Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
625 FXCA62 TJSJ 182046 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 446 PM AST Wed Sep 18 2024 .SYNOPSIS... Generally fair weather this evening, with shower activity increasing later tonight. A wetter pattern is anticipated in the coming days, driven by rising instability and moisture under weak steering winds. Excessive heat will continue to be a concern for vulnerable individuals, especially those outdoors without adequate cooling or hydration. Additionally, a moderate risk of rip currents will remain over the next few days. && .SHORT TERM...Tonight through Friday... Radar, satellite, and surface analyses indicate generally fair, warm-to-hot conditions, with skies becoming progressively partly to variably cloudy. Shower activity began this morning with isolated showers over eastern Puerto Rico, followed by streamers downwind of the Sierra de Luquillo and the local islands. The strongest showers, accompanied by isolated thunderstorms, developed over the island`s interior and western areas in the afternoon. Radar estimates indicated the highest rainfall totals, around 2 inches, between Lares, Las Marias, and Maricao in western Puerto Rico, prompting the issuance of a Flood Advisory. Surface observations report consistently hot temperatures up to 93-96F, with heat indices rising into the mid-100s, while higher elevations recorded temperatures in the lower 80s. Winds were predominantly from the east to east-northeast at 10-15 mph, with gusts up to 25 mph along coastal areas. The short-term forecast highlights a period of significantly below-normal precipitable water (PWAT) levels for tonight, with values dropping to as low as 1.2-1.4 inches, which is about two standard deviations below normal for September. PWAT will rapidly increase to above-normal levels on Thursday, reaching 2.0-2.2 inches, before returning to typical values around 1.8-2.0 inches by Friday. A deep layer high to the west-northwest will be replaced by a tropical upper tropospheric trough (TUTT) moving in from the east-northeast in the coming days. This shift will increase instability and the potential for isolated to scattered thunderstorms as the trade wind inversion weakens and 500 mb temperatures drop to around -6C. At the surface, east- northeasterly winds of 10-15 mph will become more northeasterly and decrease to 5-10 mph on Thursday before shifting to southeasterly on Friday, influenced by the remnants of Gordon located far to the northeast over the central subtropical Atlantic. Despite the expected moisture and wind variations, intense heat will persist, with 925 mb temperatures exceeding two standard deviations above normal thresholds for September, which may require the issuance of Excessive Heat Advisories or Warnings. Tonight`s forecast calls again for generally fair weather conditions to shift towards increased shower activity over northern and eastern sections as high moisture airmass reaches the region behind a drier-than-normal airmass later tonight. However, flooding impacts are not expected, with rainfall totals ranging from trace to around a quarter of an inch. Expect gentle to light and variable winds, with overnight lows ranging from the upper 60s in higher elevations to the lower 80s in coastal areas of eastern Puerto Rico and the local islands. Further increase in moisture and favorable conditions will lead to more showers and isolated thunderstorms on Thursday and Friday, with a broader area likely experiencing limited to elevated flooding and lightning risks. For details on excessive heat, excessive rainfall, or other potential hazards in the coming days, please refer to the Experimental Graphical Hazard Weather Outlook at https://www.weather.gov/erh/ghwo?wfo=sju. .LONG TERM...Saturday through Tuesday... From previous discussion...issued at 500 AM AST Wed Sep 18 2024/ 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 1.6 inches. Heights will begin to increase as the axis of an upper-level trough moves away, leaving the subsiding portion over our region, which will help limit rain development a little. While there wont be enough moisture for widespread rainfall, local effects and diurnal heating will still promote afternoon convection in some areas, particularly in the interior, northwestern, and north-central Puerto Rico. In contrast, the U.S. Virgin Islands are expected to experience mostly isolated showers. Similar conditions are anticipated to persist into Sunday. By Monday and Tuesday, moisture will increase from the south, resulting in better rain chances across the islands. During this period, winds are expected to remain very light and somewhat variable. As a result, urban and small stream flooding may occur due to locally induced slow-moving showers and thunderstorms with heavy rainfall, especially in the central and western sections of Puerto Rico. Hot conditions will likely dominate over the long-term period, with potentially heat indices remaining elevated. Over the weekend, guidance are suggesting that temperatures at 925 mb will remain two standard deviations above normal. The combination of high temperatures and humidity could create hazardous heat conditions, particularly in urban and low-elevation areas of the islands. Therefore, it is likely that Heat Advisories and Excessive Heat Warnings will be issued. Residents and visitors are encouraged to monitor the forecast, 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. && .AVIATION... (18Z TAFs) Mainly VFR conditions are expected to prevail across all terminals through the forecast period. However, locally induced SHRA/TSRA over portions of the islands may cause tempo MVFR cigs thru 22z. Also, SHRA is expected to increase aft 19/12z in and around TJSJ/TIST. Winds will continue to gradually turn from the NE and become lighter through Thursday. && .MARINE... East-northeast winds will dominate, gradually becoming variable and weakening from Thursday throughout the weekend. Showers are expected over the local Atlantic waters and Caribbean passages later tonight, with a chance of isolated thunderstorms off the western coast of Puerto Rico each afternoon. Increased showers with isolated thunderstorms over the local waters are likely on Thursday and Friday. Overall, seas will range from 3 to 4 feet, occasionally reaching 5 to 6 feet in some areas, while southern coastal regions of the islands will experience lower seas in the coming days. && .BEACH FORECAST... The risk of rip currents will remain low at most local beaches, except for northern Puerto Rico, where a moderate risk will persist. By Friday and into the weekend, a moderate risk will return for beaches in western Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands. For a more detailed forecast, please consult 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...DSR/CVB LONG TERM....ICP AVIATION...DS/CVB PUBLIC...MRR/MMC