This weeks' record and near record heat might suggest the forecast for this weekend would be a simple one. Not quite. The end of the heat wave is likely to begin Saturday and end Sunday. Endings, whether in relationships or weather patterns, can be messy...especially when 'heat' is involved...
From the Crows' Nest
A massive ridge of high pressure extending into the upper reaches of the atmosphere where we find weather (the troposphere) is paired with what we call a thermal trough near the surface; a finger of hot, dry, low pressure air extending from the interior valley of California up into the Puget Sound area. Because higher pressure has existed east of the Cascades, dry offshore winds have amplified the warming. Little motion of either feature is likely Friday, with the exception of the coast. In that area, and in all likelihood, that area only, the eastward shift of the thermal trough will allow extensive low clouds and fog Friday morning, and again Saturday morning. Elsewhere...continued hot and dry.
The big 'Ridge-meister' will shift eastward overnight Friday into Saturday, shifting the flow aloft from north-northwest to south-southwest. That will be paired by the eastward shift of the thermal trough, and an expanding onshore push of cooler, ocean air. But in the process, moist and unstable air from the south will trickle into western Washington Saturday. That will bring increasing clouds as the day wears on, and likely showers or even thundershowers late Saturday afternoon or evening.
Those thundershowers could persist into Sunday, then end as cooler, moist ocean air floods into the Puget Sound area...making the airmass more stable. The potential for showers will diminish as the day wears on.
On the Trail
This will be a weekend to take rain gear and a rain fly...and choose a campsite that won't leave you perilously exposed to lightning! Skies will likely begin mostly sunny early Saturday, but increasing clouds are likely during the afternoon and certainly during the evening! Showers will be possible in the afternoon, but as the atmosphere continues to become less stable (what begins to rise is more likely to keep rising!) we will likely see showers and thundershowers develop. The snow level will be near 13,000' and pass temperatures will remain in the 80's...perhaps slightly cooler than Friday, 'dropping' into the low to mid 80's. Pass winds will be light northeasterly at 5 to 10 mph, with little change found on higher ridges.
Thundershowers could persist into Sunday morning, then taper off later in the afternoon. Pass temperatures will likely drop into the low to perhaps mid 70's, with a snow level near 12,000 feet. Pass winds will be westerly at perhaps 10 mph, shifting to southwesterly on exposed ridges at 10 to 15 mph.
On the Water
Saturday morning will see low clouds along the coast and very possibly through the Strait; elsewhere partly to mostly sunny skies are likely. But not for long. Clouds will increase from the south, with a potential for showers and thundershowers, first along the coast, south Sound and south Hood Canal...expanding to the north and east later in the afternoon...and probably persisting through the evening and overnight. Coastal winds will be light southwesterly, but pick up through the Strait (especially Saturday afternoon at 15 to 30 knots. Winds from Port Gardner/Everett northward will probably be southwest at 5 to 10 knots in the morning, increasing to 10 to 15 knots Saturday afternoon. Winds over the Sound will be fluky, ranging from 5 to 15 knots.
Sunday could begin wet. You will pack your rain gear, won't you? Also check the security of your anchor. Although winds generally won't be that strong (with the exception of the Strait)...any thundershower can produce focused, strong winds...even if briefly. Showers are likely to diminish later in the day. Winds over the Sound will shift to the southwest at 5 to 15 knots. From Port Gardner/Everett north, south winds will increase to 10 to 20 knots. Westerly winds will continue to rip through the Strait at 15 to 30 knots, strongest in the central and eastern Strait. Along the coast, winds will remain fairly light from the southwest at 5 to at most 15 knots.
Wear sunscreen (especially Saturday), be safe, have fun...