Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
783 FXUS66 KSEW 161604 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 904 AM PDT Mon Sep 16 2024 .SYNOPSIS...Upper level trough well to the south today while an approaching front will stay north of Vancouver Island. Front moving southeast tonight and Tuesday moving through Western Washington Tuesday evening. Rain out ahead of the front spreading over the area Tuesday. Weak upper level ridge Wednesday into Thursday. Unsettled weather Thursday night through the weekend with the ridge not strong enough to prevent systems from reaching the area. High temperatures remaining below normal except for today with highs near normal. && .SHORT TERM /TODAY THROUGH WEDNESDAY/...No significant changes to the forecast this morning. The previous discussion can be found below: Some stratus banked along the west slopes of the Cascades this morning along with some patchy fog preventing skies from being totally clear. Sunny skies for the most part today as the cold front along the British Columbia coast moves slowly southeast and the upper level trough continues to move south. The fog and stratus will dissipate this morning. Even with plenty of sunshine, the cool start to the morning will keep highs near normal, in the mid 60s to lower 70s. Front continuing to move southeast tonight but still well offshore by 12z Tuesday. Even the clouds out ahead of the system not arriving until the early morning hours. Could be a colorful sunrise Tuesday morning. Lows tonight in the upper 40s to mid 50s. Front offshore near the coastline by 00z Wednesday with rain along the coast in the morning spreading inland during the afternoon. There is a chance the precipitation will not reach the Cascades by late Tuesday afternoon. Strong jet digging south offshore behind the front will slow down and weaken the feature as it gets close to Western Washington. With plenty of cloud cover and rain highs will be cool Tuesday, around 60. Front trying to move through Western Washington Tuesday night but with the digging jet offshore chance the front just dissipates over the area. Upper level trough behind the front will keep showers in the forecast overnight. Lows Tuesday night in the 50s. Wednesday starting to look like Sunday with the upper level trough digging south and drying northerly flow aloft developing late in the day. Air mass unstable enough to keep a chance of showers in the forecast. Temperatures will remain cool with highs in the lower to mid 60s. .LONG TERM /THURSDAY THROUGH SUNDAY/...Confidence in the extended forecast not very high this morning. Pretty good agreement in the models Thursday with a flat upper level ridge over the area for a dry day. Weak shortwave riding by to the north Thursday night into Friday for a slight chance of showers. Model solutions for the weekend very inconsistent. The GFS builds an upper level ridge over the area for dry weather. The ECMWF has a much weaker ridge with a long fetch of moisture extending all the way back to the dateline. The first system embedded in this fetch of moisture reaching Western Washington Sunday. Ensembles show a little different story with the ECMWF solutions not as wet as the operational run but the GFS solutions wetter than the operational run. The trend in the ensembles from last night`s run is there are more wet solutions this morning. With the lack of confidence for now will stay with a broad brush mostly cloudy/partly sunny chance of showers forecast through the weekend. Felton && .AVIATION...Light east to northeast flow aloft continues over Western Washington today as an upper level low over northern California shifts eastward into the Great Basin. The flow aloft will shift to westerly late tonight as another upper trough and associated frontal system approach the region. Light surface winds and low level moisture driving patchy LIFR/IFR in fog/stratus across parts of the interior this morning. This will scatter out by around 19Z for VFR areawide this afternoon and evening. Increasing onshore flow tonight will produce widespread low MVFR or IFR ceilings across Western Washington on Tuesday morning. KSEA...The stratus deck that developed early this morning with ceilings 1500 to 1800 feet looks to be a bit more persistent - have updated the forecast to reflect a later transition to VFR continuing through this evening. Surface winds N/NE 7 knots or less becoming northwesterly this afternoon then backing to south/southwesterly tonight. 27/Cook && .MARINE...A broad surface ridge remains centered well offshore with lower pressure over the interior. Onshore flow will increase somewhat this evening. Winds may briefly get close to small craft advisory levels in the central/east strait tonight. The surface ridge over the offshore waters will weaken tonight and a trough and associated front will shift into the coastal waters on Tuesday. This system is unlikely to generate any headlines. The trough will weaken as it gradually shifts southward into the Oregon coastal waters on Wednesday. High pressure rebuilds offshore late in the week before another front is expected over the upcoming weekend. 27 && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$