Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
499 FXCA62 TJSJ 050835 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 435 AM AST Fri Jul 5 2024 .SYNOPSIS... Residual moisture from the tropical wave (Invest 96L) will lead to occasional passing showers across the region through at least this afternoon. A drying trend is expected during the weekend along with a Saharan Air Layer. Warm to hot conditions will continue. The next tropical wave will reach the islands Monday night into Tuesday increasing the potential for shower activity once again. && .SHORT TERM...Today through Sunday... Last night, a tropical wave caused showers and thunderstorms in the local waters. These showers and thunderstorms then moved inland across the US Virgin Islands, as well as the south and east of Puerto Rico. Additionally, the arrival of a Saharan Air Layer led to hazy skies, and the GOES-E infrared channel detected cloudy skies. These two factors - the suspended dust particles and cloud cover - blocked outgoing longwave radiation, preventing nighttime cooling and resulting in warmer-than-normal low temperatures. The winds were mainly from the east-southeast at 10 mph or less. From this morning through this afternoon, the trailing moisture associated with the tropical wave will maintain high low-level moisture values, which, combined with the maximum air temperatures, will result in hazardous heat index values along PR and the USVI`s coastal and urban sections. Therefore, a Heat Advisory is in effect for most of these locations. The skies will also be hazy due to moderate to high suspended Saharan dust particles. The proximity of a mid to upper-level trough north of the islands will promote enough instability, resulting in periods of showers and thunderstorms across the local waters, eastern Puerto Rico, and the Virgin Islands. The combination of the lingering tropical moisture, excessive heating, suspended dust particles, topography, and sea breeze variations will promote some isolated strong thunderstorms, especially during the afternoon. Rain activity will tampered off late this afternoon into the evening as the concentrations of the suspended African dust particles increase, making the haziness more evident and promoting stable conditions. The dry, stable, and hazy weather pattern will extend into the weekend. However, a surface high-pressure building across the central Atlantic and extending into the northeast Caribbean will promote breezy easterly winds, advecting occasional patches of clouds and showers, especially across the windward sections of the US Virgin Islands and Puerto Rico. A trade wind perturbation may increase the potential for showery weather on Sunday, as a sharp increase in moisture would be possible. && .LONG TERM...Monday through Friday... On Monday, residual Saharan dust particles will remain in low concentrations across Puerto Rico and the U.S. Virgin Islands. Nonetheless, by late Monday, expect an increase in moisture and shower activity as a tropical wave, along with a wind surge, reaches the islands on Tuesday. Precipitable water models show values in the 50th percentile or around normal climatological levels for this time of the year with the passage of this tropical wave. This wave will move relatively fast across the region, particularly on Tuesday. With it, expect the passing showers to increase across the islands during the morning hours, with moderate to heavy showers and isolated thunderstorms expected across the western and northwestern portions of Puerto Rico during the afternoon hours. Trailing moisture associated with the tropical wave will prevail over the long-term period. The latest model guidance continues to suggest a strong surface to mid-level high-pressure system over the central Atlantic. This weather feature will maintain the wind flow out of the east- southeast through at least Friday. With these weather conditions, we anticipate warm weather conditions with temperatures in the 90s along the coastal and urban areas of the islands and in the upper 70s to low 80s in the mountainous areas. As mentioned in previous discussions, the mixture of warm temperatures and the available moisture will result in higher heat indices across the region, surpassing the mid-100s. && .AVIATION... (06Z TAFS) Although VFR conds will prevail for most of the period. SHRA/TSRA will affect the surrounding waters of USVI and may move inland, affecting ISX or IST through at least 05/18z; then this activity may spread into PR, affecting JSJ/JPS or JBQ between 05/16-05/22z, promoting brief MVFR conds. Hazy skies will remain possible as a SAL enters the region. The winds will remain calm to light and variable through 12z, returning from the E to ESE at 15-20 kt with gusts between 20 and 30 kt near TSRA and sea breeze variations after 12z. && .MARINE... A departing tropical wave will continue to bring showers and thunderstorms across the regional waters through this morning. A strengthening surface high pressure over the central Atlantic will extend into the northeast Caribbean by this afternoon through the weekend, leading to moderate to fresh easterly winds. The next tropical wave will be approaching the islands by late Monday night into Tuesday, increasing the chance of showers across the local waters. && .BEACH FORECAST... Today, the rip current risk should remain moderate along the southern beaches of Puerto Rico, Vieques, Culebra, Saint Thomas, Saint John, and Saint Croix. Elsewhere should remain low. For additional information, consult the Surf Zone Forecast (SRFSJU) issued by the NWS San Juan Office. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001-003-005-007-008-010-011. VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ002. AM...None. && $$ SHORT TERM/AVIATION/MARINE...CAM LONG TERM/BEACH FORECAST...MMC