617 FXCA62 TJSJ 130055 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 855 PM AST Sun Sep 12 2021 .UPDATE...Although the reasoning behind the forecast and analysis has not changed, some adjustments were made to the weather grids following the most recent model guidance and observations. Although drier air has filtrated the 750-500 mb layer as depicted in the 13/00Z sounding, and radar Doppler indicated a decrease in shower and thunderstorm across the area, enhanced tropical moisture associated to a passing tropical wave now moving into the eastern Caribbean interacting with an upper level low to our northwest will set the conditions to support further shower and thunderstorm development later tonight and continuing through at least Monday night. In fact, model guidance suggests precipitable water values around 2.10 inches with 500 mb temperatures between -7 and -8 degrees Celsius between 13/16Z and 14/08Z. This will definitely lead to another active afternoon, with shower and thunderstorm development favoring the interior and western sections of Puerto Rico, as well as portions of the San Juan metropolitan area. In the meantime, shower and thunderstorm activity is expected to favor the regional waters during the overnight and early morning hours, moving into eastern Puerto Rico and the US Virgin Islands from time to time. Moderate to locally heavy rains over already saturated soils will result in ponding of waters to urban and small stream flooding and possible mudslides near areas of steep terrain. Overnight lows are forecast to drop into the lower 70s to around 80 degrees Fahrenheit, while daytime highs are expected to peak into the lower 90s with heat indices reaching the lower to mid 100s. Winds will continue from the east, but at lower speeds during the overnight and early morning hours, increasing to 10-20 mph with higher gusts during the day. && .AVIATION...The combination of a tropical wave and an upper level low moving across the region through Monday will promote shower and thunderstorm development with VCSH/VCTS at most terminals during the next 24 hours. The focus of the activity will affect Leeward, USVI and TJSU terminals through 13/12Z and after 13/22Z, and all PR terminals between 13/15-22Z. This may result in brief MVFR conditions with SCT-BKN layers between FL025-050 and reduced visibilities. Winds will be light and variable through 13/12Z, turning from the ESE at 10-20 knots with occasionally higher gusts thereafter. && .MARINE...No changes to the marine and surf zone grids. Please refer to the Marine discussion below for more information. && .PREV DISCUSSION... /issued 454 PM AST Sun Sep 12 2021/ SYNOPSIS...A Tropical Upper Tropospheric Trough (TUTT) low will continue to move westward and stall over Hispaniola through early next week. A tropical wave will continue to pull further away from the Mona Passage tonight. A surface trough will move across the local area on Monday from the east. Precipitable water content is forecast to remain above 2 inches through the first part of the workweek. A mid-level ridge and a Saharan Air Layer should promote drier air and hazy skies during the second part of the workweek. SHORT TERM...Today through Tuesday... The TUTT-low is expected to meander over Hispaniola through the rest of the short term period. This will continue to promote unstable conditions and provide divergence aloft to enhance shower and thunderstorm activity across the local area. Furthermore, a surface trough east of the Windward Islands will bring a surge of low-level moisture on Monday, and linger through Tuesday. Therefore, showers and thunderstorms are expected each day across portions of the regional waters and enhanced in the diurnal cycle over the mountain ranges of PR, as well across the San Juan metro area and western PR. Urban and small stream flooding is expected due to saturated soils across much of PR. Across the USVI, scattered showers are expected mainly, but thunderstorms across the nearby coastal waters could move at times over land areas and cause ponding of water in poor drainage areas. LONG TERM...Wednesday through Sunday... From Previous Discussion /issued 428 AM AST Sun Sep 12 2021/ An induced surface trough is expected to develop just east of the Bahamas by Wednesday. This feature will drag additional deep tropical moisture into the area with precipitable water values expected to range between 2.0-2.2 inches. This in combination with a weakening TUTT expected to be located over the island of Hispanola will continue to result in favorable conditions for additional enhanced shower and thunderstorm activity, with the potential for urban and small stream flooding across areas that receive the heaviest and most persistent rainfall activity. Thursday is expected to be a transition day. As the aformentioned TUTT moves further westward, a mid and upper-level ridge will begin to build overhead. This feature will be introducing drier air aloft. However, abundant low-level moisture will still remain in place that another round of afternoon convection appears likely, mainly over the northern half of Puerto Rico. The ridge aloft pattern will hold through the rest of the long term period, limiting the potential for widespread convective activity. Nevertheless, sufficient low-level moisture will still be present to combine with strong daytime heating and local effects to generate afternoon convection, mainly across the western half of Puerto Rico. A Saharan Air Layer, according to the NASA Dust Extinction Aerosol Model, will be over the area between late Wednesday and Friday bringing light to moderate dust concentrations and somewhat hazy skies. Dust concentrations look to diminish by next weekend as the Saharan Air Layer moves away. AVIATION...The combination of a tropical wave and a upper low moving across the region thru Monday will promote shower and Isold Tstorm dvlpmnt with VCSH/VCTS at most terminals during rest of the period. Winds will be mainly fm ESE 10-15KT with occasionally hir gusts, becoming light and variable aft 12/23Z. SCT-BKN lyrs nr FL025... FL050... FL100. BKN ocnl OVC btw FL200-FL280. Incr cloud development ovr the ctrl mountain range of PR will cause brief Mtn Top Obscr with aftn convection. Max Tops btw FL300-FL350 w/TSRA. VCSH/VCTS psbl at most terminals til 12/23Z. SFC wnds will bcm lgt/vrb aft 12/23Z. MARINE...Seas across the coastal waters of Puerto Rico and the USVI should range between 2-4 feet during the next several days. Across the offshore waters and passages, seas are expected to range in general between 3-5 feet, with up to 6 feet on Monday across the offshore Atlantic waters. Trade winds in general should range between 10-15 knots, with locally higher winds at times across the Anegada Passage and offshore Atlantic waters. A moderate risk of rip currents will prevail across most of the north and east facing beaches of the islands through at least Tuesday. && .PRELIMINARY POINT TEMPS/POPS... SJU 77 89 78 89 / 50 60 40 50 STT 79 88 78 91 / 60 60 40 60 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...ICP LONG TERM....DS PUBLIC DESK...FRG