Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
031 FXCA62 TJSJ 232109 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 509 PM AST Mon Sep 23 2024 .SYNOPSIS... Light winds will prevail over the next few days with showers and thunderstorms each day, but generally less on Wednesday. A low risk of rip currents will be seen tonight and Tuesday due to light winds. Winds will increase somewhat on Wednesday through Friday when a mild tropical wave is expected to move across the forecast area. && .SHORT TERM...Tonight through Wednesday... Calm conditions in terms of rainfall prevailed in the morning across the islands, accompanied by light winds with a southerly component. The primary concern has been the heat, with heat index values generally ranging from 108 to approximately 115 degrees F across the coastal and lower-elevation areas of Puerto Rico and the U.S. Virgin Islands, validating the Heat Advisory and Excessive Heat Warning. Maximum values reached as high as 120 and 123 degrees F in Vega Baja and St. Croix, respectively. After midday, satellite imagery indicated the development of cloud cover across the Cordillera Central, leading to the formation of showers and thunderstorms throughout the interior of Puerto Rico. According to radar estimates, around 1 to 1.5 inches of rainfall have fallen. At the moment, two Special Weather Statements have been issued due to strong thunderstorms. At least for the next hour, shower activity due to diurnal heating is expected to continue mainly across western sectors of Puerto Rico. For tonight, expect rain activity to subside in coverage, but some isolated showers across the waters and local passages, U.S. Virgin Islands, and eastern sectors of Puerto Rico cannot be ruled out of the forecast. During the remainder of the short term-period, a similar pattern with showers and thunderstorms can be expected, particularly across the interior each day. This pattern will be influenced by diurnal heating, local effects, and the presence of an upper-level trough moving from the western Atlantic. At the surface, light ESE winds will continue to lead higher rainfall accumulations over the same areas, increasing the potential for flooding and mudslides in areas of steep terrain. On Wednesday, winds will turn more form the east and increase a little, helping to move showers faster. In addition, an elevated to significant heat risk will persist throughout the short-term period. Residents and visitors should monitor the forecast and take precautions to stay hydrated by drinking plenty of water, wearing light and breathable clothing, and avoiding prolonged sun exposure, especially during peak heat hours. && .LONG TERM...Thursday through Monday... .PREV DISCUSSION... /issued 529 AM AST Mon Sep 23 2024/ High pressure in the lower levels will be found just north of the area and this will induce a more typical easterly surface flow. That high pressure will move westward and a mild tropical wave will move through the U.S. Virgin Islands, Puerto Rico and most of the eastern Caribbean south of the area Friday afternoon and Friday night. On Saturday a shearline will approach the islands from the north but remain north of 20 north. This will maintain easterly flow and given that the moisture does not really diminish we will continue to see typical shower and thunderstorm formation Saturday and Sunday. On Monday flow becomes southerly and very light and temperatures will begin to climb a few degrees. This will lead to stronger showers and thunderstorms in central and western Puerto Rico. At upper levels a weak TUTT low will be found just south of Puerto Rico on Thursday. When it moves west, a second TUTT will dive in from the northeast and will create a environment only somewhat favorable to active convection given that 500 mb temperatures will rise during the period--especially after the tropical wave passes. Strong high pressure at upper levels will hold over Cuba and then move over southern Florida adding to the stability over the weekend and into Monday. With light southerly surface winds beginning then, we will see a warming trend. && .AVIATION... (18Z TAFs) SHRA/TSRA will develop along the Cordillera Central, spreading over the W, N and E coastal municipalities of PR thru 24/01z. Therefore, we expect mtn obsc and MVFR conds over the interior through this evening. TSRA will impact JSJ between 23/18-20z, creating MVFR or even IFR conditions. Elsewhere, expect VCTS. SHRA/TSRA could develop downwind from the USVI into the surrounding waters. Expect southerly winds with sea breeze variations around 10 knots. Winds will become calm to light and variable aft 23/23z. && .MARINE... A weak high pressure ridging in over the region from the east northeast will maintain light and variable winds through tomorrow. Then, winds will become gentle to moderate and with an easterly component. Showers and isolated thunderstorms will develop from time to time across the regional waters, and some will move into the coastal waters of Puerto Rico each afternoon. A northeasterly swell will increase seas to around 4 to 5 feet during the latter part of the week. && .BEACH FORECAST... The Rip Current Risk will remain low tonight and Tuesday, then the risk will increase across the north and northeast beaches of Puerto Rico, Culebra, and the U.S. Virgin Islands around Wednesday through Friday. Rip current risk could be high on Friday due to a 4 to 5 foot swell reaching the area from the northeast. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...RAM/YZR/MNG LONG TERM...WS AVIATION...CAM/RAM PUBLIC...ICP/MNG