Olympics about to be hammered… and more snow for the Cascades

November 18, 2009
3:47 P.M.

Hello everybody! The rain from the approaching front has started falling around the sound, and, as I and every single news station around Seattle has been forecasting, this storm is going to be a wet one. Take a look at the precipitation graphic for the next 24 hours (ending at 4 P.M. tomorrow). There is a large area of 5-10 inch amounts of rain in the Olympics. Thankfully, although this is serious, there have been bigger Pineapple Express events, and the freezing levels with this one will not nearly be as high. Some Pineapple Express events have freezing levels around 11,000 feet. This one is looking like it will boast freezing levels of around 5,000 feet or so, which is great news for both the rivers and skiers alike.

What is exactly causing this pattern for us? The answer, on one aspect, is fairly straightforward; we have a strong jet stream thrusting powerful systems into our area every couple days or so. A more specific reason why we have seen all this stormy weather and rain though has been the fact that a baroclinic band has been stalled over us. Remember the “firehose” reference I made in one of my earlier posts? That is what the baroclinic band is. It is a steep temperature gradient along which precipitation occurs and atmospheric “waves” of varying strength ripple upon. It is these waves that bring us periods of enhanced rainfall, and, to a greater extent, wind.

One thing that sets these baroclinic bands apart from other storm patterns is that they usually remain nearly stationary over the area for long periods of time. While most fronts swing through after 6-12 hours, baroclinic bands associated with Pineapple Express events have been known to stay over the area for 24 to even as much as 48 hours! Combine that with the extraordinarly high freezing levels that are usually associated with Pineapple Expresses (thankfully not as much for this one), the heavy rainfall (since the air is tropical in origin, it is more moist), and the strong winds aloft (which enhance rainfall totals on mountain slopes, and consequently, limit it on leeward slopes due to rain shadowing), stalled baroclinic bands often result in extreme flooding for the area. Thankfully, the case is not so here (although we will see some minor flooding, perhaps even moderate flooding on the Skokomish).

The mountains have a winter storm watch for 1-3 FEET of snow over 4,000 feet, with higher amounts in the northern Cascades. It would seem that Snoqualmie Pass, located at an elevation of 3,000 feet, would only see rain. However, an easterly flow through the passes (remember, since the low pressure is offshore, winds flow to it (east to west in our area across the passes)) will keep temperatures there at or even slightly below freezing, resulting in mainly snow there, maybe occasionally mixed with rain. This will be heavy, wet snow too, which isn’t quite that good for a powder day but is wonderful when trying to build a substantial base early in the season.

Winds will also be high on the coast and north interior. We have high wind warnings there with gusts to 80 on the coast and to 70 on the north interior. Here in Seattle, we have a lesser wind advisory as winds will gust to 45 miles per hour. The wind will come in two phases – the first phase is occuring now and is associated with a ~975 mb low headed up to the Queen Charlottes. The winds from this will die down later tonight at around midnight after peaking in the late evening hours. The winds from the second phase will be associated with a surface low that will track up the Washington coast into northern Vancouver Island. If it was deeper, there would be the potential for some serious winds since it is so much closer, but it doesn’t look to be too deep, and it looks to loose energy rapidly as it heads north along our coastline.

There is a possible storm of interest Sunday as moisture from a decaying front over the north Pacific interacts with a powerful jet stream off of our coast and spins up an intense area of low pressure. However, the low pressure does two things as it approaches us: 1.) It slows down and rapidly decays, and 2.) it heads to our south (weaker pressure gradients and winds are found on the northern side of extratropical storms in the northern hemisphere). In any event, the mountains will get a decent shot of snow with this storm.

Models show some more rain over the area Sunday. After that, we dry out for a couple days! Models show some rain returning after Thanksgiving but there is nothing of note yet.

I’ll keep you posted.

Charlie

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