 feet, occasionally to 6 feet across offshore waters while 3 to 4 across coastal waters. Winds will gradually subside Friday into the weekend, though occasionally breezy conditions may persist across the more exposed waters.

BEACH FORECAST

Issued at 300 PM AST Thu Aug 20 2026

Tonight into Friday, there is a moderate risk of rip currents along the beaches of northern and eastern Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands, meaning life-threatening rip currents are possible. A generally lower risk will prevail elsewhere. However, remember that even when the risk is low, life-threatening rip currents can occur near groins, jetties, reefs, and piers.

By the weekend into early next week, with slightly lighter winds, the risk is forecast to fluctuate between low and moderate across exposed beaches. From Tuesday into midweek, winds are expected to gradually become breezier, with a moderate risk expanding to most local beaches. Additional hazards will include heat across all areas and isolated thunderstorms, particularly across the western beaches of Puerto Rico during the afternoons.

FIRE WEATHER

Issued at 300 PM AST Thu Aug 20 2026

Surface weather stations have been recording minimum relative humidity values in the mid 50s and easterly winds near 15 mph, with gusts up to 25 to 28 mph, particularly across the southern plains and hills of Puerto Rico and St. Croix. An elevated fire danger threat prevails in these areas due to a combination of dry atmospheric conditions, ongoing rainfall deficits, stressed vegetation, and dry fuels. Conditions will remain favorable for rapid fire ignition and spread. Therefore, a Fire Danger Statement (RFD) remains in effect until 5 PM AST this afternoon, particularly across the southern plains and hills of Puerto Rico. Continue to monitor the forecast, as additional fire weather products may be needed in the coming days.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007- 008-010>013.

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

AM...None.