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Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
096 FXCA62 TJSJ 151707 AAA AFDSJU Area Forecast Discussion...UPDATED National Weather Service San Juan PR 107 PM AST Mon Jul 15 2024 .UPDATE...A few showers have developed around the northern U.S. Virgin Islands. These are moving with the lower level flow at about 11 knots from the east (086 degrees). A streamer has also formed west of Saint Croix. Owing to the much drier air above the marine layer, cloud formation has been delayed. Precipitable water values were 1.14 inches. Some moisture is expected to come in over the southeast corner of the forecast area. The air over western Puerto Rico is quite dry, but it is still too early to rule out at least some showers and a thunderstorm during the late afternoon. Some adjustments have been made to PoPs due to an approaching trough/tropical wave. Minor adjustments were also made to the temperature grids. && .AVIATION... (12Z TAFS) VFR conds expected to continue for the next 24 hours except for some SHRA activity in wrn interior PR btwn 15/18-21Z and ovr El Yunque aft 16/08Z. E flow to gradually bcm ESE 10-15 kts. Aft 15/22Z land breezes at less than 10 kt are expected. Winds aft 16/14Z 10-15 kt with gusts to 23kt in sea breezes. && .PREV DISCUSSION... /issued 456 AM AST Mon Jul 15 2024/ SYNOPSIS... Tutt and induced low level trough will continue to cross the region today trough Tuesday, with trailing moisture to affect the area as the east northeast winds become more southeasterly by midweek. Hot, hazy and humid conditions will persist with another extensive plume of Saharan dust forecast to affect the region by mid week. Good daytime heating along with local sea breeze variations and the proximity of the upper trough will aid in enhancing afternoon shower and thunderstorm development particularly across the interior and southwest sections of Puerto Rico, as well as over the coastal waters and local passages today. SHORT TERM...Today through Wednesday... Current satellite derived precipitable water (PWAT) values indicate a dry air mass approaching the region from the northeast. Lows last night were in the upper 70s to low 80s at lower elevations and in the upper 60s to low 70s in higher elevations and valleys. Surface winds were generally from the east northeast at 5 to 10 mph. Showers moved across the area during the overnight hours, leaving mostly minimal accumulations over northern, interior and eastern Puerto Rico since midnight. Radar estimated rainfall indicate up to 0.20 to 0.24 in at localized sectors of north-central and western interior Puerto Rico since midnight. Throughout the day, drier air will plummet available PWAT to below normal values for this time of the year. PWAT values could even decrease to 2 standard deviations below normal, around an inch. For today, only patches of more humid air reaching the region, and afternoon convection can promote local areas of more normal PWAT values. Instability will be present, however, as a TUTT and an induced surface trough continue to cross the region today through Tuesday. Daytime heating, sea breeze convergence and these features will aid in enhancing afternoon shower and thunderstorm development. Surface winds will be from the east northeast veering and becoming lighter from the east later in the afternoon. Winds will then continue to veer to become east-southeasterly, while also increasing, by Tuesday and Wednesday as the trough retrogrades westward and a surface high-pressure continues to builds over the central Atlantic. Current model guidance indicate PWAT values increasing and reaching above normal values, above 2 inches, by late Tuesday morning and persisting throughout most of the period. Afternoon showers and thunderstorms today will concentrate over the interior to the southwestern quadrant of Puerto Rico. For tomorrow and Wednesday, afternoon activity should focus mainly on the northwestern quadrant of Puerto Rico as steering winds become more east-southeast. The risk of minor urban and small stream flooding will persist in isolated areas mainly over the central interior and west sections of Puerto Rico steered by low level winds. Patches of moisture and showers will affect some windward sectors of the islands during the overnight to morning hours each day. Lingering, but overall decreasing, Saharan dust will be present today and tomorrow. However, a more prominent Saharan Air Layer (SAL) will approach the islands by Wednesday morning, prompting hazy skies and decreased air quality to end the short term period. Aerosol Optical Depth (AOD) values can reach around 0.40. This can exacerbate respiratory conditions in immunocompromised, vulnerable and sensitive groups. Maximum temperatures will be in the upper 80s to around the low 90s in lower elevations of the islands. The Saharan dust can also inhibit nighttime cooling (especially with more prominent dust like on Wednesday), prompting warmer than normal nights. Heat indices in coastal, urban and lower elevation areas can reach above 108 degrees Fahrenheit. This will be further exacerbated by east-southeast winds from tomorrow onwards. Hot temperatures and high humidity may cause heat illnesses. LONG TERM...Thursday through Tuesday... Recent model guidance suggests a strong surface high pressure will remain anchored across the Atlantic and north of the region to maintain moderate to strong east southeast winds on Thursday. Winds are then to become more east northeast by Friday into the following weekend as a tropical wave will enter the eastern Caribbean and cross the forecast area. This is expected to bring increasing moisture and instability to the region along with a wind surge and moderate to strong southeasterly winds by Saturday afternoon, as the tropical wave crosses the region. Accompanying the wave and thereafter another extensive plume of dense Saharan dust is forecast to quickly spread across the region Sunday through Monday then gradually diminish from Tuesday onwards. By then winds are to become more easterly with yet another Tutt and induced surface trough forecast to approaches from the east. All in all the most impactful period will so far be late Thursday through Saturday, when enhanced overnight and afternoon convection is expected due to the tropical wave along with good moisture convergence and cooler advective temperatures aloft. This will be followed by hazy/dusty conditions and a somewhat drier airmass due to the forecast dense Saharan dust particulates which will maintain hot and humid conditions. Temperatures are expected to continue slightly above normal and elevated to significant maximum daytime heat indices are forecast to be reached at times in some coastal and urban areas. Marine conditions area also expected to deteriorate by Friday and into the weekend due to the tropical wave and the increasing trade winds. Therefore precautionary statements and small craft advisories may be required for all or portions of the local waters and passages by that time. AVIATION... (06z TAFs) Mainly VFR conditions across the terminals. Winds will be ENE, bcmg E later in the afternoon then ESE by tomorrow. VCSH AND VCTS are forecasted for the interior and SW PR, including TJPS, at around 15/17Z-22Z. This can promote brief MVFR conditions. Brief VCSH across other terminals possible during the period. Lingering but decreasing saharan dust present, visibility P6SM. MARINE... Surface high pressure across the western Atlantic will promote light east to northeast winds today, then become more southeasterly and increase to moderate on Tuesday and become breezy by midweek due to the arrival of a tropical wave and accompanying wind surge. As the wind increases, choppy seas will be likely across the local waters and passages, resulting in seas up to 7 feet and winds up to 20 knots. Small craft advisories and precautionary statement will be required by then. BEACH FORECAST... The rip current risk will be low today, however life-threatening rip currents often occur in the vicinity of groins, jetties and piers. By Tuesday and for the remainder of the workweek, there will be a moderate risk of rip current for most beaches and therefore life-threatening rip currents will be possible in the surf zone. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>003-005- 008-010-012-013. VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002. AM...None. && $$ SHORT TERM/MARINE/AVIATION...WS PUBLIC....GRS UPPER AIR...MNG