Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
002 FXCA62 TJSJ 302043 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 443 PM AST Thu May 30 2024 .SYNOPSIS... The primary concern for the short-term period will continue to be heat, at least through this weekend. Diurnally induced afternoon showers and thunderstorms are expected each day, mainly across the interior and western Puerto Rico. Additionally, some Saharan dust is expected by tomorrow and will persist through the weekend, contributing to hazy skies and an elevated to moderate heat risk. && .SHORT TERM...Tonight through Saturday... As anticipated, thunderstorm activity developed by early afternoon, primarily affecting the northwestern quadrant of Puerto Rico leaving around 1 to 2.5 inches of rain. Strong thunderstorms eventually moved over the local waters. Weather conditions are expected to improve after sunset due to diminishing surface heating. Once again, sweltering temperatures were observed today as southerly winds persisted, advecting warmer temperatures and combining with higher-than-normal moisture across the area due to the passage of a tropical wave, although the bulk of moisture remained mostly south throughout the day. Passing showers are expected to move in over windward coastal areas this evening and late tonight as the tropical wave continues its westward movement. The overall synoptic pattern for the remainder of the short-term forecast suggests that the current typical weather pattern will persist. On Friday, a slight decrease in moisture content is expected, with precipitable water values dropping to slightly below 2.0 inches. However, by Saturday, a slight increase in precipitable water is anticipated due to the approach of another tropical wave currently situated at 40W. This pattern will maintain moisture content within the 75th and 90th percentiles, or slightly higher than the climatological norm, throughout the period. Dynamics aloft will remain seasonable, with 500 mb temperatures staying close to -5 and -6 degrees Celsius, and both 850-700 mb and 700-500 mb lapse rates remaining near normal for this time of year. Therefore, under an east-southeast steering wind flow, expect showers to move over eastern/southeastern portions of Puerto Rico and smaller islands during the morning and night hours, with convective activity clustering over the northwestern quadrant of Puerto Rico each afternoon. Streamers may develop downwind of the smaller islands and El Yunque into the San Juan metro area. Model guidance suggests that the highest rainfall accumulations are expected during the afternoon shower activity, with southwestern and south-central coastal areas of Puerto Rico observing the least amount of rainfall accumulations throughout the period. However, the GEOS-5 model suggests the arrival of suspended Saharan dust particulate over the forecast area by Friday through the weekend, which could slightly suppress shower development in some areas, bringing hazy skies across the northeastern Caribbean. The 925 mb temperatures will remain well above normal for this time of year through the weekend. High temperatures combined with above-normal moisture could promote hazardous heat conditions, particularly across coastal and urban areas. Therefore, it is very likely that Heat Advisories and possibly Excessive Heat Warnings may need to be issued daily for some of these areas. && .LONG TERM...Sunday through Thursday... .PREV DISCUSSION... /issued 455 AM AST Thu May 30 2024/ There will still be some Saharan dust over the region as the Saharan Air Layer weakens during the weekend. A tropical wave is forecast to cross the islands on Sunday. This will bring an increase in shower activity first across the U.S. Virgin Islands, then spreading across Puerto Rico during the peak time of afternoon convection. Some thunderstorms could be strong over western PR. For next week, a mid-to upper-level trough is expected to move from the western Atlantic into the northeastern Caribbean, while a deep layered ridge builds and hold just east of the Leeward Islands. This will promote divergence aloft and a south to southwesterly wind flow at lower levels. In addition, a surface front associated to the upper trough will sink southwards but should remain above 20N. However, pooling of moisture is expected in general across the northeastern Caribbean through the long term period with global models suggesting between 2.10-2.40 inches of precipitable water content over the local area, peaking by midweek. The southerly winds and high humidity content will also promote warm to hot conditions across the islands, particularly across St. Croix and most coastal municipalities of PR. && .AVIATION... (18z TAFs) SHRA/TSRA will remain possible across NW-PR thru this evening. VFR conditions will prevail across the local flying area. However, the trade winds will bring pulses of SHRA from time to time. A SAL will arrive later tomorrow, lowering VIS, remaining P6SM. SFC wnds mainly fm E/ESE at 15 kt or less with sea breeze variations, bcmg calm to lgt/vrb aft 30/23Z. Expect E/ESE winds at 10-15 kt with sea breeze variations and ocnly hir gusts aft 31/14Z. && .MARINE... A moderate east to southeast wind flow will prevail across the region over the next few days. A tropical wave will continue to move far to the south of Puerto Rico and the U.S. Virgin Islands between tonight and early Friday, increasing winds and seas across the Caribbean Offshore Waters. Thunderstorms are expected to develop once again tomorrow, between the late afternoon and evening hours across the western waters of Puerto Rico. && .BEACH FORECAST... A low risk of rip currents will continue tonight across most beaches of Puerto Rico, St. Thomas, and St. John while a moderate risk is expected across Vieques, Culebra, and St. Croix. The risk of rip currents is expected to remain between low and moderate throughout the upcoming weekend. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM/MARINE/BEACH FORECAST...CVB/YZR LONG TERM...DSR AVIATION...CAM/RAM