692 FXUS66 KSEW 132228 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 328 PM PDT Mon Sep 13 2021 .SYNOPSIS...Weak ridging through tonight will allow for dry conditions before a weak frontal system pushes across the area Tuesday and Tuesday and yields the next chance for light rain. Conditions then briefly dry out again Wednesday and Thursday before the arrival of the first significant frontal system of the season. Widespread rain is expected Friday, followed by an unstable atmosphere through the weekend, bringing the threat for thunderstorms and high elevation snow. && .SHORT TERM /TONIGHT THROUGH THURSDAY/...Recent water vapor imagery this afternoon shows weak mid/upper level ridging building into W WA, replacing a weak shortwave trough that can now be seen moving into SE WA and the ID panhandle. Downstream from these features resides weak cyclonic flow over the northern tier of the country into the Western Great Lakes, while well upstream a noticeably strong cyclone and associated deep trough reside over the Bering Sea into Alaska and its adjacent Gulf. Near the surface, high pressure dominates the Pacific Northwest, with a frontal system noted offshore over the Pacific up towards Haida Gwaii. East of the Rockies, a frontal boundary extends across the Central Great Plains towards the Southern Great Lakes, with expansive high pressure over much of the South. A pleasant day continues over W WA this afternoon as weak upper level ridging and sfc high pressure remain the dominant sensible weather features. Areas of extensive morning cloud cover have largely eroded into a scattered cumulus field, with some locations having experienced mostly sunny conditions for much of the day. Scattered clouds will continue into the evening hours, with high temps peaking over the next few hours in the upper 60s to lower 70s. The upper level ridge influence and associated height rises will peak this evening into the early overnight period, with a weak frontal system (previously mentioned over the Pacific) approaching the region late tonight. Can expect increasing mid level cloud cover from west to east overnight tonight into Tuesday ahead of the frontal boundary. Clouds will then gradually lower through the day, with rain reaching the upper coast and north interior by the afternoon. The front will gradually sink SE through the evening and overnight period, with precipitation also waning in time as the parent system pushes well to the north over Northern BC and Northern Alberta. Areas around Central Puget Sound may even get rain shadowed for much of this event as light rain continues into Wednesday morning. Rainfall amounts will be highest over the North Coast and across San Juan, Whatcom, and Skagit Counties with around a quarter of inch possible, whereas elsewhere the large majority will receive less than a tenth of a inch of rain. The front will continue to the SE through the morning hours Wednesday. This will allow most locations to dry out and maybe even see some afternoon sun. The exception will be the areas typically in the convergence zone-a PSCZ is likely to develop Wednesday morning in the wake of the front. It is expected to dissipate into the afternoon. Larger scale troughing will deepen over Western Canada on Thursday, placing the PNW in stronger W/NW flow aloft. The lack of a substantial weather system will allow for an overall dry day with high temps in the mid to upper 60s. Kovacik .LONG TERM /FRIDAY THROUGH MONDAY/...The long term forecast period is set to be quite active as a pattern change takes hold. Strong troughing will deepen over the Pacific on Friday, orienting the jet stream towards the PNW. A lead shortwave perturbation within this developing larger scale flow will eject east into BC, pushing a more fall-like frontal system towards W WA. Widespread stratiform rain is expected to affect the entire area quickly from west to east on Friday. Rainfall may be moderate to heavy at times, with rainfall amounts Friday-Friday night potentially ranging from 3/4ths-over an inch in the lowlands to 1-2 inches in the higher terrain. Breezy winds are also anticipated. The front will then push east of the area Friday night into Saturday, with the deep upper trough also pushing east into the region in its wake. Ensemble members are in good overall agreement with the placement and intensity of the trough on Saturday for confidence in this pattern to be high. The placement of this trough will allow for quickly cooling mid level temperatures to -20C or below, yielding an unstable environment. This combined with plenty of PVA supplied by smaller scale perturbations within the trough will allow for development of widespread convective showers most of Saturday. With the unstable conditions, thunderstorm potential remains amidst a possibly moderately sheared environment. Will continue to monitor some of these finer-scale details in the coming days but currently can begin to plan for the potential for locally heavy downpours (on top of rainfall amounts from the previous day) and lightning. Winds will also continue to be breezy. Unsettled conditions will persist into Sunday, where the potential for thunderstorms may continue. In addition to the thunderstorm threat, snow levels will take a tumble down towards 6000 feet Saturday and Sunday. With widespread convective showers likely, snow in the higher elevations is expected. This will be important to keep in mind, as this will be the first snowfall in several months. Those planning to recreate in the mountains this weekend need to prepare for colder and more winter-like conditions. Discrepancy in the pattern is then quite noticeable into early next week, with a debate between the return to upper level ridging vs continued weak troughing. At present time, it seems more ensemble solutions favor troughing so will leave the potential for unsettled conditions in through Monday. Kovacik && .AVIATION...Mainly VFR conditions are expected through the period. Light winds and scattered clouds this afternoon will give way to clearing overnight. Some IFR-MVFR conditions are possible towards daybreak Tuesday with patchy fog and low clouds developing but this would dissipate by 18Z Tue and return to VFR conditions after. Mid level clouds will increase after 18Z Tue in advance of an approaching system. KSEA...Light and variable northerly winds this afternoon less than 8 knots will gradually shift southerly overnight less than 6 knots. Scattered afternoon clouds will dissipate overnight. A few scattered low clouds towards daybreak Tuesday but not expecting a ceiling at this time. VFR on Tuesday with southwest winds 4-8 knots and mid level clouds increasing in advance of an approaching system. Johnson && .MARINE...Onshore flow bringing increasing winds to likely peak around 20 kts this evening in the Strait. Winds will flip around to the south and increase across area waters on Tuesday with the approach of another front. Winds will swing around to the northwest behind the front, with advisory level winds expected offshore and across portions of the inland waters. A more significant front will approach area waters by the end of the week, bringing another round of strong winds and increasing seas. Northwest swell 3 to 5 feet through Tuesday. Increasing swell again midweek and this weekend. Johnson && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ www.weather.gov/seattle