Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
733 FXCA62 TJSJ 142010 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 410 PM AST Fri Jun 14 2024 .SYNOPSIS... An induced surface trough passing through the region will promote passing showers and afternoon convection tonight through tomorrow, however mid to upper- level ridging will limit widespread rainfall. A plume of Saharan dust of moderare to high concentration is forecast to spread across the region, causing hot and hazy conditions that last through the second half of the weekend into the next workweek. An Excessive Heat Watch is in effect for tomorrow. && .SHORT TERM...Tonight through Sunday... A surface trough will exit the region overnight, heading northwestward which will begin a veering of the surface to lower level winds to a south-southeasterly direction by tomorrow lasting through Sunday. Meanwhile, a mid to upper-level ridge continues to anchor across the central Caribbean, providing overall stable conditions. A dry airmass containing Saharan dust begins to move northward across the eastern Caribbean basin late Saturday into Sunday, however plenty of moisture hung up from the short wave trough will sit over the area keeping rain possible in the forecast through the weekend. Most of the expected rainfall will be across eastern Puerto Rico through the day and night in the form of passing showers, however during the afternoons convection is likely across northern central to northwestern Puerto Rico with the southerly component in the wind flow. The combination of suspended Saharan dust particulates and the mid to upper level ridging will favor subsidence and good compression of the warm air in the low layers throughout this weekend. Daytime high temperatures will be in the low 90s especially along the north and west coastal areas with maximum heat indices expected to again exceed 100 degrees, resulting in excessive heat impacts particularly along the west north and east coastal and urban areas. Currently a Heat Advisory is in effect for most of the islands tomorrow besides the interior and southern coastline of Puerto Rico. && .LONG TERM...Monday through Thursday... /from previous discussion/ Although a decrease in saharan dust is forecast to start the workweek, another pulse will continue to promote hazy conditions through most of the long term period. Current model guidance suggest Precipitable Water (PWAT) values at generally normal values to start the period with a patch of below normal PWAT values reaching the area on Tuesday. For Wednesday and Thursday, columnar moisture will increase once again to normal to above normal values as moisture related to a surface trough and a possible wave, although uncertainty remains. A developing upper level trough near the area will also increase instability. Although model guidance suggest moisture over the mid levels, a more notable increase is suggested by Wednesday. Most available moisture to start the workweek will remain below 850 mb, however. East to east- southeast surface winds should affect the islands through Thursday, when they become more easterly and increase. This will steer warmer tropical air towards the islands, promoting overall warm to hot conditions. Heat related products will possibly be issued for the islands to start the period. 925 mb temperatures are forecast to be above normal to start the workweek. && .AVIATION... (18Z TAFS) Scattered SHRA expected to prevail across eastern PR and TJBQ through 22Z, with periods of reduced VIS and low ceilings possible. Additional activity expected to increase after 22Z for the Caribbean waters and USVI terminals as well. Winds are shifting from the ESE at around 10-12 knots, becoming more from the south above the SFC to FL050 on 15/12Z. Hz due to Saharan dust is expected to increase after 12/16Z for the USVI, but VIS should remain at P6SM. && .MARINE... Surface high pressure over the Atlantic and an induced trough over the area will promote light to moderate east to southeast winds. Moist air will allow showers and isolated thunderstorms to develop over portions of the local waters tonight through the weekend. Winds will continue to diminish and veer, becoming more southeasterly through the weekend with increasing Saharan dust expected across the region through Sunday. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory until 6 PM AST this evening for PRZ001-005-008-010. Excessive Heat Watch from Saturday morning through Saturday afternoon for PRZ001-002-005-008-010-012-013. VI...Excessive Heat Watch from Saturday morning through Saturday afternoon for VIZ001-002. AM...None. && $$ WS/YZR/RC