Issaquah, WA Weather

Synopsis

An offshore storm system moving into area today will very gradually shift south and east through Monday morning. This will bring widespread rain and breezy conditions through Sunday morning followed by showery unsettled weather into the start of the week. The pattern looks to remain very seasonal for the last week of the month - cooler with a series of systems bringing periods of unsettled weather nearly every day.

Short Term

Rain remains on track this evening with a minor update to the short-term below, and aviation section. -HPR

The offshore system that has been sliding down the British Columbia coast continues to move off the Washington coast, with light warm frontal precipitation spreading into the north Olympic Peninsula, and into British Columbia this evening. REFS guidance showing the cold front slowly moving onshore overnight as the parent system takes it's time moving south and east. Precipitation with the cold front will be heavier that the lighter rain Saturday. While this ultimately is a fairly typical October storm system, it is the first fall-like system to move into the area, and will definitely feel different as such. Heavier precipitation amounts are expected along the coast and Olympic Peninsula with 24 hour amounts through Sunday evening in the neighborhood of 1-2 inches, and 0.50 to 1 inch along the I-5 corridor eastward. In addition to the rainfall, increasing south winds along the coast this evening will spread inland ahead of and with the front, remaining breezy through Sunday. Peak wind gusts of 20 to 35 mph are possible for many areas.

The parent upper level system is expected to continue its slow south-southeast track through the day Sunday into Monday. Ensembles showing more agreement in the track of the upper level low than the surface low. The higher resolution REFS ensemble hinting at a more northerly track with the ensemble mean showing the surface low moving into Grays Harbor late Monday morning, while many of the other higher resolution ensembles like the HREF and UW ensemble systems show a much more southerly track. In any case, with the upper level low over the area Sunday into Monday morning, scattered, but widespread showers will linger well into Monday before shifting east with the upper level support late Monday afternoon.

High temperatures will trend near normal in the mid to upper 50s, while overnight lows will trend cooler than normal in the in the low 30s to low 40s.

Long Term

The grand ensemble remains generally consistent early in the week with the upper level trough associated with the weekend system shifting into the northern Intermountain area Tuesday morning. Ensembles all hint at a trailing shortwave sliding down the British Columbia coast, with variability in the strength and timing. The consistent message is that the bulk of the relatively light precipitation looks to be focused mainly south of western Washington Tuesday. Ensembles very consistently showing a more consolidated upper level low sliding down the BC coast Tuesday night and into western Washington Wednesday. This looks scenario very similar in many ways to this weekend's system, with cooler temperatures and widespread rain over much of the Pacific Northwest. The pattern looks to remain unsettled Thursday into Saturday with a broad positively tilted upper level trough along the Western US. Some scenarios showing a more consolidate upper level low over California. For the Pacific Northwest the pattern looks to remain unsettled - but seasonal - through the last few days of the month.

Aviation

Southwest flow aloft increases into Sunday, becoming more southerly by Sunday afternoon as an upper trough digs offshore. Widespread VFR mid and high clouds this evening across western Washington ahead of a frontal system. However, MVFR has been observed along the coast and BLI as rain continues to slowly spread eastwards. Low-end VFR to high-end MVFR cigs expected for most southern and central Puget Sound terminals by early Sunday. Lower VFR cigs for interior, with continued MVFR for portions of the Olympic Peninsula on Sunday afternoon. Vsby reductions possible in areas of steadier rain into Sunday, dropping to 3 to 6SM, lowest along the coast. Increasing S/SE winds for the coast, and E/NE winds for BLI into tonight, remaining light for KPAE to KTCM into Sunday.

KSEA...VFR continues through tonight with mid and high clouds. Cigs slowly lowering towards MVFR by Sunday AM. At this time, the probability for MVFR cigs peaks between 13 to 18z Sunday as rain arrives, with probabilities around 25 to 30%. Rain at times into Sunday PM, with brief reductions to vsbys possible. VFR for Sunday afternoon with continued OVC mid clouds. Light west winds this afternoon, with a light E/NE component tonight into Sunday.