475 FXCA62 TJSJ 081542 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 1142 AM AST Fri Oct 8 2021 .UPDATE...Satellite imagery showed the Saharan Air Layer moving across the northeast Caribbean region and a tropical wave well south of Puerto Rico this morning. The suspended aerosols are producing hazy skies across the local islands today. As a result, the visibilities are lower than normal, ranging between 6 to 9 miles in Puerto Rico and the U.S. Virgin Islands. In terms of precipitation, limited shower activity is anticipated across most of the forecast area except the central interior and western Puerto Rico where local induced showers and thunderstorms are possible late in the afternoon. The drier pattern will continue the rest of the weekend, followed by a wetter pattern likely for the next week. No changes were introduced to the forecast package as it remains in line with the latest thinking. && .AVIATION...VFR conditions will continue through Sunday. Visibilities are reduced due to the presence of Saharan Dust. VIS have ranged between 6-9 SM in PR/USVI, with lower VIS in TNCM/TKPK. SHRA possible near TJBQ and TJMZ between 19Z-22Z. Winds will remain from the east at 10-20 kts. && .MARINE...Local buoys are reporting typical marine conditions with seas ranging between 3-5 feet. Seas will increase slightly Sunday and early next week when a long period northerly swell moves across the waters, increasing the risk of rip currents to high. && .PREV DISCUSSION... /issued 500 AM AST Fri Oct 8 2021/ SYNOPSIS...A drier than normal air mass with a Saharan Air Layer will prevail across the local area into the upcoming weekend with convection limited mostly to western Puerto Rico during the afternoon hours. A wet weather pattern appears likely during most of next week. SHORT TERM...Today through Sunday... Saharan dust is moving in, which will cause hazy skies today and on Saturday, although the Saharan dust concentration is lower on Saturday and thus is not expected to be quite as hazy as today. A moderate easterly wind flow will prevail through the short term period. Along with the Saharan dust, a much drier than normal air mass is expected through the short term forecast period, with Precipitable Water values generally around the 25th percentile of normal. However, a few brief patches of moisture will be moving through but the impact is not expected to be more than brief showers over the local waters, USVI and portions of eastern and northern Puerto Rico during the overnight and early morning hours. An upper level trough is expected to prevail over the local area through the short term period, which could aid in the development of brief thunderstorms. That being said, the areas with showers and thunderstorms may be limited with as much dry air as we are expecting. The areas with the best chance of thunderstorms are across western PR in areas of sea breeze convergence in the afternoon hours. With the possible shower and thunderstorm activity this afternoon, ponding of water in poor drainage areas is expected, but some urban flooding is also possible. As far as temperatures, we can expect near normal to slightly higher than normal highs and lows. Daytime highs in the lower 90s across the lower elevations, with heat indices near or slightly above 100 in some areas. Overnight lows in the mid 70s to near 80 across the lower elevations. The higher elevations could observe highs and lows about 5 to 10 degrees cooler than the lower elevations, even slightly cooler in a few isolated areas. LONG TERM...Monday through Saturday... A wet weather pattern continues to appear likely during the long- term period. On Monday, a vigorous tropical wave will move over the area. This will result in widespread shower and thunderstorm activity, with activity concentrated across eastern Puerto Rico and the USVI during the morning hours followed by enhanced convective activity over mainland Puerto Rico during the afternoon hours. By Tuesday, a mid and upper-level trough will become cutoff and amplify over Cuba, inducing a broad low-level trough north of Hispanola. This feature will induce a deep southerly flow, tapping deep tropical moisture from the Intertropical Convergence Zone (ITCZ) into the local area. This overall pattern is expected to prevail through the rest of the long term period, helping to support a moist and unstable air mass. In fact, model guidance suggests precipitable water values fluctuating between 2.1 and 2.5 inches during the period, which are between the 75th to 99th percentile for this time of the year. Therefore, conditions at this time appear favorable for deep and organized convective activity to develop and affect the local area with the potential for portions of the area to observe significant rainfall activity with an enhanced threat for flooding and mudslides. Given that this potential rainy pattern is still in the long term period, there are still some uncertainties in the overall details of where the heaviest rainfall activity will occur and how significant it will be. AVIATION...VFR conditions expected across the local terminals through the forecast period. However, -SHRA will be observed across the local flying area, causing VCSH through the morning at the local terminals. After 08/17Z SHRA and ISOLD TSRA expected across western and PR, possibly causing VCTS at TJBQ. Winds will continue either variable or from the east at 10 KT or less through 08/12Z, increasing up to 15 KT, mainly from the east but with sea breeze variations after 08/14z. MARINE...Mainly tranquil marine conditions with seas of up to 5 feet and winds of up to 15 knots will prevail across the regional waters for today. Choppier marine conditions can be expected during the upcoming weekend as a northeasterly swell with a 12-14 second period invades the area. For beachgoers, there is a moderate risk of rip currents for most of the local beaches, except for those beaches located from the western to south- central coast of Puerto Rico, where the risk will be low. The rip current risk will increase to high across the north-facing beaches of Puerto Rico and Culebra as well as the eastern tip of Saint Croix during the upcoming weekend as the aformentioned northeasterly swell invades the local waters. FIRE WEATHER...Latest surface observations indicate KBDI values across Cabo Rojo and Guanica at 550 and 568, respectively. A drier air mass is expected to prevail for today and this will cause relative humidity values to drop into the lower to middle 50. Although sustained winds may remain below 15 mph, frequent gusts of greater than 20 mph can be expected. As a result of all of these factors, there is an elevated fire danger risk in the southwestern coastal plains of Puerto Rico for today. && .PRELIMINARY POINT TEMPS/POPS... SJU 91 78 90 77 / 30 10 20 20 STT 89 79 89 78 / 20 20 20 20 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...EM LONG TERM....OM PUBLIC DESK...OM