148 FXCA62 TJSJ 080137 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 937 PM AST Thu Oct 7 2021 .UPDATE...Locally numerous showers and strong thunderstorms were observed across the northwestern sections of PR during the late afternoon and early evening hours. The doppler radar estimated between 2-3 inches of rain with the heaviest showers over portions of the western interior. Meanwhile, a line of showers with isolated thunderstorms developed from the northern USVI/Culebra into the northeastern coastal sections of PR around sunset. This activity was enhanced by the upper level trough north of the islands, and a surge in low level moisture from the east. A mid-level drier air mass with suspended Saharan dust is expected to gradually move from the Leeward Islands into our region on Friday. Below this layer, patches of moisture will move in a moderate easterly wind flow, and trade wind showers cannot be ruled out through at least Friday morning across portions of the islands. For Friday afternoon, the upper level trough should aid in the development of showers and thunderstorms over portions of the western interior and northwest coast of PR. No significant changes were made to the previous forecast. && .AVIATION Update...Mainly VFR conditions are expected to prevail across all terminals through the forecast period. Trade winds showers should cause mainly VCSH across all terminals overnight. The 08/00z TJSJ sounding indicated east winds at 7-18 kt blo FL090. && .MARINE Update...Coastal buoys were indicating seas up to 4 feet and east winds around 15 kt or less. Seas up to 5 feet are expected mainly across the outer waters. A moderate risk of rip currents will continue for most of the east, north and southeast facing beaches of the islands. && .PREV DISCUSSION... /issued 412 PM AST Thu Oct 7 2021/ SYNOPSIS... Saharan dust will result in hazy skies through at least early in the weekend. However, afternoon activity may still develop over the northwestern quadrant of Puerto Rico. Unsettled weather conditions are expected for the first half of the next workweek. Seas are gradually improving, but still remaining a little choppy. SHORT TERM...Tonight through Saturday... A surface high pressure over the central Atlantic will maintain a moderate east-southeast wind flow through the next several days. At the mid-levels, a ridge holds just west of Puerto Rico, while at the upper levels, a trough lingers north of the islands. Recent infrared satellite imagery shows small areas of clouds advancing toward the region. The high resolution models have some of these areas reaching portions of the U.S. Virgin Islands and east- southeast Puerto Rico, but with rainfall accumulation mainly below one inch. On Friday, a drier air mass east of the Leeward Islands and evident in Total Precipitable Water from GOES-16 will reach the local islands, with values falling to 1.3 to 1.5 inches. This air mass also contain Saharan dust, that will linger at least into early in the weekend, hence hazy skies are expected. Each day on Friday and Saturday, passing showers may move over portions of eastern Puerto Rico and the U.S. Virgin Islands through the day, then in the afternoon showers with isolated thunderstorms may develop over the interior and northwestern Puerto Rico. LONG TERM...Sunday through Friday.../from prev discussion/ For Sunday, model guidance continues to suggest a more stable weather pattern. A dry air mass will move in over the area decreasing precipitable water content at or below normal levels. More specifically, relative humidities within the 850-700 mb layer are expected to drop between 30% to 40%, which is two standard deviations below normal, serving as a limiting factor for shower development. However, some patches of moisture embedded in the easterly trades are expected to move across the area from time to time. Therefore, expect isolated showers over U.S. Virgin Islands, Vieques, Culebra and eastern third of Puerto Rico during the morning hours and, in the afternoon, isolated to scattered showers with minimal rainfall accumulations. However, in localized areas across northwestern Puerto Rico, thunderstorm development due to local effects cannot be ruled out. From Monday onwards, discrepancies persist regarding the potential of a very wet weather pattern, more specifically on the timing. Both ECMWF and GFS models agree on the arrival of a vigorous broad tropical wave by Monday. The differences lie on an upper cut-off low that develops over Florida and its evolution throughout the week. GFS and most of its ensemble members continue to put the cut-off low moving eastern enough to induce a surface trough by Tuesday night with axis over Hispaniola. This feature will induce a southerly flow, tapping deep moisture from the ITCZ into the area, further invigorating the tropical wave and supporting a very moist unstable weather pattern. The GFS Galvez-Davison Index maintains very high values throughout the week indicating the potential of scattered to widespread thunderstorm activity across the area. High rainfall accumulations can be expected with this setup. ECMWF maintains the cut-off low and induced trough solution further west of the area. Low-level moisture from the tropical wave will be there from Monday through Wednesday but ECMWF still has ridging aloft, and hence drier air, throughout that period not supporting a widespread thunderstorm activity as GFS does. Some ECMWF ensemble members agree with the GFS solution though, but the bulk of precipitation is still locating it further south of the forecast area. Interestingly, both global models agree on a deepening mid level trough pattern from Thursday to Friday with deeper moisture continuing over the area. To sum up, the signal of a wetter pattern for next week is definitely there between models. However, uncertainty persists regarding the evolution and timing of the mid to upper level weather conditions, and on the overall impacts across the area. And thus a continued low confidence in the forecast details remains at this time. Stay tuned! AVIATION...VFR conditions expected to prevail at all terminals through the forecast period. However, brief MVFR conds in SHRA and ISOLD TSRA will remain possible across western PR including JBQ, and possibly JSJ. Winds up to 15 KT with sea breeze variations becoming light and variable overnight. MARINE... Nearshore buoys have seas 3 to 4 feet with winds at 10 to 15 knots, and up to 6 feet at the offshore buoy. Moderate winds will continue for the next several days. There is a high rip current risk for the northern coast of Puerto Rico and Culebra, then by tonight, the risk will be moderate to the northern, eastern, southeastern coast of Puerto Rico, most of Vieques and Culebra, and also for most beaches in the U.S. Virgin Islands. && .PRELIMINARY POINT TEMPS/POPS... SJU 79 90 78 90 / 50 30 20 20 STT 80 89 79 88 / 50 20 20 30 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...DSR LONG TERM....ERG PUBLIC DESK...ICP