Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
809 FXCA62 TJSJ 212033 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 433 PM AST Sat Sep 21 2024 .SYNOPSIS... Fair weather conditions are expected to return a few hours after sunset and continue tonight. 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 tonight. && .SHORT TERM...Tonight through Monday... A col area to our north continues to promote light and variable winds across the islands. The col will shift gradually around the region through the rest of the short-term period as a broad surface low pressure lingers to our northeast and a weak surface high pressure north of Hispaniola extends into the Mona Passage. Currently, a low-to mid-level high-pressure east of the Lesser Antilles is providing a weak southerly steering wind flow, but an ill-defined flow is expected to prevail in general. Therefore, the diurnal afternoon convection will be driven by a weak sea breeze and should favor shower and thunderstorm development due to orographic lift over the central portions of the islands while slowly drifting over the coastal areas as the sea breeze loses its influence late in the afternoons into the evening hours. Slow-moving storms will lead to urban and small stream flooding, with some main rivers possibly reaching minor flooding at night. An upper-level trough over the eastern half of the CWA will continue to move eastward into the Anegada Passage and over the Leeward Islands during the next 24 hours or so as an upper-level ridge builds briefly from the west over the region. However, a significant warming of the 500 mb temperature is not expected, and thunderstorms are expected to produce frequent lightning again today. On Monday, an upper-level trough from the western Atlantic is expected to move over the region, further enhancing shower and thunderstorm development. Light winds and relatively drier conditions during much of the morning and early afternoon will cause excessive heat conditions in most coastal and urban areas along the islands` lower elevations. .LONG TERM...Tuesday through Friday... From previous discussion issued at 512 AM AST Sat Sep 21 2024 The inherited forecast remains on track. According to the latest model guidance, a deep-layered trough is expected to move just north of the Caribbean islands until at least late Tuesday into Wednesday. This weather feature will result in a weak steering wind flow, with speeds ranging from 1 to 5 knots and gradually recovering to around 10 knots after its passage. Tuesday is expected to be a day with variable weather conditions, as the trough will be closest to the islands allowing the surface moisture to slightly filter across all levels of the atmosphere. The 500 mb temperatures are expected to be around -7 to -8 degrees Fahrenheit on Tuesday, which could lead to an increase in instability and the potential for thunderstorm development particularly during the afternoon hours. Despite the passage of the trough, occasional patches of moisture are anticipated to move across the forecast area throughout the period, resulting in showers reaching the coastal areas of Puerto Rico and the Virgin Islands each day. The strongest shower and thunderstorm activity is expected each afternoon across the Cordillera Central, with the activity then drifting toward the coast. Due to weak winds, it is difficult to determine which area will be more affected, aside from the interior, which should receive the bulk of the activity each day. Towards the end of the workweek, a tropical wave will reach the islands. We anticipate occasional ponding of water on roads and poorly drained areas from Tuesday into Friday. Temperature-wise, above-normal temperatures are expected to persist over the region, with the threat of excessive heat each day. This heat will mainly impact those without effective cooling, adequate hydration, or prolonged exposure. && .AVIATION... (18Z TAFs) SHRA/TSRA with mtn obscurations and MVFR/IFR conds over interior and western PR should last till arnd 21/21Z. This may cause tempo MVFR to brief IFR conds at TJSJ/TJBQ late in the afternoon. Across the USVI terminals mostly -RA/VCSH should prevail thru mid-afternoon. Winds less than 10 kt with a few higher gusts in mostly sea breezes and near TSRA. SHRA/TSRA ovr land expected dissipate around 21/23Z. VFR conditions should prevail overnight thru early Sunday afternoon. && .MARINE... A weak surface low pressure to the northeast of the region will maintain weak and variable winds into the early part of 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 moderate rip current risk will persist for the northern and eastern coast of Puerto Rico and Culebra tonight. Elsewhere is low. The risk will drop to low tomorrow, but a moderate risk will return by midweek. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001- 003-005-007-008-010-011. Heat Advisory until 5 PM AST this afternoon for PRZ002-004-012- 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...DSR/RAM LONG TERM....ICP AVIATION...DSR/RAM PUBLIC...GRS/ICP