138 FXCA62 TJSJ 161624 AAA AFDSJU Area Forecast Discussion...UPDATED National Weather Service San Juan PR 1224 PM AST Sun Oct 16 2022 ...New UPDATE, AVIATION... .UPDATE... Issued at 1211 PM AST Sun Oct 16 2022 Showers formed before noon in the usual places, including over Guaynabo, southern San Juan and Trujillo Alto. Coamo and Utuado also are beginning to see showers. An approaching upper level trough and a low level trough from the other side will make things interesting during the next 36 to 48 hours. Will especially be watching the Culebrinas river if thunderstorms reappear this afternoon. Sounding results were inconclusive due to a software error, but moisture has increased during the last 24 hours. Urban and small stream flooding are just a few hours away. Still some time to enjoy the low risk of rip currents at the beaches. && .SYNOPSIS... Near-normal moisture and increased instability will result in an active afternoon today and tomorrow, with the typical shower activity increasing through this time. Showers and thunderstorms are expected for western and interior portions of Puerto Rico, especially. A light steering flow will result in slow-moving showers, which could lead to locally high rainfall totals, increasing the potential for flooding conditions to develop. Decreasing moisture and stability are expected for Tuesday into the end of the week. && .SHORT TERM...Today through Tuesday... The available moisture is expected to remain fairly steady today and tomorrow, with precipitable water near 1.8 inches but perhaps peaking to 1.9 inches on Monday. Then, a slight decrease to around 1.7 inches is forecast. We also have a diffluent pattern in the upper levels and an upper trough moving in from the NW, which will cause an increase in instability by this afternoon and through Monday. The slightly above normal moisture is causing widely scattered showers across the local waters in the early morning hours, some of which also affected portions of eastern PR and will continue for the rest of the day. Brief showers could also affect the USVI, though none have so far tonight. In the afternoon hours, the available moisture is expected to combine with the local effects and diurnal heating to cause showers and thunderstorms to develop. The central to western sections of PR as well as portions of the San Juan Metro have the best chance of showers, with central to western Puerto Rico having more persistent and longer lasting showers and thunderstorms. This activity, combined with the shower activity observed in the past few days, will likely cause some could cause urban and small stream flooding, while isolated flash flooding will not be ruled out. Elsewhere in PR, scattered showers with isolated thunderstorms are forecast, and some ponding of water in areas of poor drainage could be observed. Across the USVI, mainly scattered showers could be observed, as well as lines of showers streaming off the islands. For Monday, there is the potential for another round of persistent thunderstorms over PR, due the approaching upper trough from the northwest, which will be in a favorable position to cause instability over the area. This is in addition to the above normal moisture with precipitable water values reaching 1.9 inches. Therefore, showers and thunderstorms are forecast over PR on Monday afternoon, while scattered showers are forecast elsewhere in the local area. The thunderstorms on Monday may persist over any one area simply because the winds are very light, so the thunderstorms that due develop in areas of convergence over PR will move very slowly. This pattern will once again cause at least urban and small stream flooding, but some isolated flash flooding should not be ruled out. By Tuesday, the moisture decreases slightly and the upper trough continues moving to the SE, with the local islands being in the subsident side of the trough by Tuesday afternoon. This will limit shower and thunderstorm development to locally induced thunderstorms over PR, which will be once again across the central to western sections of PR, while isolated to scattered showers are forecast elsewhere. As far as temperatures, daytime high temperatures will be near normal, with highs in the upper 80s to low 90s across the lower elevations. The heat index values will continue reaching or surpassing the 100s degrees Fahrenheit during short term period, especially across north central Puerto Rico. && .LONG TERM...Wednesday through Sunday... Ridging is expected in the mid- to upper-levels, maintaining stability over the region through at least the end of the workweek. Meanwhile, drier than typical air will be over the area into Friday, as well, though patches of near-normal (on the low end of the normal range) moisture are likely to pass through the region from time to time. Still, local effects will combine with available moisture to result in showers and isolated thunderstorms in a typical pattern over the region. Forecast confidence is medium, largely due to uncertainties in the direction of a slow steering flow and the timing of moisture patches. Forecast certainty decreases at the end of the week. Both the Euro and GFS suggest that an upper-level low will approach from the northeast, absorbing the upper-level low that will have been effectively stationary east of the Windward Islands over the previous days. This trough will then affect the region, resulting in an increase in shower activity. However, there is disagreement on timing, and just how close the low will get to the area. As such, there is the potential for active weather next weekend, but it is essentially equally likely that there will be seasonally-typical conditions. Forecast confidence becomes low by late Friday/early Sunday. && .AVIATION... (12Z TAFS) VCTS is expected at TJBQ after 16/17Z. TJSJ may have VCTS at some point between 16/17Z and 16/20Z, as SHRA have already dvlpd south of the terminal in the VCNTY. Winds will be generally easterly 10-15 knots with sea breeze influences. && .MARINE... Winds of up to around 10 to 15 knots will continue across the local waters. Seas will remain generally 4 feet or less for the offshore Atlantic waters, and 3 feet or less across the rest of the waters. There is a low risk of rip currents. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. && $$ SHORT TERM...WS LONG TERM....GRS AVIATION...ERG