(Sherlock Holmes and the Olympic tree fall will return on Sunday---and the answer may be in hand!)
With high pressure offshore, northerly winds have developed in the lower atmosphere, as seen in the time-height plot above Sea-Tac airport (below, red is temperature, heights in pressure--850 is about 5000 ft, time increasing to the left). Over the last 24 h, temperature at 850 hPa (again about 5000 ft) has dropped from 3C to -4C.
The cool air will continue to spread southward today and, as shown in the plot below, will reach the Oregon border by 7 AM (1500 UTC) tomorrow morning. Note that there is a large pressure change (gradient) in northern CA associated with the leading edge of the cool air. This makes sense, since cooler air is more dense than warm air, thus a gradient in temperature produces a pressure gradient.
Expect frosty temperatures on Saturday AM with cold air aloft and clearing skies (which allows good radiational cooling to space).
On Sunday morning an upper level trough will approach our region, bringing clouds and some precipitation. I would be talking about the potential for lowland snow, except the trough is going too far offshore and south of western Washington....not quite the right set up for Puget Sound.
The 24-h precipitation total ending 4 PM Sunday shows plenty of precipitation along the coast, but little over Puget Sound (due to rain shadowing from the Olympics and Mountains of Vancouver Island under NW flow).
On Sunday morning an upper level trough will approach our region, bringing clouds and some precipitation. I would be talking about the potential for lowland snow, except the trough is going too far offshore and south of western Washington....not quite the right set up for Puget Sound.
The 24-h precipitation total ending 4 PM Sunday shows plenty of precipitation along the coast, but little over Puget Sound (due to rain shadowing from the Olympics and Mountains of Vancouver Island under NW flow).
The forecast snow total for the same period is disappointing. Not much on the coast because it is too warm there and nothing over Puget Sound. Only over far NW Washington and southern BC, will there be sufficiently low temps and enough moisture to get a dusting. And not much good for the mountains.
During the past few days and this weekend, we have had an area of high pressure offshore, with lower pressure inland, similar to the typical La Nina pattern. But only similar... strong La Nina's have the high pressure farther offshore. Here is the typical 500 hPa (upper level) height anomalies (difference from normal heights) for La Nina years, with red indicating above normal heights (pressures), and blue/purple the opposite.
Compare that pattern with the forecast pattern for 4 AM Sunday (below). Similar, but the features are displaced eastward. And it looks like La Nina's days are numbered....the latest model forecasts show a transition to neutral (or La Nada) conditions by summer.
In any case, there should be lots of sun on Saturday.
(Again, the third and final blog on the Olympic Mountain mystery tree fall will be released on Sunday....)
