Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
819 FXCA62 TJSJ 240944 CCA AFDSJU Area Forecast Discussion...CORRECTED National Weather Service San Juan PR 544 AM AST Fri May 24 2024 SYNOPSIS... Cloudy conditions with the chance of showers across eastern Puerto Rico continues through today with an increase of rain during the afternoon hours. Wet weather conditions are expected to persist throughout next week as a tropical wave approaches from the southeast by Monday. In addition, hot days are expected over this weekend with heat indices in the upper 100s mainly for coastal sections. && .SHORT TERM...Today through Sunday... The night was tranquil for the most part over land areas, with some isolated to scattered showers occurring over the local waters, affecting windward coastal areas of Puerto Rico and the US Virgin Islands at times. The strongest activity was located over the western quarter of the offshore Atlantic waters, with heavy showers and strong thunderstorms associated with the deep-layered trough that had been affecting the Dominican Republic in the past few days. This activity will gradually approach the area during the morning hours. The forecast had to be adjusted slightly to account for some of the latest tendencies in the recent model cycles. Challenges arise due to the forecast area being situated between the deep-layered trough to the northwest and a mid- to upper-level ridge southeast of the region. The TJSJ 24/00z sounding showed dry air within the 850-500 mb layer, with mid-level relative humidities at 28%. This, along with ridging aloft, inhibited convective activity in the last 18-24 hours. However, the 700-500 mb CIRA Layered Precipitable Water (LPW) product shows a broad area between the Dominican Republic and Puerto Rico with high moisture content gradually making its way towards us. This is partly eroding the mid-level ridge while increasing mid- level relative humidities across the region. At the same time, the 850-700 mb CIRA LPW shows a moist slot over the USVI moving northwestward. The latest model guidance suggests increased moisture convergence and instability as these features approach today, promoting the development of shower and thunderstorm activity, particularly over the northern half of Puerto Rico and smaller islands this afternoon. For this weekend, the mid-level ridge will continue to move eastward as the deep-layered trough weakens and settles in. Marginal instability aloft will persist, with precipitable water content exceeding and remaining above the 90th percentile of the climatological normal. Sufficient dynamics will combine with well above normal moisture every day, potentially leading to heavy showers and isolated thunderstorms, mostly during the afternoon hours. A weak southeast steering flow will cause convective activity to cluster mostly over interior and northwestern sections of Puerto Rico, with streamers developing downwind of the smaller islands and El Yunque into the San Juan metro area. Slow-moving showers could enhance rainfall accumulations in flood-prone areas. Therefore, we can expect a limited to elevated flooding threat to persist through the weekend. Southerly winds will persist, advecting warm temperatures over the local islands through the short-term forecast. Extensive cloud coverage moving in will somewhat inhibit temperatures from increasing substantially today. However, we can still anticipate heat index values above 100 F across coastal and urban areas. But, for this weekend, heat index values are projected to reach Heat Advisory Criteria in those areas during peak hours of the day before shower activity begins. Therefore, it`s important to stay hydrated if engaging in outdoor activities. && .LONG TERM...Monday through Friday... Model guidance indicates that a wet pattern will persist through the long-term forecast due to a moist airmass across the region as a tropical wave approaches from the east. As this tropical wave moves over the islands by late Monday, causing moisture content to rise above 2.0 inches, an increase of unstable weather is expected to last through the long term period. Weak ridging aloft throughout the week will help keep conditions somewhat decent, yet the continuation of the passage of weak short-wave troughs through Friday will maintain above-normal moisture and marginal instability. The National Blend of Models hints for heavier rainfall during the afternoon hours particularly across western/northwestern Puerto Rico under an east-southeast steering flow. The Galvez-Davison Index also suggests a similar scenario, with a high potential for observing isolated to scattered thunderstorms capable of producing heavy rainfall each day. With saturated soils and elevated streamflows from previous days of rainfall, the flooding risk will remain elevated. Therefore, we could expect urban and small stream flooding, as well as flash flooding and landslides in areas of steep terrain. Hot days are also to be expected with heat indices reaching in the 100s to 110s through the week due to the high RH values and heating from the summer sun, therefore heat advisories or warnings may be issued during this time. Saharan dust particulates are expected to arrive by early in the week. Expect hot and hazy conditions to prevail across sections with less rainfall. && .AVIATION... (06z TAFS) Areas of SHRA and Isold TSRA over Hispaniola will move close to TJBQ aft 24/10Z. Aft 24/16z, SHRA and VCTS could affect all remaining terminals. These features will reduce VIS and CIG possibly producing MVFR/IFR conds and mtn obscurations. Sfc winds ESE 10-15 kts with higher gusts near SHRA till 24/22Z. && .MARINE... Surface high pressure across the central and northeast Atlantic and an induced surface trough just northwest of the region will promote moderate to fresh southeast winds through tonight. In addition, the proximity of a deep layer trough over the Dominican Republic will aid in enhancing thunderstorm development across portions of the regional waters and local passages through at least today. Winds are forecast to diminish overnight and then become more easterly over the weekend. && .BEACH FORECAST... Low to moderate rip current risk will persist throughout the period. In the presence of a moderate rip current risk for St. Croix, there is still the possibility of life-threatening rip currents in the surf zone. Please remember to heed the advice of the local beach patrol and flag warning systems. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...CVB LONG TERM....RC