Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
526 FXCA62 TJSJ 220904 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 504 AM AST Sun Sep 22 2024 .SYNOPSIS... Expect the humidity water values to peak by the late afternoon and early evening each day of the short term period when showers and thunderstorms grow over interior Puerto Rico. 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...Today through Tuesday... Currently, radar and satellite imagery show scattered showers over the Atlantic waters and south of the forecast area and isolated showers over the local Caribbean waters. Isolated thunderstorms continued to pop off and then dissipate, but tops are not as high as yesterday and the showers are not reaching the level where their tops can blow off. Most clouds and showers have dissipated over land. 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 6 mph or less with many areas becalmed. The TPW product showed a drier strip of air over the local Caribbean waters. It continues to squelch showers over the area including Ponce and Saint Croix. This strip is no longer making any northward progress. Also there is a narrow band of much better moisture just south of the forecast area that is allowing showers and thunderstorms to fire off from time to time. Some moisture is also seen over the Atlantic outer waters in the center of our forecast area. Lower level winds are quite light with winds 10 kts or less below 28 kft. This is due to high pressure over the Canary Islands being blocked by a pair of weakening 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, the U.S. Virgin Islands, Puerto Rico and over to Hispaniola. At upper levels the TUTT low is found over and north of the Leeward Islands and the 30 knot jet from the north northwest continues to push across the area today and Monday while increasing to 50 knots over the eastern Caribbean tonight. 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 has been and will continue to be quite variable, which is one reason why neither the dry band in our local Caribbean waters nor the band of moisture just south of it have made no further northward progress. This lack of winds and the continued moisture over the local islands is part of the reason heat advisories have been issued for all lower elevation locations. Some relief from the heat is expected in all but coastal spaces as showers and thunderstorms, fueled by the continued humidity, redevelop over the area. All of these warm-to-hot conditions will hold on, and precipitable fueled by the continued humidity water values peak late afternoon and early evening each day of the short term period when showers and thunderstorms grow over interior Puerto Rico. The GFS 1000-850 mb thickness values are also seen to increase each day until Tuesday when the highest value for the week reaches 1426 m. && .LONG TERM...Wednesday through Saturday... For the long term, we anticipate calm and warm weather conditions with the development of showers and thunderstorm activity, particularly in the afternoon hours. On Wednesday, the steering wind flow will gradually recover to around 10-15 knots as the deep-layered trough continues to exit the region. The latest model guidance suggests that all the available moisture should remain below 700 mb through at least Thursday, meaning that patches of moisture will move across the forecast area from time to time, 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. From Friday into the upcoming weekend, we foresee that a tropical wave will move well south of Puerto Rico and the U.S. Virgin Islands. However, we anticipate a slight increase in moisture across the area, which could enhance shower development. With the proximity of this tropical wave and the everyday afternoon shower activity, we forecast occasional ponding of water on roads and poorly drained areas through Saturday. 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 22/15Z 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 22/17Z till arnd 22/21Z. 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 22/22Z. SHRA/TSRA ovr land will dissipate btwn 22/21-23Z. High density altitudes at TAF sites are expected today due to heat and humidity. && .MARINE... A surface low pressure to the northeast of the region over the Central Atlantic will maintain light and variable winds into the early part of next week. Showers and isolated thunderstorms will develop from time to time across the regional waters, particularly off the western and southern coastal waters of Puerto Rico each afternoon. && .BEACH FORECAST... The rip current risk will remain low across the islands through at least Wednesday, when the moderate risk returns for the northern coastal areas of Puerto Rico. && .CLIMATE... Yesterday San Juan tied the record warm minimum temperature for the day at 81 degrees. Today we will not, as the minimum temperature has already dropped to 81 degrees, with 82 degrees as the record. Should the minimum remain at 81 degrees, it will be the second warmest minimum in its own right. Although it may not seem like there is much difference, today is the first official day of Autumn and when the shortening of the daytime hours reaches its greatest speed. The sun will cross the equator at 8:44 AM AST this morning. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007- 008-010>013. VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002. AM...None. && $$ SHORT TERM/AVIATION...WS LONG TERM/MARINE/BEACH FORECAST...MMC