Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
887 FXCA62 TJSJ 150048 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 848 PM AST Fri Jun 14 2024 .UPDATE...Little change to the current package. Have removed haze from the weekend forecast. Although some Saharan dust will enter the area the model has backed off on the amount expected and the RGB Dust product shows most of the dust is to our south. This will allow better chances of rain, but quantities are still expected to be light except where intense thunderstorms form in western and interior Puerto Rico. Have raised the local overnight temperatures for tonight in the vicinity of San Juan on the basis of persistence, but will hold onto the high temperatures tomorrow area-wide on account of the south southeast winds expected in the area tomorrow. The heat watch is continued. For more details see the NPW Weather Message. && .AVIATION...Numrs SHRA and isold TSRA will linger thru 15/04Z tonight--mainly ovr NW and W PR where MVFR/IFR conds may be expected. Then VFR conds should prevail through 15/17Z. Aft 15/17Z SHRA/TSRA will redvlp in NWrn PR with MVFR/IFR conds some of which may affect TJBQ. Wind flow of 6-12 knots will turn to the SE overnight, but land and sea breeze patterns will override this flow during the midday hours. VSBYs in Saharan dust are expected to remain above 10 miles thru the period. && .CLIMATE...Clouds and showers prevented temperatures from warming to yesterday`s heat, but San Juan`s minimum temperature today is likely to remain at 82 degrees, breaking the record warmest minimum for the date of 80 degrees set in 2023 and three previous years. && .PREV DISCUSSION... /issued 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...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. && $$ SHORT TERM/AVIATION...WS MARINE/PUBLIC....RC UPPER AIR/PUBLIC...YZR