Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
375 FXCA62 TJSJ 110926 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 526 AM AST Thu Jul 11 2024 .SYNOPSIS... Moisture is forecast to gradually decrease, with patches of more humid and drier air reaching the region, through Friday night and the establishment of mid to upper level ridging over the region. More breezy easterlies, during the second half of the week, with showers and possible afternoon thunderstorms in the forecast, mainly due to diurnal heating and local effects, today and Friday. Another pulse of suspended Saharan dust particulates will reach the islands on Friday and persist through Sunday. An elevated heat risk will also likely persist. Moisture from another tropical wave is forecast to reach the islands by Friday night leading to an increase in showers and thunderstorms throughout the weekend. On Monday, another TUTT will approach the region enhancing instability. && .SHORT TERM...Today through Saturday... Today, moisture levels will decrease slightly to near-normal levels. Meanwhile, the TUTT that impacted the area the previous day will continue to move westward as ridging aloft settles in. This will promote a less active day, with some showers reaching windward coastal areas during the morning hours. In the afternoon, diurnal heating and local effects will promote showers and isolated thunderstorms across western and northwestern portions of Puerto Rico under an east-southeast steering flow. However, this activity is expected to have less areal coverage and last for a shorter period. For the rest of the short-term forecast, weather conditions will vary. On Friday, another pulse of suspended Saharan dust particulates will reach the islands, promoting hazy skies for the most part. However, the leading edge of a tropical wave will also arrive, increasing moisture content to above-normal levels, particularly during the afternoon hours. This raises the potential for isolated to scattered showers and some isolated thunderstorms in localized areas, mostly across interior and western Puerto Rico. By Saturday, model guidance suggests the arrival of a wind surge, which could bring squally weather for a very short period. Therefore, after 12z, expect an increase in thunderstorm activity with the potential for frequent lightning and gusty wind conditions. Behind the wind surge, a broad area containing higher concentrations of suspended Saharan dust particulates will engulf the forecast area. This could shorten the period of shower activity in some areas, bringing back hazy skies by late Saturday evening. Warmer-than-usual conditions will persist in the coming days. Consequently, some urban and coastal areas may experience a limited to elevated excessive heat threat, increasing the possibility of heat exhaustion with prolonged exposure, particularly on Friday and Saturday. && .LONG TERM...Sunday through Wednesday... Based on the latest models, during the beginning of the long-term period, we will continue to have the presence of Saharan dust across the region exacerbating health conditions for immunocompromised individuals, vulnerable populations, and those sensitive to air quality issues. During Sunday afternoon and evening, the Saharan dust should be receding. Residual moisture from a tropical wave will keep precipitable water mainly above normal values, with a slight decrease to normal on Sunday. On Monday, PWAT should increase again as a Tropical Upper- Tropospheric Trough (TUTT) will approach the region and for now looks like it could be the more active day of this forecast period. This feature will also be evident in the mid and low levels of the atmosphere, with winds first from the northeast on Monday, and then shifting from the southeast on Tuesday and Wednesday. Early next week, enough instability will be available to generate active afternoons across the interior and western Puerto Rico. Temperatures in the 500 MB are expected to drop to around -7 degrees Fahrenheit on Sunday and Monday and that could be beneficial for thunderstorm development. Trade winds will carry some showers across portions of Puerto Rico and the Virgin Islands too, but the higher risk of flooding and mudslides will be for the interior and west of Puerto Rico. Potential impacts range from ponding water on roads to flooding in poorly drained areas and urban and small stream flooding. In terms of temperatures, warmer-than-usual conditions are expected for most of the period. On Monday, if the rain materializes, the temperature should stay a few degrees lower. Generally, daytime highs will range from the lower 90s in lower elevations to around 80 degrees in higher elevations. With abundant moisture in the air, heat indices in coastal and urban areas may soar above 108 degrees Fahrenheit. && .AVIATION... (06Z TAFS) VFR conditions will likely prevail across all terminals through the morning hours. However, afternoon SHRA/TSRA may result in MVFR to brief IFR conditions, with reduced visibility and lower ceilings, particularly for TJBQ. The remaining terminals may experience the occasional VCSH/VCTS. Light to calm and variable winds will become more E-ESE and increase to 14-18 knots between 11/13-23Z, accompanied by higher gusts and sea breeze variations. Higher wind gusts are likely near the SHRA/TSRA. && .MARINE... A surface high pressure across the Atlantic Ocean will promote moderate to fresh easterly to southeasterly winds for the next several days. An approaching drier airmass will result in less rain chances across the waters today, but some thunderstorms could develop and move across the western waters of Puerto Rico during the afternoon hours. Moisture from another tropical wave is expected to reach the northeastern Caribbean by Friday night leading to an increase in showers and thunderstorms throughout the weekend. && .BEACH FORECAST... There is a moderate risk of rip currents for beaches of Saint Croix today, low risk elsewhere. However, by tonight, the moderate risk of rip currents is forecast to spread to the rest of the local islands and remain moderate through the upcoming weekend. For additional information, consult the Surf Zone Forecast (SRFSJU) issued by the NWS San Juan Office. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001-003-005-007-008-010-011. VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ002. AM...None. && $$ SHORT TERM/AVIATION...CVB/CAM LONG TERM/MARINE/BEACH FORECAST...YZR