Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
977 FXCA62 TJSJ 192100 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 500 PM AST Thu Sep 19 2024 .SYNOPSIS... Light and variable winds will persist over the next few days, promoting slow-moving showers and thunderstorms. The risk of flooding will be elevated nearly every afternoon, while hot temperatures are expected to continue across the islands. && .SHORT TERM...Tonight through Saturday Explosive convection prevailed during the afternoon hours, with very high values of Vertically Integrated Liquid (VIL) and Geostationary Lightning Mapper (GLM) data indicating strong thunderstorms featuring frequent lightning, torrential rain, and the potential for pea-sized hail. Clusters of thunderstorms pulsed in intensity throughout the afternoon, gradually redeveloping and propagating westward across interior Puerto Rico. Several Flood Advisories and Special Weather Statements were issued. Excessive runoff caused multiple USGS river sensors to reach action stage, with Rio Tanama briefly hitting minor flood stage. Although showers and thunderstorms have subsided, excessive runoff will continue through the evening, with several rivers remaining at elevated streamflows. The unstable and wet weather pattern will persist through the rest of the short-term forecast. Precipitable water content will fluctuate between 1.8 and 2.2 inches through Friday before dropping below 1.7 inches on Saturday. Conditions aloft will remain conducive for deep convection as a TUTT (Tropical Upper Tropospheric Trough) stays anchored over the northeast Caribbean. Remnants of Gordon in the central Atlantic will continue influencing the wind pattern across the forecast area. A low-level col region, currently southeast of St. Croix, will gradually move northwest and linger nearby through the weekend. This will promote a mostly light and variable wind pattern, exacerbating rainfall accumulations due to slow-moving showers and thunderstorms each afternoon, particularly on Friday. The 925 mb temperatures will rise, especially on Saturday. As a result, the combination of the TUTT to the northeast of the Caribbean, abundant moisture, and strong surface heating will support diurnally induced convective activity, mainly over interior Puerto Rico each afternoon, though less widespread on Saturday. A limited to elevated flooding threat, along with the potential for frequent lightning and gusty winds, is expected with this activity. As mentioned above, 925 mb temperatures will exceed the 90th percentile over the next few days. Therefore, before afternoon convection, heat index values could reach Heat Advisory or Warning criteria, particularly in urban and coastal areas. Be sure to drink plenty of fluids and avoid direct sun exposure during peak hours of the day. .LONG TERM...Sunday through Wednesday... .PREV DISCUSSION... /issued 350 AM AST Thu Sep 19 2024/ The pressure gradient will remain weaken late on the weekend and early next week, with a low level steering flow coming from the south-southwest of 1 to 4 knots. These light winds are due to a series of trough over the western Atlantic. At the surface a small pocket of moisture is expected to reach the islands. This will act to fuel afternoon showers and thunderstorms along the Cordillera Central and then spreading toward the coastal areas of Puerto Rico. Some of this shower activity, although at a lesser scale, are also expected for the Virgin Islands. On Tuesday, the combination of a low pressure system developing over the western Caribbean, and an upper level trough near the Bahamas will escort an area of mid to upper level clouds into the region. Conditions aloft will be favorable for showers and thunderstorms too, so if enough sunshine filters in, another active afternoon will be likely for the Cordillera Central, and with the showers then drifting toward northeastern Puerto Rico and the San Juan metro area. These showers will later move into the vicinity of the Virgin Islands. Late Tuesday and Wednesday, the trade wind will recover, gaining an easterly component, and at 7 to 11 knots. The unsettle weather pattern is expected to persist, as an upper level low develops near the islands. The risk for flooding will be elevated all these days. Temperatures will be high too, but the excessive risk will be dependent on how deep and widespread the cloud coverage finally materialize. && .AVIATION... (18Z TAFs) Mainly VFR conditions are expected to prevail across all terminals through the forecast period. However, SHRA/TSRA over mainland PR may cause tempo MVFR cigs across the area terminals until sunset. Light winds with a northerly component will prevail through 20/08z. A southerly component and lighter winds are forecast for Friday. && .MARINE... A weak surface low pressure lingering northeast of the region will continue to shift winds, gradually becoming variable and weakening throughout the weekend. Showers and isolated thunderstorms will persist across the regional waters, particularly off the western coastal waters of Puerto Rico each afternoon. && .BEACH FORECAST... Low to moderate rip current risk will persist throughout the period. A long-period northwesterly swell could reach northern exposed beaches by midweek next week, potentially increasing the rip current risk at some beaches. For a more detailed forecast, please refer to the Surf Zone Forecast for Puerto Rico and the U.S. Virgin Islands (SRFSJU). && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM/MARINE/BEACH...CVB LONG TERM...PREV DISC AVIATION...DSR/RAM PUBLIC...CAM