000 FXCA62 TJSJ 232029 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 429 PM AST Fri Oct 23 2020 .SYNOPSIS...A tropical wave will bring a peak in moisture that ends abruptly Saturday morning, with moisture below or at the lower end of normal through the start of next week. Moisture will then increase gradually from 1.35 inches of precipitable water to over 2.2 inches on Wednesday. This will be accompanied by a broad lower-level trough that passes through the area during the midweek. && .SHORT TERM...Tonight Through Sunday... Minimal shower activity has occurred thus far ahead of a band of moisture, which is located just behind a tropical wave that extends from the eastern Caribbean to just north of St Kitts & Nevis. This band is expected to pass through the U.S. Virgin Islands around midnight tonight and west Puerto Rico tomorrow morning. This will bring an increased chance of showers over the Caribbean around the U.S. Virgin Islands and over eastern Puerto Rico tonight. Based on satellite imagery, precipitable water is currently about 1.6 inches across the local islands; the moist band has a maximum of 1.7-1.9 inches. After the passage of this weak tropical wave, model guidance is suggesting a minimum in TPW around 1.35 inches over San Juan tomorrow morning. This will clear most showers out of the area during the early morning on Saturday. However, local effects and diurnal heating could give way to localized convection along and west of the Cordillera Central. Moisture will gradually increases to about 1.75 inches by Sunday morning. This will increase somewhat the chance for shower activity across the islands. Therefore, showers and isolated thunderstorms are expected across much of Puerto Rico, with a focus on the western interior, especially during the afternoon. The U.S. Virgin Islands can expect tranquil weather conditions with the typical passing showers at times. Low Level winds were east southeast today, but sea breezes were able to over come the southerly flow in San Juan and as of 1 PM AST the peak gust was 29 mph. They will be turning more easterly tomorrow so it is likely that high temperatures will not increase tomorrow and will likely be a degree or two cooler on Sunday under the influence of better moisture. .LONG TERM...Monday through Saturday...(From the previous discussion issued at 5:18 AM AST) Recent model guidance now suggests that a moist and unsettled weather pattern will dominate the local weather conditions through the long term forecast period. First, a trade wind or easterly disturbance will enhance moisture advection with frequent overnight and early morning trade wind showers, and the potential for increased locally and diurnally induced activity on Monday. In the meantime, an amplifying mid to upper-level trough will spread southward across the eastern Caribbean, with a weak cutoff low and surface-induced trough lingering over the region through early Thursday. Another, but stronger cutoff upper level low will quickly follow, sinking south just northwest of Puerto Rico by Thursday evening, then drifting westward through early Friday. To support this scenario, deep tropical moisture with model- estimated precipitable water above 2 inches by 12Z on Tuesday and 18Z on Wednesday along with favorable conditions aloft with 500 mbar temperatures running between -6 and -8 degrees C will aid in the development of shower and thunderstorm activity. Lastly, a tropical wave is forecast to reach the local islands between Friday and Saturday. Overall, a deepening surface pressure will maintain a generally moderate to fresh easterly wind flow through the period. This forecast follows GFS model guidance closely. However, there is only medium confidence in this scenario since the ECMWF suggest a different scenario with relatively drier air and ridging aloft by the end of the workweek. && .AVIATION...VFR conds will likely continue across all the TAF sites thru at least 22Z. Then, a weak tropical wave will bring some SHRA near TKPK and TNCM during the evening and near TIST and TISX late tonight. Cannot rule out brief MVFR conds as these SHRA pass by but the confidence remains low. SE winds of 15-20 kt will continue below FL100. && .MARINE...Seas have bottomed out at around 5 feet at the inner buoy and bounced back to 6 feet at the outer buoy. seas had been over forecast by the models but this should rectify by Saturday afternoon or evening. Seas will begin to increase again late Saturday morning and onward. Then, confused seas due to a swell from the distant Epsilon and wind-driven seas will deteriorate even more marine and coastal conditions by Sunday morning. Seas will continue to be hazardous as wind increase over the area Saturday through Monday. For the beachgoers, a high risk of rip currents will continue for north facing beaches of Puerto Rico, the eastern tip of Saint Croix. A high risk of rip currents will also return for the northwestern beaches of Saint Thomas by Sunday. && .PRELIMINARY POINT TEMPS/POPS... SJU 79 92 78 90 / 50 20 20 30 STT 79 89 79 88 / 50 20 30 30 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...High Rip Current Risk through late Saturday night for Culebra- North Central-Northeast-Northwest-San Juan and Vicinity. High Rip Current Risk until 6 PM AST this evening for Mayaguez and Vicinity. VI...High Rip Current Risk through late Saturday night for St Croix. high rip current risk for Saint Thomas till 6 PM AST. AM...None. && $$ SHORT TERM...ER LONG TERM....WS PUBLIC DESK...ER