Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
356 FXCA62 TJSJ 210912 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 512 AM AST Sat Sep 21 2024 .SYNOPSIS... Slightly drier conditions will prevail across the islands today. 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 for the next few 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...Today through Monday... Currently, radar and satellite imagery show some isolated showers over the local waters. Most clouds and showers have dissipated over land except for some tops that have blown south from earlier thunderstorms over the Atlantic waters. Low temperatures this morning are running from the mid 70s to the lower 80s in the lower elevations and from the upper 60s to the mid 70s in the mid to upper elevations. Winds have quieted down to 5 mph or less with many areas becalmed. The TPW product shows a drier strip of air crawling very slowly northward. It has reached Ponce and Saint Croix and will spread across Saint Thomas and some of Puerto Rico during the day. Lower level winds are quite light with winds 10 kts or less below 25 kft. This is due to high pressure over the Canary Islands being blocked by a pair of low pressures between 25 and 30 north and between 40 and 60 west. Weak high pressure can be seen north of the Mona Passage and across the local area. At mid levels a broad area of high pressure extends across the tropical Atlantic. At upper levels the TUTT low over the area tonight is edging east and will allow a 30 knot jet from the north northwest to push across the area Saturday through Sunday. Low pressure will bring troughing to our area Monday night, but its associated low will remain north of 30 north and will just barely be able to impinge on the high pressure over the west central Caribbean Sea. Flow at lower levels and specifically the mean flow between 1000 and 700 mb will be southerly and will bring the drier air to our south over the area and allow temperatures to warm more than yesterday. For this reason Excessive Heat Warnings for most of the lower elevations of Puerto Rico and Saint Croix have been issued. Heat Advisories have been issued for the remaining lower elevations including the interior valleys in eastern Puerto Rico. All of these warm-to-hot conditions will fade a little on Sunday when better moisture arrives, and precipitable water values peak around 7 PM AST each day of the short term period when showers and thunderstorms grow over interior Puerto Rico. But warm to hot conditions will persist through the short term and into the long- term before meaningful relief occurs. && .LONG TERM...Tuesday through Friday... 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... (06Z TAFs) Mainly VFR conditions ovr land. Aft 21/15Z Clds and SHRA to dvlp ovr interior PR followed by SHRA/TSRA with mtn obscurations and MVFR/IFR conds over interior and wrn PR aft 21/17Z till arnd 21/21Z. TAF sites will have only brief unfavorable conds. Winds less than 10 kt with a few hir gusts in mostly sea breezes and nr TSRA till aft 21/22Z. SHRA/TSRA ovr land will dissipate btwn 21/21-23Z. High density altitudes at TAF sites are expected today due to heat and humidity. && .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 moderate rip current risk will persist for the northern and eastern coast of Puerto Rico and Culebra. Elsewhere is low. && .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/AVIATION...WS LONG TERM/MARINE/BEACH FORECAST...MMC