Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
272 FXCA62 TJSJ 170820 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 420 AM AST Wed Jul 17 2024 .SYNOPSIS... Tutt low and associated low level trough will continue to influence the local weather pattern today but will gradually weaken while moving westward away from the region. Low level winds will be generally from the southeast as high pressure builds across the west and central Atlantic. Hot and humid conditions will continue the rest of the workweek with an extensive layer of Saharan dust forecast to spread and linger across the region through the end of the week promoting hazy skies. A tropical wave will bring additional moisture and instability Friday through Saturday. Moderate to locally strong east to southeast winds will bring breezy conditions later today and through the weekend. && .SHORT TERM...Today through Friday... Current satellite derived precipitable water (PWAT) show normal to slightly above normal values (above 2 inches) across the islands. Overnight low temperatures were in the upper 70s to low 80s at lower elevations and in the upper 60s to low 70s at higher elevations of Puerto Rico. East-southeast to southeast winds steered showers, that left radar estimated rainfall since midnight, across southern, eastern interior and eastern Puerto Rico, St. John, Vieques and Culebra. A prominent Saharan Air Layer (SAL) will reach the islands later today 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 0.60. These conditions can exacerbate respiratory conditions in immunocompromised, vulnerable and sensitive groups. A TUTT low will linger north of the region today while gradually moving away, another upper low will remain well east of the area through the second half of the workweek. A mid to upper level ridge will also gradually build across the eastern Caribbean during the period. Normal to above normal PWAT values will continue through Thursday morning. Model guidance suggests PWAT values decreasing to up to high end normal values from Thursday morning through Friday morning. By Friday afternoon onwards, the moisture field of an approaching tropical wave will once again promote increasing PWAT values to up to above normal to end the period as the wave crosses Friday night into the long term period. The nearby presence of the TUTT low, an induced surface trough and a tropical wave will instability during the period, which 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 to southeast during most of 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. 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. Daytime highs will be in the upper 80s to around the low 90s in lower elevations of the islands. Saharan dust will also inhibit nighttime cooling, 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...Saturday through Thursday... 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 through Saturday as a tropical wave and associated moisture field enters the eastern Caribbean and crosses the forecast area. This increasing moisture will bring unstable conditions to the region followed by a wind surge and moderate to strong southeasterly winds by Saturday afternoon and early Sunday as the tropical wave exits the region. Trailing the wave an extensive layer of dense Saharan dust will quickly spread across the region the rest of the weekend and linger through Monday then diminish from Tuesday onwards. Low level winds are then expected to become more easterly as another Tutt and induced surface trough is forecast to approach the region from the east. All in all Hot, humid and hazy conditions will likely be the dominant weather pattern through the weekend and into the early part of the following week with some locally and diurnally induced afternoon convection each day. Recent model guidance continued to initialize well and suggests the most impactful period to be Friday through Saturday due to increasing moisture convergence and instability. That said there will be a better chance for enhanced overnight and afternoon convection as the tropical wave and somewhat cooler advective temperatures aloft are across the region. As previously mentioned considerably hazy conditions and fairly drier airmass will follow due to the dense layer of Saharan dust particulates. Local temperatures will remain slightly 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 through the weekend due to the tightening of the local pressure gradient and a wind surge which will increase the trade winds. Precautionary statements and or small craft advisories may therefore continue for portions of the local waters and passages due to the choppy wind driven conditions. Sunday through Monday are forecast to be the driest period with typical summertime weather and hot and hazy conditions. 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 will again increase the chance for more frequent overnight passing showers followed by afternoon convection across parts of the islands. Based on the Tropical Weather Outlook from 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 to SE at up to 12 to 18 kts with sea breeze variations and higher gusts. VCSH and VCTS are forecast for the interior and NW PR, including TJBQ, at around 17/17Z-22Z with streamers that can also affect TJSJ at that time. This can promote brief MVFR conditions. Brief VCSH across other terminals possible during the period. Increasing Saharan dust after 17/18Z. && .MARINE... A surface high pressure anchored across the west central Atlantic and an upper trough and associated induced low level trough crossing the region will aid in maintaining a light to moderate east southeast wind flow promoting occasional showers and isolated thunderstorms across the local waters and passages. Winds will become more southeasterly and increase to moderate to locally fresh later today. As the wind increases, choppy seas will be likely across the local waters and passages, resulting in occasional seas up to 7 feet and winds up to 20 knots. Precautionary statements will continue for the local waters and Small craft advisories may be required for some areas by late Thursday. && .BEACH FORECAST... For the remainder of the week, 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...None. VI...None. AM...None. && $$ SHORT TERM...MRR LONG TERM....RAM AVIATION...MRR