000 FXCA62 TJSJ 160830 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 430 AM AST Fri Apr 16 2021 .SYNOPSIS... Moisture will increase today due to the passage of an old frontal boundary. As a result, showers and thunderstorms are expected to develop, with the potential for heavy rain. Drier air, along with Saharan dust is expected for the weekend and the first part of the next workweek. && .SHORT TERM...Today through Sunday... The remnants of an old frontal boundary, are moving across the Caribbean waters into the Anegada Passage and across portions of the islands. This will cause a surge in moisture and increase shower activity in general through the evening hours. An upper level jet will move across the Atlantic waters later today, enhancing the potential for isolated thunderstorm development and for afternoon heavy showers. Urban and small stream flooding is expected with the heaviest showers. As steering winds are from the southeast today, shower development along the Cordillera Central is expected to drift northwards this afternoon. The remnants of the frontal boundary are expected to slowly move into the Atlantic waters by tonight, and moist southerly winds should continue to carry showers across the southern and eastern sections of PR. Across the USVI, cloudy skies and scattered shower activity are expected through much of the day. For Saturday, the remnants of the frontal boundary are expected to move over the offshore Atlantic waters and away of the region, while a building ridge east of the Leeward Islands will promote drier air intrusion and the arrival of a Saharan Air Layer (SAL) should cause hazy skies and limit shower activity to diurnal activity over northwestern PR. Even drier conditions are expected on Sunday, and no significant rainfall accumulations are anticipated at this moment across the islands. Overall, the weekend should be hazy due to the SAL and maximum temperatures should range from the high 80s to low 90s across coastal areas. .LONG TERM...Monday through Friday... As the workweek begins, the mid level ridge will continue to hold over the eastern Caribbean. At the surface, high pressure extends from the central toward the western Atlantic Ocean, generating a southeasterly wind flow over the local islands. The latest guidance still shows dry air at the low and mid levels. This air mass contains Saharan dust, hence hazy skies will prevail through at least Tuesday. By mid-week, the high pressure at the surface will tighten the gradient along the area, resulting in an enhanced wind flow, but so far, the models are only showing moderate winds over the regional waters. The mid-level ridge will begin to move into the central Atlantic by Wednesday into the latter part of the week, while an upper level trough develops to the northeast of the Lesser Antilles. Although the trough should continue to deepen, the islands will remain on its subsidence side. As the ridge begins to lose its influence, fragmented groups of clouds embedded in the trade winds will increase in frequency. These moisture fields will be a little deeper, reaching up to 700 mb. As a result, the atmosphere will become more favorable for shower development, with passing showers affecting eastern Puerto Rico and the U.S. Virgin Islands, and convective activity developing in the afternoon mainly over the interior and western sections of Puerto Rico. There is still some uncertainty in the timing of arrival of these groups of clouds, but so far, both the GFS and ECMWF are suggesting this increase in moisture by sometime on Thursday and Friday. && .AVIATION...The remnants of an old front will move across PR and the USVI through the day. This will cause tempo MVFR cigs across all the local terminals, as well as intermittent periods of -SHRA. In addition, +SHRA/TSRA is expected to develop this afternoon over central PR, drifting to the north and impacting the northern terminals through at least sunset with possible MVFR to brief IFR conditions. Winds are expected from the SE at 10-15 kt, with higher gusts at times in SHRA/TSRA. && .MARINE... Recent observations are showing a dominant period of 10-11 seconds over the northern areas of Puerto Rico, with seas 3 to 5 feet. The outer buoy, 41043, also shows 10 seconds at 6 feet. This weak northerly swell is expected to continue today, while another one is expected to arrive during the weekend. For the beaches, there is a moderate rip current risk for the northwest, northern, and northeast coast of Puerto Rico, all of Culebra, eastern Vieques, most of St. Thomas, northwest St. John, and the eastern tip of St. Croix. && .PRELIMINARY POINT TEMPS/POPS... SJU 87 76 89 76 / 40 40 20 20 STT 86 77 84 76 / 50 50 20 20 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...DS LONG TERM....ERG