Mercer Island, WA Weather

Synopsis

A weather system offshore will drift southwards into Monday and move onshore across NW Oregon resulting in continued scattered showers. Unsettled conditions Tuesday with continued troughing. A frontal system midweek will bring increased lowland rain and mountain snowfall into late week. Cooler temperatures next weekend.

Short Term

Steadier precipitation continues this afternoon over portions of the Cascades and coast as a low pressure system offshore slowly drifts southwards. Unsettled conditions with periodic showers continue through Monday as the system weakens and moves onshore into NW Oregon by Monday afternoon. Snow levels remain near 5000 feet for high elevation mountain snowfall in heavier showers. Otherwise, winds will slowly subside for breezy areas of the coast and Admiralty Inlet into tonight. Highs remain in the 50s for most areas this afternoon and again Monday.

Troughing persists over the Pacific Northwest on Tuesday, with lingering moisture and a few showers over the Cascades throughout the day. The next frontal system also approaches from BC later Tuesday into Tuesday night bringing increasing precipitation and locally breezy winds. Highs continue in the mid 50s Tuesday.

Long Term

The aforementioned frontal system moves across Western Washington Tuesday night into Wednesday. This looks to be the first noticeable mountain snowfall event of the season as colder air filters into the area with this system. Snow levels during the bulk of the precipitation ranges between 3500 to 4500 feet, allowing for snow at the higher elevation Cascade Passes. NBM probabilities for greater than 6 inches of snow (24 hours ending 500 PM Wednesday) is approximately 70% at Heather Meadows (Mt. Baker), 15% at Rainy Pass, 20% at Stevens Pass, and 90% at Paradise, with most of the snowfall between Tuesday evening into early Wednesday. Will need to monitor the need for any headlines for this event, both due to early season, and higher snowfall amounts above 4500-5000 feet. Otherwise, lowland rainfall is expected on Wednesday, with a 15% probability for thunderstorms for most of the lowlands.

The associated upper low will then deepen offshore Thursday and linger over the WA/OR waters into late week for continued unsettled conditions. Ensembles are generally consistent the troughing digs even further southwards late Friday into next weekend, lowering the precipitation potential. However, cooler temperatures are then expected over the weekend, and will need to monitor any frost or freeze concerns during the morning. JD

Aviation

Southwesterly flow aloft will become southerly later today then southeasterly overnight as an upper low offshore moves southward and then onshore over northwest Oregon on Monday. Latest radar shows the heaviest bands of precipitation moving into the Cascades and inland along the coast this afternoon, with some light shower activity across the interior lowlands. Showers will continue across the area into Monday. Ceilings remain a mixed bag of mostly low-end VFR to MVFR across the region this afternoon as a result. MVFR ceilings will become more widespread by early Monday as ceilings lower overnight.

KSEA...Mostly a mix of MVFR to low-end VFR ceilings this afternoon in shower activity. This trend will continue through tonight before a more widespread lowering of ceilings occurs overnight into Monday. Latest probabilistic guidance indicates a 45% chance of ceilings dropping to MVFR by 12Z, with a 10-20% chance of ceilings dropping to IFR at the terminal. SE winds primarily between 5-10 kt this afternoon will become more NE tonight and ease to 5 kt or less overnight into Monday.

14