Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
138 FXCA62 TJSJ 120847 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 447 AM AST Wed Jun 12 2024 .SYNOPSIS... Excessive heat continues. Excessive Heat Warnings are in effect from 9 AM to 6 PM AST for lower elevations and coastal areas of Puerto Rico. Heat Advisories are in effect from 9 AM to 6 PM AST for valleys and urban areas of the eastern and western interior sections of Puerto Rico, Vieques, Culebra, and the US Virgin Islands. Since dangerous hot conditions will likely continue, an Excessive Heat Watch is now in place for Thursday. A tropical wave south of the area, frontal remnants and an upper trough are expected to increase moisture and the potential for showers and isolated thunderstorms. Still, this activity will remain localized and amplified by daytime heating, especially across western sections of Puerto Rico today and Thursday. Southeasterly winds and saharan dust will promote a hot and hazy weekend. && .SHORT TERM...Today through Friday... An overnight radar and satellite imagery analysis indicated mainly clear to partly cloudy skies, with persistent mid-to-upper level clouds moving over the eastern half of the forecast area. A few light showers moved into eastern sections of Puerto Rico, leaving rainfall totals up to a quarter of an inch. Reports again highlighted warm overnight low temperatures above 80 degrees Fahrenheit in coastal areas of the San Juan metropolitan area, eastern and southern Puerto Rico, and the local islands. Coastal observation sites and local buoys have also recorded warm low temperatures, hovering around 82-84 degrees Fahrenheit. Although slightly lower than yesterday`s lows, this situation offers little overnight relief and will exacerbate the region`s ongoing heatwave situation. Winds were mainly light to calm and variable. Weather conditions throughout the forecast period should slightly deviate from the persisting stable and shower-less pattern that has dominated the region recently, primarily due to increased instability yielded by an upper-level trough moving in from the west and increasing moisture levels from the remnants of an old front, steered by an approaching surface trough from the east. Driven by a surface high pressure meandering to the north of the local islands and the subsequent influence of the approaching surface trough, the general steering flow will undergo some changes. Easterly trade winds today, with peak daytime speeds ranging from 15-20 mph, will gradually weaken to 10-15 mph and shift from the east-northeast by tonight into early Thursday. Winds will further diminish to 5-10 mph, gradually veering from the east on Thursday and from the southeast on Friday. Precipitable water values will stabilize throughout the forecast period, rising from around 1.4 inches this morning to about 2.0 inches by Thursday afternoon and settling within typical seasonal thresholds of around 1.8 inches on Friday. This trend and marginally unstable conditions aloft suggest a higher probability of showers with isolated thunderstorms. Still, this activity will remain localized and amplified by daytime heating, especially across western sections of Puerto Rico today and Thursday. Additionally, expect increased passing showers with a few isolated thunderstorms over the local waters tonight, with some moving into eastern Puerto Rico and the local islands as remnants of an old front approach the area. Regarding flooding impacts, ponding on roadways and poorly drained areas, with localized urban and small stream flooding, is likely with the heaviest and most frequent rains. Dangerously hot conditions are likely again today, prompting Excessive Heat Warnings for lower elevations and coastal areas of Puerto Rico from 9 AM to 6 PM AST. These conditions pose a serious risk to anyone without adequate cooling or hydration. Heat Advisories are also in effect for valleys and urban areas of the eastern and western interior sections of Puerto Rico, Vieques, Culebra, and the US Virgin Islands, indicating a significant risk for individuals sensitive to heat. When outdoors, take extra precautions: wear lightweight, loose-fitting clothing and limit strenuous activities to early morning or evening. Staying hydrated and recognizing signs of heat exhaustion are crucial. Since dangerous hot conditions will likely continue, an Excessive Heat Watch is now in place for Thursday. .LONG TERM...Saturday through Wednesday... Current model guidance suggests precipitable water (PWAT) at below normal to normal values this weekend, with only few patches of more humid air moving through the islands, last remnants of a past frontal boundary pushed towards the region by a high pressure system over the Atlantic. This system will also promote southeasterly surface windflow this weekend. This will steer warmer tropical air towards our area and promote overall warm conditions for the islands. Heat related products will likely be issued for the islands later on. 925 mb temperatures are forecast to be above normal this weekend. Saharan dust particles will increase to start the period as well, promoting hazy skies during the weekend at least. Most available moisture will remain below 850 mb. Although model guidance suggest some moisture over the mid levels, a more notable increase is suggested by Tuesday and into Friday. This increase in moisture, will lead to increasing PWAT values Tuesday and Wednesday, in part due to the possible passing of a tropical wave. High pressure aloft will also strengthen over the Western Caribbean through at least the weekend. Model guidance suggests more easterly winds to start the workweek as the Atlantic surface high weakens, uncertainty remains. && .AVIATION... (06Z TAFS) VFR conditions will likely prevail across all terminals during the next 24 hours. However, SHRA/TSRA may result in brief MVFR conditions at TJSJ and TJBQ between 12/16-22Z and after 12/22Z at TJSJ and USVI terminals. Light to calm and variable winds will become more easterly and increase to 14-18 knots between 12/13-23Z, accompanied by higher gusts and sea breeze variations. && .MARINE... Surface high pressure over the Atlantic will promote moderate to locally fresh easterly winds. A tropical wave, frontal remnants and an upper trough are expected to increase the potential for showers and isolated thunderstorms. Moderate winds will back and become east northeast tonight into early Thursday, veering to become east for the rest of Thursday. Winds will then continue to diminish and veer, becoming east-southeast to southeast throughout the weekend. && .BEACH FORECAST... There is a moderate risk of rip currents for St. Croix, NW to north-central PR, and SW Puerto Rico today. Although the rip current risk decreases to low for Puerto Rico tonight, a moderate risk will persist for easternmost St. Croix. Low risk elsewhere today and tonight. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning from 9 AM this morning to 6 PM AST this evening for PRZ001>003-005-007-008-010-011. Excessive Heat Watch from Thursday morning through Thursday afternoon for PRZ001>003-005-007-008-010>013. Heat Advisory from 9 AM this morning to 6 PM AST this evening for PRZ004-009-012-013. VI...Excessive Heat Watch from Thursday morning through Thursday afternoon for VIZ001-002. Heat Advisory from 9 AM this morning to 6 PM AST this evening for VIZ001-002. AM...None. && $$ SHORT TERM...ICP LONG TERM....MRR