Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
145 FXCA62 TJSJ 162057 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 457 PM AST Fri Aug 16 2024 .SYNOPSIS... Expect excessive heat hazard risks over the next 5-7 days, along with excessive rainfall and lightning risks beginning on Saturday and non-thunderstorm wind risks starting on Sunday. There is a chance for afternoon showers and thunderstorms to develop each day, even during the driest periods, with increased frequency and intensity during the weekend. Moderate rip current risks will continue through the weekend for most north-facing beaches, with a high risk expected early next week. Fresh winds will generate choppy seas for small craft by Sunday into early next week. Pulses of Saharan dust will reach the region by tonight into Saturday morning and again from Sunday onward. Warmer-than-normal temperatures are anticipated to persist in the coming days. && .SHORT TERM...Tonight through Sunday... Satellite and radar observations indicated sunny to partly cloudy and hazy skies, with nearly no shower development over land areas. Daytime high temperatures ranged from the mid to upper 80s in higher elevations and reached the mid to upper 90s in lower elevations. Meanwhile, heat index values soared above 112 degrees, particularly in the lower elevations and urban areas of northern, southern, and western Puerto Rico, where Excessive Heat Warnings were in effect. In the U.S. Virgin Islands, daytime highs ranged from 88 to 94 degrees, with heat indices soaring into the lower 110s in Saint Croix. Winds generally blew from the south at 10-15 mph, occasionally reaching 20-30 mph during peak sea-breeze influence. The short-term forecast continues to highlight a drier-than-normal air mass tonight, along with the interaction of a mid-to-upper- level trough to the northeast and a weak tropical wave passing on Saturday and Sunday. With precipitable water levels as low as 1.4-1.6 inches, generally fair weather is expected tonight, with only a 20% chance of precipitation for eastern Puerto Rico and the U.S. Virgin Islands. However, by Saturday and continuing through Sunday, favorable conditions from the nearby mid-to-upper level trough and increasing PWAT levels, reaching up to 2.0-2.1 inches due to the tropical wave, will lead to increased showers and isolated thunderstorms. Precipitation chances will rise to 60-70% over western areas during the day and windward sections, including the local islands, at night. The forecast indicates a limited to elevated risk of excessive rainfall and lightning hazards on Saturday and Sunday, especially in the afternoon. Warmer-than-usual conditions will persist in the coming days. These conditions present significant health risks and may require the issuance of Excessive Heat Advisories and Warnings. It is crucial for residents and visitors, especially those participating in outdoor activities, to take necessary precautions to stay safe. .LONG TERM...Monday through Friday... From previous discussion issued at 440 AM AST Fri Aug 16 2024 No major changes to the long term forecast. Model guidance indicates precipitable water (PWAT) values decreasing to below normal and normal values to start the workweek, coinciding with a pulse of saharan dust that will also serve to promote hazy skies. PWAT values are forecast to further decrease on Tuesday before gradually increasing to above normal values (above 2 inches) through the second half of the workweek. This last increase will be due to a tropical wave that, in addition to increasing moisture, will increase the potential for enhanced shower activity across the area. Surface high pressure across the Atlantic will spread westward on Monday as Ernesto moves farther north. This will promote more east to east northeast winds through midweek, becoming more easterly by the second half of the workweek. Heat risks will persist through the workweek in addition to a generally typical weather pattern. This pattern will include periods of passing overnight and early morning showers across windward sectors of the islands (especially windward sectors of Puerto Rico), followed by afternoon showers and thunderstorms mainly over the interior to western Puerto Rico each day. This activity will be somewhat enhanced, Wednesday onwards. && .AVIATION... (18Z TAFs) VFR conds thru 17/12Z although some mtn topping may occur btwn 17/08- 17/12Z in the Luquillo range. SHRA psbl VCTY TJBQ/TJMZ btwn 16/18- 16/21Z. Winds SE 6-12kt with land breeze influences btwn 16/22-17- 11Z. Outlook: better chc of SHRA/TSRA wnw PR aft 17/18Z. && .MARINE... Moderate east-to-southeast winds will continue through the weekend, providing favorable conditions for small craft. However, swells from Ernesto and moderate to locally fresh winds from a tropical wave will lead to choppy seas for small craft early next week. && .BEACH FORECAST... The rip current risk will remain moderate for most local beaches tonight, except for north-central Puerto Rico. The moderate risk will spread to most north-facing beaches of Puerto Rico, Culebra, and Saint Thomas on Saturday, with a high risk likely early next week as swells from Hurricane Ernesto and moderate to locally fresh winds from a tropical wave impact the local waters. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001- 005-008-010-011. Heat Advisory until 5 PM AST this afternoon for PRZ002>004-007- 012-013. VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002. Heat Advisory until 5 PM AST this afternoon for VIZ001. AM...None. && $$ SHORT TERM/AVIATION...WS/DSR LONG TERM....ICP PUBLIC...CVB/MMC