Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
319 FXCA62 TJSJ 190750 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 350 AM AST Thu Sep 19 2024 .SYNOPSIS... Winds will weaken over the islands in the upcoming days, with slow moving showers and thunderstorms developing each afternoon. The risk for flooding will be elevated almost each afternoon. Hot temperatures are also expected to persist across the islands. && .SHORT TERM...Today through Saturday... Currently, radar and satellite imagery show calm, fair-weather conditions across the islands, with light and variable winds. Isolated showers across the local waters. Temperatures in coastal and lower-elevation areas of Puerto Rico range from the upper 70s to lower 80s, while higher elevations experience cooler temperatures in the mid to upper 60s. In the U.S. Virgin Islands, temperatures are currently in the upper 70s to lower 80s. Today, a patch of moisture moving in from the northeast will rapidly increase precipitable water values (PWAT) to above-normal levels, reaching between 2.0 and 2.2 inches. Winds at the surface will have a northeasterly component while becoming lighter, around 3 to 6 knots. This moisture will combine with local effects to generate showers across the islands this afternoon. As a result, minor urban and small stream flooding may occur due to locally and diurnally induced slow-moving showers with heavy rainfall and thunderstorms, especially across the Cordillera Central and western sections of Puerto Rico. Similar conditions are expected to prevail on Friday. Latest satellite-derived precipitable water values indicate a patch of drier air moving into the area around next Saturday, with PWAT values expected to range from 1.4 to 1.7 inches. Heights will begin to increase as the axis of an upper-level trough moves away, leaving a subsiding portion over our region. However, local effects and diurnal heating will still promote afternoon convection in some areas, particularly in the interior, northwestern, and north-central Puerto Rico. The U.S. Virgin Islands are expected to experience mostly isolated showers. Northeast winds will help keep temperatures somewhat cooler today, but warm to hot conditions will likely continue in the coming days, with heat indices remaining elevated. Guidance shows temperatures at 925 mb two standard deviations above normal. Residents and visitors should 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. As a summary, the increase in moisture and favorable conditions aloft will lead to more showers and isolated thunderstorms today and Friday, with a broader area likely experiencing limited to elevated flooding and lightning risks. Better conditions are anticipated on Saturday, but local effects and diurnal heating will still promote afternoon convection in some areas. Warm to hot conditions will likely continue in the coming days. .LONG TERM...Sunday through Wednesday... 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... (06Z TAFs) Mainly VFR conditions are expected to prevail through the morning forecast period, with VCSH likely across TJSJ and U.S. Virgin Islands TAF sites. After 19/1700Z, additional activity is anticipated to develop in the Cordillera Central, causing mountain obscuration and likely affecting TJPS. Winds will be from the NE at 10 knots or less, becoming calm or light/variable after 19/2300Z. && .MARINE... East-northeast winds will dominate, gradually becoming variable and weakening throughout the weekend. Showers are expected over the Atlantic waters today, with a chance of isolated thunderstorms off the south and west coast today. && .BEACH FORECAST... Low rip current risk is expected today for most of the local beaches, but the risk will increase to moderate tonight and will stay like that until Sunday. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...YZR LONG TERM....ERG