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Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
298 FXCA62 TJSJ 160931 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 531 AM AST Tue Jul 16 2024 .SYNOPSIS... Tutt and associated low level trough will continue to cross the region today. Fragments of trade wind moisture will affect the area as the low level winds become more east southeast then southeasterly by Wednesday. Hot and humid conditions will continue the rest of the period with an elongated plume of Saharan dust forecast to affect the area into the upcoming weekend promoting hazy skies. Good daytime heating along with local sea breezes and the proximity of the upper trough will aid in enhancing afternoon shower and thunderstorm development each day. A tropical wave will bring additional moisture and instability to the islands Friday through Saturday. Breezy conditions with moderate to locally strong east to southeast winds will return to the region by Wednesday and continue into the weekend. && .SHORT TERM...Today through Thursday... Current satellite derived precipitable water (PWAT) values indicate a dry air mass over the region. Overnight minimum temperatures were in the upper 70s to low 80s at lower elevations and in the upper 60s to low 70s at higher elevations. Steering winds gradually veered to become east-southeast. Showers moved across the area during the overnight hours, leaving mostly minimal accumulations over eastern Puerto Rico, Vieques and St. Croix since midnight. Radar estimated rainfall indicate up to 0.68 to 0.95 inches at sectors of Ceiba since midnight. A TUTT low will linger north of the region today while moisture gradually increases. More normal PWAT values will be seen late this morning. A patch of moisture related to an induced surface trough will increase PWAT values to above normal, above 2 inches, this evening and tomorrow. The presence of these features will promote instability, and, alongside with daytime heating, sea breeze convergence and local effects, will aid in enhancing afternoon shower and thunderstorm development. Afternoon showers and thunderstorms today will concentrate over the northwestern quadrant of Puerto Rico, as well as possibly downwind of El Yunque. Surface winds will generally be from the east-southeast during the period, increasing to become more breezy for the second half of the workweek, as the upper trough retrogrades westward and a surface high-pressure continues to builds over the central Atlantic. Model guidance suggest PWAT values staying at normal values for Thursday. Patches of moisture, showers and potential t-storms will affect windward sectors of the islands. Strong showers and thunderstorms can lead to periods of heavy rainfall that will cause urban and small stream flooding. Lingering Saharan dust will be present today. However, a more prominent Saharan Air Layer (SAL) will approach the area by tomorrow morning, prompting hazy skies and decreased air quality for the rest of the short term period. Aerosol Optical Depth (AOD) values can reach around 0.40 to 0.55. Highs 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 in part due to east-southeast winds and available moisture. .LONG TERM...Friday through Wednesday... Strong surface high pressure will remain the dominant feature across the Atlantic and north of the region. This will maintain moderate east to northeast winds Friday into Saturday while 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 and on Sunday, as the tropical wave exits the region. Accompanying and trailing the wave is an extensive plume of dense Saharan dust which will quickly spread across the region the rest of the weekend through Monday before diminishing from Tuesday onwards. Winds are then to become more easterly while another Tutt and induced surface trough approaches from the east. Hot,humid and hazy conditions will likely persist through the weekend. Model guidance continued to initialize fairly well so far and still suggests the most impactful period to be Friday through Saturday due to increasing moisture convergence and instability with enhanced overnight and afternoon convection expected due to the tropical wave and cooler advective temperatures aloft. This will be followed by hazy conditions as mentioned and a drier airmass due to the dense layer of Saharan dust particulates which will maintain unseasonably hot and humid conditions. Local temperatures are expected to continue above normal with maximum daytime heat indices forecast to be elevated to significant at times especially along the coastal and urban areas. Marine conditions area also forecast to deteriorate Friday and into the weekend due to the tightening of the local pressure gradient and a wind surge which will increase the trade winds. Precautionary statements and small craft advisories may be required for all or portions of the local waters and passages through Saturday. Sunday through Monday are expected to improve with typical summer time weather and hot and hazy conditions. So far only isolated to scattered locally and diurnally afternoon convection expected mainly over the west and northwest sections of Puerto Rico. By late Monday through the rest of the period, the upper ridge will erode as a Tutt low is forecast to retrograde across the region and low level moisture gradually increases. This in turn will increase the chance for more frequent overnight passing showers and afternoon convection across parts of the islands. Otherwise based on the tropical weather outlook for the National Hurricane center... Tropical cyclone formation is not expected during the next seven days. && .AVIATION... (06Z TAFS) Mainly VFR conditions across the terminals. Winds will be ESE. VCSH and VCTS are forecast for the interior and NW PR, including TJBQ, at around 16/17Z-22Z. Streamers can also affect TJSJ at that time. This can promote brief MVFR conditions. Brief VCSH across other terminals possible during the period. Lingering saharan dust present, visibility P6SM. && .MARINE... A surface high pressure centered across the west central Atlantic and an upper trough and associated induced low level trough crossing the region will maintain a light to moderate east to southeast wind flow with occasional passing showers across the local waters and passages. Winds will become more southeasterly and increase to moderate to locally fresh by Wednesday becoming breezy and thus creating choppy conditions. Small craft operators should exercise caution across the offshore waters and local passages today. && .BEACH FORECAST... 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...Excessive Heat Warning from 11 AM this morning to 5 PM AST this afternoon for PRZ001-005-008-010. Heat Advisory from 11 AM this morning to 5 PM AST this afternoon for PRZ002-003-007-011>013. VI...Heat Advisory from 11 AM this morning to 5 PM AST this afternoon for VIZ001-002. AM...None. && $$ SHORT TERM...MRR LONG TERM....RAM AVIATION...MRR