Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
038 FXCA62 TJSJ 172000 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 400 PM AST Tue Sep 17 2024 .SYNOPSIS... Fair weather conditions will continue this evening, followed by increased shower activity later tonight. A typical shower pattern is expected over the next few days, heightened by the end of the week due to increased instability and available moisture. Excessive heat levels in the coming days will pose a risk to vulnerable individuals, especially those outdoors without proper cooling or hydration. A moderate rip current risk will also persist for the next few days. && .SHORT TERM...Today through Thursday... Radar, satellite, and surface analyses indicate generally fair and warm-to-hot conditions under mostly sunny to partly cloudy skies throughout the day. Afternoon showers began developing around 2 PM AST over parts of the western interior and western Puerto Rico. So far, the highest radar-estimated rainfall totals, around half an inch, have been recorded in Las Marias, Aguada, and Moca in western Puerto Rico. Surface observations show consistently hot temperatures of 90-95 degrees Fahrenheit across most lower elevations of Puerto Rico and the local islands, with heat indices climbing into the mid 100s. In contrast, higher elevations have reported temperatures in the upper 70s to lower 80s. Winds were predominantly from the east to east-northeast at 15-20 mph, with gusts reaching up to 25 mph along coastal areas. Radar imagery also indicated an intense wildfire, reported by emergency managers within Camp Santiago`s facilities in Salinas. The short-term forecast highlights two periods of significantly below-normal precipitable water (PWAT) for September: tonight and Wednesday night, with values dropping as low as 1.2-1.4 inches. PWAT levels will likely rise to typical or above-normal ranges on Wednesday and Thursday, reaching 2.0-2.2 inches. Additionally, intense heat is anticipated, with 925 mb temperatures exceeding two standard deviations above normal, potentially necessitating Excessive Heat Advisories or Warnings. A meandering deep layer high to the west-northwest will be gradually replaced by a deepening tropical upper tropospheric trough (TUTT) moving in from the east-northeast. This transition will increase instability and the potential for isolated to scattered thunderstorms as the trade wind cap inversion weakens and 500 mb temperatures decrease to around -6C. At the surface, easterly winds will gradually shift from the northeast and weaken on Wednesday, with lighter and more variable steering flow expected on Thursday. Tonight`s forecast calls for fair weather conditions to shift towards increased shower activity over northern and eastern sections as a patch of low-level moisture moves through the area. Flooding impacts are not expected, with localized rainfall totals potentially reaching around a quarter of an inch. For those planning to view tonight`s Super Harvest Moon partial Lunar Eclipse, skies should remain sufficiently clear until approximately 2 AM AST. 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. While the increase in available moisture on Wednesday will likely enhance afternoon convection, the risk of flooding remains localized and limited. If any areas are at risk, they would be in the western-southwestern quadrant of Puerto Rico, influenced by northeasterly winds. Fair weather conditions will likely return on Wednesday night with the arrival of a drier air mass. However, an additional increase in moisture will lead to increased and slow- moving showers with isolated thunderstorms on Thursday, with a broader area likely experiencing limited flooding 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...Friday through Monday... From previous discussion issued at 500 AM AST Tue Sep 17 2024 Recent model guidance indicates that an upper-level trough will deepen and move southward between Puerto Rico and the northern Leeward Islands by Friday. This will help destabilize the upper levels and increase the likelihood of early morning and afternoon convection across the interior and western sectors of Puerto Rico. Additionally, the latest satellite-derived precipitable water values show an increase from mostly below normal to normal levels. Therefore, while there wont be enough moisture for widespread rain, there will be enough to develop showers. Throughout 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 and diurnally induced slow-moving showers and thunderstorms with locally heavy rainfall, especially in the central and western sections of Puerto Rico. By the weekend, surface winds are expected to shift to a more south- southeasterly direction and remain very light. The latest models suggest that the area of convergence associated with the upper- level trough will be over the region, which may help to limit rain chances. However, daytime heating and local effects will continue to enhance afternoon convection in some areas, particularly in the interior, northwestern, and north-central Puerto Rico. In contrast, the U.S. Virgin Islands is expected to experience mostly isolated showers. Hot conditions are likely to persist throughout this period, with temperatures potentially reaching the upper 90s and heat indices remaining elevated. Temperatures at 925 mb will remain well above normal through the weekend. The combination of high temperatures and moisture could create hazardous heat conditions, particularly in urban and low-elevation areas. Therefore, it is very likely that Heat Advisories and possibly Excessive Heat Warnings will be issued daily for some areas. 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. && .AVIATION... (18Z TAFs) VFR conds will cont across the area thru 18/15Z except in the vcnty of isold SHRA that are expected to dvlp in WNW PR btwn 17/18-22Z and sct SHRA alg the N coast of PR east of Dorado aft 18/09Z. No significant impacts expected at TAF sites. Wind flow is ESE 10-16 kt with sea breeze influences and gusts up to 25 kt but will bcm less than 10 kt aft 18/00Z with land breezes. Sea breezes return with E flow 10-15 kt aft 18/14Z. Max winds E-ESE 15-23 kt btwn FL011-105. && .MARINE... A weak surface high-pressure system will maintain gentle to moderate easterly winds. Beginning Tuesday, winds will shift from the northeast and gradually weaken, becoming more variable by mid- week. Increased shower activity is expected over the Atlantic local waters and Caribbean passages tonight. There is also a possibility of a few thunderstorms off the western coast of Puerto Rico each afternoon. Overall, seas will range from 3 to 5 feet, occasionally reaching up to 6 feet in most local waters, with lower seas along coastal areas over the next few 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...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001- 005-008-010. Heat Advisory until 5 PM AST this afternoon for PRZ002-003-007- 011>013. VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002. Heat Advisory until 5 PM AST this afternoon for VIZ001. AM...None. && $$ SHORT TERM...WS/CVB LONG TERM....ICP AVIATION...WS/CVB PUBLIC...MRR/MMC