000 FXCA62 TJSJ 101517 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 1117 AM AST Wed Aug 10 2022 .UPDATE...Mostly sunny skies prevailed across the islands during the morning hours. Showers were noted after sunrise across the Atlantic waters. By 11 AM, the temperatures were already in the low 90s across coastal areas of Puerto Rico and the U.S. Virgin Islands. The heat index was in the low 100s, mainly in St. Croix and along the northern coastal municipalities of PR. For this afternoon, the combination of the available moisture with daytime heating and the sea breeze convergence will lead to the development of showers with possible isolated thunderstorms over western PR. Urban and small stream flooding is possible with this activity. Across the USVI and eastern PR, streamer- type of shallow convection is expected to develop. && .AVIATION...Mainly VFR conditions expected across all terminals through most of the period. However, SHRA/TSRA expected to develop across western PR this afternoon, which may cause brief MVFR cigs at TJBQ. The 10/12z SJU sounding indicated east winds up to 23 kt blo FL050. && .MARINE...Seas will continue between 3-5 feet in general across the regional waters, and winds up to 15 knots should prevail through the day. However, locally fresh trades are expected across the Atlantic coastal waters of PR, and small craft should exercise caution there. A moderate risk of rip currents continues for most of the beaches of PR and the USVI. && //PREVIOUS DISCUSSION ISSUED AT 545 AM AST Wed Aug 10 2022// .SYNOPSIS... High temperatures and maximum heat indices in the low 100s will continue through the workweek. A weak perturbation will move across the forecast area today through Thursday increasing the potential for some showers with a chance of isolated thunderstorms over the regional waters and mainly western Puerto Rico. Lingering Saharan dust will result in slightly hazy skies through the end of the week. A tropical wave is forecast to move across the region on Saturday followed by a stronger and vigorous tropical wave late Sunday through Monday. && .SHORT TERM...Today through Friday A mid to upper level ridge continues to hold east of the Lesser Antilles. The circulation of the upper level high is causing winds to shift from the southwest at these levels, which is bringing some high level clouds with it. The mid level high is sustaining a trade wind cap inversion around 800 mb. At the low levels, infrared satellite imagery shows an easterly perturbation advancing into the local islands. A narrow band of showers was evident in the Doppler radar, with the bulk of the activity crossing the Atlantic waters. The low level moisture associated with this feature should combine with local effects to generate showers this afternoon across western Puerto Rico. With enough diurnal heating, isolated thunderstorms cannot be ruled out. Ponding of water on roadways and low-lying areas will be possible with this activity. East of this perturbation, the air mass looks drier, with Total Precipitable Water from GOES-16 showing values around 1.3 inches. This is below the climatological value, so once this air mass filters in, the probability of precipitation will decrease. This period of fair weather should not last too long, since a tropical wave wave should cross the Caribbean Waters early Thursday. The mid and upper level ridge will also weaken in response to a trough digging in from the north. With the available moisture and better support aloft, the potential for isolated thunderstorms increases. In the afternoon hours, widespread activity is anticipated, with the stronger convection developing across the Cordillera Central, northern and northwestern Puerto Rico, and downwind from El Yunque and from the Virgin Islands. Once this wave departs into the central Caribbean, another drier air mass will filter in across Puerto Rico and the U.S. Virgin Islands. Some saharan dust will be embedded within the air mass, hence skies could turn hazy. Under an easterly wind flow, near normal or just above normal temperatures will persist into the area. This in combination with dewpoints between 75 and 80 degrees, heat indices will soar between 102 to 107 degrees today, and a little higher through the end of the workweek. && .LONG TERM...Saturday through Wednesday... As previously mentioned the drier airmass is expected across the region on Friday, but a weak tropical wave will quickly move across the region on Saturday. This will bring an increase in moisture and better chance for showers and isolated thunderstorms mainly over the coastal waters and the west sections of Puerto Rico and the U.S. Virgin Islands during the afternoon hours. Some saharan dust and breezy conditions will accompany this wave, but widespread showers and thunderstorm activity is not expected during the day. However, by late Sunday and through Monday of next week, recent model guidance continued to suggest increasingly moist and unstable conditions with the arrival of a strong and more vigorous tropical wave, which will affect the region through at least Tuesday due to trailing moisture. That said the potential widespread showers and isolated thunderstorms will increase as guidance continue to suggest precipitable water values to exceed 2.0 inches during that time. Consequently urban and small stream flooding will be likely across the islands with the best chance late Sunday or early Monday through Tuesday. Additionally, as the wave crosses the region by Monday, the low level winds are expected to shift and become more southerly resulting in sufficient moisture being lifted up across the region favoring afternoon convection each day along with maximum heat indices above the 100 degrees in some area. By Wednesday conditions are expected to gradually improve. Please stay tuned and continue to monitor as this so far expected weather pattern unfolds. && .AVIATION...Mainly VFR conditions expected to persist through the forecast period. Some Saharan dust remains, but VIS are expected to remain P6SM. VCSH are expected to develop around TJBQ after 17Z with brief periods of low ceilings possible. Winds will be out of the east at 10 to 15 knots, with stronger gusts due to sea breezes. Winds are expected to slow down after 10/23Z. && .MARINE...Recent data from the surrounding buoys suggest seas will continue up to 5 feet and mainly from the E between 10 to 20 knots. Marine conditions will continue to gradually improve throughout the day, but will remain choppy for the rest of the work week especially over the offshore waters and local passage, where small craft operators should exercise caution. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. && $$ DSR/MRR/MMC