FXCA62 TJSJ 301959 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 359 PM AST Fri Jul 30 2021 .SYNOPSIS... Saharan dust along with drier air is expected to filter in for the weekend, limiting the shower activity overall. Conditions should change by early next week as more moisture reaches the islands. Choppy marine conditions are expected to prevail for the next several days. && .SHORT TERM...Tonight through Sunday... Heavy showers and thunderstorms developed over the northwestern quadrant of Puerto Rico during the afternoon hours, leaving rainfall accumulation around two inches. Additional showers were also observed across the U.S. Virgin islands, Vieques and Culebra, but with limited rainfall amounts. Total Precipitable Water from Goes-16 currently shows an area of enhanced moisture entering the islands, which should result in passing showers through the eastern half of the forecast area for the evening and overnight hours. The synoptic pattern shows an upper level trough (TUTT) north of Hispaniola, with a high pressure moving from the east. At the mid levels, a ridge also dominates to the north of the eastern Caribbean, while a tropical wave east of the Lesser Antilles approaches from the east. However, the bulk of the wave should remain over the Caribbean waters as the system progresses westward. Due to the presence of the mid-level ridge, the GFS forecast sounding are showing a stronger trade wind-cap around 850 mb through Sunday, with most of the moisture confined to the lower levels of the atmosphere. The aerosols models also have some Saharan dust embedded in the air mass, which should result in hazy skies. Additionally, a surface high pressure over the eastern Atlantic will maintain a tight pressure gradient, resulting in a moderate east- southeast wind flow. Taking all these factors into consideration, mainly tranquil weather conditions are expected to prevail this weekend, but trade wind showers cannot be ruled out early in the day over the eastern municipalities of Puerto Rico and the U.S. Virgin Islands. Then, in the afternoon, diurnal heating may aid in the formation of a few showers over western Puerto Rico. However, rainfall accumulation are expected to remain moderate at best. In terms of temperatures, expect very warm temperatures, with highs climbing to the low 90s, and heat indices surpassing the 100 degree mark. .LONG TERM...Monday through Saturday... A wetter pattern is expected for most of the long term period. At the surface, a wind surge with precipitable water values near normal will reach the local islands, increasing the potential for shower development. In the upper levels, a retrogressing TUTT-low will continue to moves westward. For Monday, model guidance suggests the subsident side of the TUTT will move over the local area. Regardless of the stability of the conditions aloft, the available moisture, along with the local effects can erode the trade wind cap and result in afternoon convection over western Puerto Rico. By Tuesday, at the surface, moisture embedded in the east- southeasterly wind flow is expected to move in. Meanwhile, better upper level dynamics are expected, both the GFS and ECMWF model guidance are suggesting a TUTT to be located just at north of the forecast area. If model guidance is correct, the TUTT will result in unstable conditions on Tuesday into early Wednesday. Therefore, shower activity is expected across the forecast area, possibly resulting in urban and small stream flooding each afternoon over the first half of the workweek. The moisture will slowly decrease over the region on Thursday, as a narrow slot of drier air moves in. This will limit shower activity through the late evening hours on Thursday. By Friday, a tropical wave is expected to move over the lesser Antilles into the local area. The bulk of the moisture is expected to remain to our south, however, above normal moisture is expected over the area. So given the expected pattern, scattered showers are expected across the local islands. As the building surface high pressure moves into the central Atlantic on Saturday, a mixture of patches of moisture and drier air will result in isolated to scattered shower activity across the local waters, USVI, and eastern PR in the morning and overnight hours, while afternoon convection would be expected somewhere across western PR. && .AVIATION...VFR conds at all TAF sites with VCSH possible in and around JBQ thru 30/22. Aft 30/22z, VCSH possible in and around the USVI terminals as well as JSJ. HZ is present but VIS expected to remain P6SM. Winds will be from the east ranging from 15 to 20 kts with higher gusts and seas breeze variations, becoming at 10 knots or less overnight. && .MARINE... Recent buoy observations show seas around 2-4 feet across the nearshore waters, and up to 5 feet offshore, and winds out of the east at 10 to 20 knots. Small craft operators should exercise caution due to winds. These conditions should linger for the next few days. Across the local beaches, the rip current risk is moderate for the northern, eastern, some in the south, as well as Vieques and Culebra, most of St. Croix and the southern coast of St. John and St. Thomas. && .PRELIMINARY POINT TEMPS/POPS... SJU 79 89 79 89 / 20 20 10 20 STT 80 89 80 89 / 20 20 10 20 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...CAM LONG TERM....ERG PUBLIC DESK...CVB