747 FXCA62 TJSJ 060835 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 435 AM AST Wed Oct 6 2021 .SYNOPSIS... A ridging pattern aloft will hold until Friday. An increase in moisture content due to remnants of Victor will aid in the development of showers and thunderstorms Thursday and Friday. Choppy to hazardous marine conditions will continue at least through early Thursday. && .SHORT TERM...Today through Friday... At lower levels, a surface high pressure north of the area will continue to promote east to east northeast winds. This will drive patches of low-level moisture into the area from time to time, resulting in brief showers across the local waters, USVI and portions of eastern and northern Puerto Rico during the overnight and early morning hours. A weak mid level ridge will continue to hold over the area through Friday. An upper level ridge will weaken as an upper low starts moving in on Thursday, and by Friday, the upper low will weaken and become a weak trough with its axis over the local islands. This will promote somewhat stable conditions, hindering widespread development of deep convection. That being said, scattered to locally numerous showers are forecast across parts of the local forecast area, but only isolated thunderstorms are expected. This overall pattern, in combination with the daytime heating and local effects, will cause shower, and isolated thunderstorm activity, across western Puerto Rico during the afternoon hours. However, due to the relatively fast steering flow, rainfall activity is not expected to be significant, therefore, the flooding potential is expected to remain low, though ponding of water in poor drainage areas is expected. As far as temperatures, we can expect near normal highs and lows, possibly within 2 degrees of normal. Highs in the upper 80s to low 90 across the lower elevations, with lows in the mid 70s to near 80. The higher elevations could observe highs and lows about 5 to 10 degrees cooler than the lower elevations, even slightly cooler in a few isolated areas. The heat index values could reach the low 100s across some areas in the lower elevations each day through the end of the workweek. .LONG TERM...Saturday through Thursday... A seasonal weather pattern is expected for this upcoming weekend with lingering moisture across the area through Sunday. A weak upper- level trough over the area will maintain some instability aloft with 500 mb temperatures remaining between -7 to -6 degree Celsius. However, the mid-level ridge will make a comeback with dry air filtering in at these levels. This will mainly support shallow convection and reduce coverage in thunderstorm activity. Therefore, isolated to scattered showers are expected during night hours and each morning across the local waters, USVI, and eastern PR. Meanwhile, diurnal heating and local effects will produce afternoon showers and isolated thunderstorms mainly across the central and western sections of PR. Model guidance diverge for next Monday throughout the week. Both GFS and ECMWF agree on the existence of a deepening cutoff low over Florida. However, discrepancies start with GFS having a much stronger upper low moving eastward while settling west of the forecast area and persisting through the week. Meanwhile ECMWF has the low weakening into a trough and staying a bit farther west. At low levels, a vigorous tropical wave will stream across eastern Caribbean with precipitable water values increasing to near and above 2.0 inches across Puerto Rico and U.S. Virgin Islands by early Monday. GFS also combines this event with the upper low pulling deeper moisture from the ITCZ into the forecast area supporting a very moist and unstable environment throughout the week. This suggests a broad coverage of shower and thunderstorm activity across the local forecast area through the end of the workweek. On the other hand, ECMWF has the tropical wave passing through but downplaying on the moisture pulling from the ITCZ, hence delaying deeper moisture in the air column and appearing much later in the workweek supporting a more seasonal weather pattern for most days. Therefore, confidence in the forecast is low. We will need to closely monitor the evolution of this forecast as there still exist the potential for a significant rain event by next week. && .AVIATION...VFR conditions expected to prevail across the local terminals through the forecast period. However, SHRA will be observed through at least 06/12Z, causing -SHRA at the local terminals. That being said, no significant impacts are expected. In the afternoon, after 06/16Z SHRA and ISOLD TSRA expected across western PR, causing VCTS at TJBQ. Winds will continue from the ENE at 10 KT or less through 06/12Z, increasing to 15 to 20 kts with gusts up to 25 kts and sea breeze variations after 06/14z. && .MARINE... Choppy to hazardous seas between 5 to 8 feet due to wind driven seas and a northeasterly swell are expected to continue across most of the regional waters through early Thursday. Therefore, there is a Small Craft Advisory in effect until Thursday morning. There is a high risk of rip currents across the northern and eastern beaches of PR, Culebra, eastern beaches of St.Croix and northern St. Thomas . && .PRELIMINARY POINT TEMPS/POPS... SJU 90 79 89 79 / 50 50 30 30 STT 90 79 89 79 / 50 50 40 40 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...High Rip Current Risk through Thursday afternoon for Culebra- North Central-Northeast-Northwest-San Juan and Vicinity- Southeast. VI...High Rip Current Risk through this afternoon for St Croix. High Rip Current Risk through this afternoon for St.Thomas...St. John...and Adjacent Islands. AM...Small Craft Advisory until 8 AM AST this morning for Caribbean Waters of Puerto Rico from 10 NM to 17N-Coastal Waters of Southern USVI Vieques and Eastern Puerto Rico out 10 NM. Small Craft Advisory until 2 AM AST Thursday for Coastal Waters of Northern Puerto Rico out 10 NM. Small Craft Advisory until 8 AM AST Thursday for Atlantic Waters of Puerto Rico and USVI from 10 NM to 19.5N. Small Craft Advisory until 2 PM AST this afternoon for Anegada Passage Southward to 17N-Coastal Waters of Northern USVI and Culebra out 10 NM-Mona Passage Southward to 17N. && $$ SHORT TERM...JA LONG TERM....CVB PUBLIC DESK...GL