Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
170 FXCA62 TJSJ 210358 CCA AFDSJU Area Forecast Discussion...CORRECTED National Weather Service San Juan PR 1158 PM AST Fri Sep 20 2024 .SYNOPSIS... Fair weather is expected to return tonight as conditions become less conducive to shower activity. A typical pattern of calm nights and mornings, followed by warm-to-hot afternoons and evenings driven by diurnal heating and weak steering winds, will prevail in the coming days. Excessive heat will continue to pose a risk to vulnerable individuals, particularly those outdoors without proper cooling or hydration. Additionally, the moderate risk of rip currents will persist over the next few days. && .SHORT TERM...Tonight through Sunday... Radar, satellite, and surface analyses indicate generally fair, warm-to-hot conditions, with skies becoming progressively partly to variably cloudy. Shower activity began late this morning, with showers developing over the far eastern and southeastern slopes of Puerto Rico, followed by showers with strong isolated thunderstorms developing over and to the north and west of the Cordillera Central. So far, radar estimates indicated the highest rainfall totals, around 1.5-2.0 inches, from southern Bayamon westward to Ciales, prompting the issuance of Special Weather Statements and Flood Advisories. Surface observations report consistently hot temperatures up to 93-95F, 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 20 mph along coastal areas. The short-term forecast indicates typical precipitable water (PWAT) levels between 1.7 and 2.1 inches, briefly interrupted by a drier-than-normal phase, with PWAT falling to 1.4-1.5 inches from early Saturday into Sunday morning. A tropical upper tropospheric trough (TUTT) will gradually lift northeastward in the coming days, allowing the trade wind cap inversion to form and promoting the entrainment of drier air. While these conditions are generally unfavorable for thunderstorm activity, peak daytime heating will still support afternoon thunderstorms. By Sunday night, the proximity of an upper-level low and increasing wind speeds aloft could create more favorable conditions for thunderstorm development. A weak surface low-pressure system to the northeast will maintain light wind speeds, mainly with a south-southeasterly component. Despite the moisture, intense heat will continue, with 925 mb temperatures rising more than two standard deviations above normal for September, potentially requiring Excessive Heat Advisories or Warnings. Tonight`s forecast suggests a shift towards more stable conditions and reduced shower activity, with clearer skies, as the environment becomes less conducive to shower development. However, isolated to locally scattered showers remain possible, especially in the southern and eastern coastal areas of Puerto Rico. Winds will be light and variable, with overnight temperatures ranging from the upper 60s in the higher elevations to the lower 80s in the coastal areas and the local islands. Calm nights and mornings, followed by warm-to-hot and stormy afternoons and evenings, are expected to continue through the weekend. Increased moisture and instability may result in more showers and isolated thunderstorms by Sunday night into early next week. For updates on excessive heat, heavy rainfall, or other potential hazards in the days ahead, visit the Experimental Graphical Hazard Weather Outlook at https://www.weather.gov/erh/ghwo?wfo=sju. .LONG TERM...Monday through Thursday... From previous discussion issued at 402 AM AST Fri Sep 20 2024 A deep-layered trough will continue to dig just north of the Caribbean islands. This through will act to maintain a weak steering flow, with speeds of 1 to 5 knots on Monday and Tuesday. The through will gradually moves away by mid-week, with winds gradually recovering speed, but only to around 10 knots. At the surface, induced patches of moisture will move from time to time through the area, although there is no particular day with a great amount of low level moisture. This moisture will act to generate some showers reaching the coastal areas of Puerto Rico and the Virgin Islands each day. However, the strongest activity, with thunderstorms, is expected each afternoon across the Cordillera Central, with the activity then drifting toward the coast. Since the winds will be so weak, it is difficult to determine which area is going to be more affected, aside than the interior that should receive the bulk of the activity each day. As has been the case in the past couple of weeks, above normal temperatures will persist in the area, with the threat of excessive heat each day. This heat will mainly impact those without effective cooling or adequate hydration. && .AVIATION... (18Z TAFs) SHRA/TSRA over mainland PR may cause tempo MVFR conds at TJBQ/TJSJ thru 22z. Thereafter, mainly VFR conditions are expected to prevail across all terminals through the forecast period. Light winds with a southerly component will prevail through Saturday. && .MARINE... A weak surface low pressure to the northeast of the region will maintain weak and variable winds throughout early next week. Showers and isolated thunderstorms will persist across the regional waters, particularly off the western and southern coastal waters of Puerto Rico each afternoon. && .BEACH FORECAST... The risk of rip currents will remain moderate for the beaches of northern Puerto Rico and Culebra on Saturday. The risk will drop to low on Sunday across all local 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/RAM LONG TERM....ICP AVIATION...DSR/RAM PUBLIC...GRS/MNG/CVB