textproduct: San Juan

KEY MESSAGES

Issued at 441 AM AST Mon Jul 20 2026

* Hazy skies associated with Saharan dust will persist through part of today, with conditions gradually improving this afternoon. Today will also be the driest day of the forecast period, with only isolated afternoon showers expected over western Puerto Rico.

* Heat indices will generally remain below Heat Advisory criteria today, although values above 100F are still expected across some coastal and urban areas. The heat threat will increase Tuesday and Wednesday, with Heat Advisories or even Extreme Heat Warnings becoming possible across portions of the lower elevations.

* A transition to a more unsettled weather pattern is expected Tuesday through Wednesday as moisture increases across the region and a trough moves across the area. Showers and isolated thunderstorms will become more likely each afternoon, especially across western Puerto Rico, with a limited risk of localized urban and small-stream flooding.

* Elevated fire weather conditions will persist today, where critically dry vegetation, low relative humidity, and breezy conditions will continue to support the rapid spread of fires.

Short Term(Today through Wednesday)

Issued at 441 AM AST Mon Jul 20 2026

Overnight conditions were generally tranquil across Puerto Rico and the U.S. Virgin Islands. A few passing showers associated with a weak tropical wave moving westward were observed over the Caribbean waters, but no measurable precipitation was detected over land. The forecast area remains embedded within a plume of moderate Saharan dust, resulting in hazy skies and reduced moisture. However, GEOS-5 guidance indicates improving conditions this afternoon as the dust plume is pushed southward by an approaching surface inverted trough induced by a TUTT low to the northeast.

In the wake of the dust plume, a pronounced dry slot is expected to overspread the region, with precipitable water values falling to near 1.0 inch, well below climatological normals and below two standard deviations for this time of year. Combined with persistent mid-level ridging, warm temperatures aloft, and relatively dry mid- levels, this will support the driest and most stable conditions of the forecast period today, limiting shower development to isolated afternoon convection over portions of the western interior. The approaching trough will also weaken the local pressure gradient, resulting in slightly lighter easterly winds compared to recent days. While temperatures will remain near seasonal normals, the drier air mass may provide some relief from heat stress, although heat indices will still approach above 100s F across some coastal and urban areas.

A transition toward a more unsettled weather pattern is expected on Tuesday as a surface inverted trough and an accompanying Tropical Upper-Tropospheric Trough (TUTT) low move across the northeastern Caribbean from the east-northeast. As the ridge weakens, deep-layer moisture will recover to near or slightly above normal, while instability aloft gradually increases. This will favor more widespread shower activity and isolated thunderstorms, particularly during the afternoon across western and southwestern Puerto Rico under an east-northeasterly wind flow. Similar conditions are anticipated on Wednesday; however, low-level winds are forecast to veer to the east-southeast behind the trough axis. This shift should focus afternoon convection across western and northwestern Puerto Rico while also promoting warmer temperatures across the islands. Consequently, the heat threat is expected to increase, with Heat Advisory or even Extreme Heat Warning conditions becoming possible across coastal and urban areas under the developing southeasterly flow and increasing moisture.

Overall, limited impacts are expected today as the combination of anomalously dry air and persistent mid-level ridging suppresses widespread convection. Hazy skies associated with lingering Saharan dust will persist through at least part of the day before gradually improving this afternoon. Isolated afternoon showers over the western interior may still produce brief periods of locally heavy rainfall. Beginning Tuesday and continuing into Wednesday, increasing moisture and instability will support a greater coverage of showers and thunderstorms, raising the potential for localized urban and small-stream flooding, particularly where showers repeatedly develop over interior and western Puerto Rico. In addition, the heat threat is expected to increase under a developing east-southeasterly wind flow and higher moisture content, potentially resulting in dangerous heat indices across coastal and urban areas, especially by Wednesday.

Long Term(Thursday through Monday)

Issued at 441 AM AST Mon Jul 20 2026

For the second half of the workweek, upper level lows will continue to linger northeast and then east-southeast of the region. A surface high over the Atlantic will reestablish itself and once again result in up to breezy conditions, especially Friday and Saturday. Current model guidance has Precipitable water (PWAT) values on a downward trend to start the long term period as moisture from a surface trough moves northeast and away from the region under an east- southeast steering flow. PWAT values should stay within normal values on Thursday, with locally above normal values being observed during afternoon convection over mainly W-NW PR. A drier air mass with below normal PWAT will then quickly move Thursday night into early Friday, before another pulse of moisture reaches the area Friday morning. More humid air masses will then move over the area through the weekend as waves continue moving close to the region, this will maintain a generally normal PWAT over the islands as showers continue to be steered towards windward sectors and afternoon convection (showers and possible t-storms) develop over mainly the western half of Puerto Rico. At 500 mb, temperatures are forecast to cool from above-normal values to near-seasonal levels, around -6C to -7C, late Thursday onwards. Most available moisture should remain below 800mb through at least Saturday while 250 mb heights remain at seasonal values.

By late Sunday through Monday, current guidance suggests that a tropical wave and wind surge will reach the area and elevate PWAT values to above normal values and result in breezy to locally windy E-SE flow. This will increase the flooding risk as quick moving passing showers reach the region and afternoon convection develops over mainly W-PR and downwind of El Yunque and the local islands. As an upper trough moves over the region, available moisture will also reach the upper levels. Warm to hot conditions will persist through at least the weekend, with an elevated heat risk forecast for coastal and lower elevations of the islands. In terms of Saharan Dust, low to moderate concentrations will continue to filter over the islands through the second half of the week and early next week, with guidance suggesting lower concentrations as the above mentioned moisture reaches the islands late Sunday and Monday. Continue to monitor the forecast for updates.

AVIATION

(06Z TAFS) Issued at 441 AM AST Mon Jul 20 2026

VFR conds will prevail at all TAF sites thru the fcst pd. HZ assocd with SAL will cont to reduce VIS to around 6 SM at times this mrng, with gradual improvement expected this aftn. ENE winds will incr to 15-20 kt with gusts up to 25 kt aft 20/13Z, bcmg lgt/vrb aft 19/23Z. VCSH psbl aft 20/18Z-23Z at TJBQ due to diurnal convection. No sig operational impacts expected.

MARINE

Issued at 441 AM AST Mon Jul 20 2026

A surface high pressure over the central Atlantic interacting with the Panama Colombia Low will maintain moderate to fresh easterly winds and moderate to locally choppy seas today. Winds and seas will gradually subside Tuesday into Wednesday before winds increase somewhat again late Wednesday into Thursday. Saharan dust will continue to promote hazy skies early today, with conditions gradually improving through the day. Increasing moisture Tuesday into Wednesday will favor more frequent showers across the region, with afternoon thunderstorms possible, particularly over western Puerto Rico.

BEACH FORECAST

Issued at 441 AM AST Mon Jul 20 2026

Life-threatening rip currents are possible today, particularly along north-, east-, and south-facing beaches. Rip current conditions improve Tuesday and Wednesday before a moderate risk returns later in the week. Showers and thunderstorms increase Tuesday, with changing weather possible each afternoon, especially across western Puerto Rico, while excessive heat becomes a greater concern through midweek. Beachgoers should remain weather-aware and check the latest Surf Zone Forecast (SRF) for specific beach conditions before heading out.

FIRE WEATHER

Issued at 441 AM AST Mon Jul 20 2026

A Fire Danger Statement (RFD) is in effect for the local area. Despite a slight decrease in winds today, elevated fire weather conditions will persist due to critically dry fuels, very low relative humidity, and prolonged rainfall deficits. Seven-day percent of normal rainfall remains below 25 percent across most of Puerto Rico, with some areas below 5-10 percent, particularly across the southern coastal plains of Puerto Rico. Any fires that develop could spread rapidly through dry vegetation, especially across southern Puerto Rico.

Fire weather conditions will continue to be monitored closely, and a Red Flag Warning may be required if conditions become more favorable for rapid fire growth. Residents and visitors are urged to avoid outdoor burning and exercise extreme caution with open flames, discarded cigarettes, or any activity capable of producing sparks.

SJU WATCHES/WARNINGS/ADVISORIES

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