Pineapple Express

Saturday, December 11, 2010
2:18 P.M.

Wait a minute…
There we go. 🙂
The word is out that we will likely see some of the biggest flooding since January 2009 on some rivers, and January 2009 was a record-breaking event for the Snoqualmie @ Carnation and saw major flooding on many other rivers. Some rivers are certain to flood, like the Skokomish, and others will probably flood also, like the Snoqualmie. Thankfully, the Green River doesn’t look like it will quite reach flood stage, although with qpf (precipitation amount) forecasts going upwards over the last few model runs, we will have to keep a close eye on the situation there. 
We have a lot to talk about, and I’m sorry I couldn’t get to it earlier. I was studying for the ACT, which I took this morning. Anyways, the leading edge of the warm front from this system is over us now and is bringing light to moderate rain to many areas, which will only increase in intensity as the weekend progresses. The radar below shows that a lot of rain has already spread over the area, and it will only get heavier as time goes on.
We also have some snow issues to deal with today. While this will mainly be a rain event, there is some cold air trapped in Eastern Washington and that cold air is causing snow from the passes eastward. Rocky Canyon, east of Cle Elum, is getting moderate snow right now.
Snoqualmie Pass is getting dumped on as well.
This cold air will work its way out as milder air from Hawaii starts to invade the area. Until then, expect heavy snows in and east of the Cascade passes. We could also encounter some sleet and freezing rain as well during the transition.
But the real story here will be the rain. Freezing levels will shoot up all the way to 8,000 feet with this storm, which means pretty much everything falling in the mountains will be rain, save the volcanoes and highest peaks. The new models showed generally 2-3 inches of rain in most parts of the lowlands in the 24 hour period between 4 P.M. tonight and 4 P.M. Sunday. You can see some areas of white in the graphic below, and those areas illustrate where there is the potential for over 10 inches of rain in 24 hours. 
This is a serious flooding event, but it isn’t too out of the ordinary. We usually get a storm like this every year or so, and this one won’t be as serious as the ones of November 2006 and December 2007, which caused catastrophic flooding on many rivers. However, I do expect major flooding on many rivers flowing off of the Cascades and Olympics for several reasons. The forecast models have been trending wetter and commonly underestimate precipitation amounts in Pineapple Express events, which is why I feel like we are going to get more serious flooding than the National Weather Service’s hydrological center is predicting. 
Looking at the forecast model above, which is the 12z GFS, you can see an intense rain shadow northeast of the Olympics. While some areas of the Olympics will likely see a foot of rain in this period, some places 60 miles away might get only .01 inches. That is 1200 times less rain! Just goes to show how pronounced the Olympic rainshadow can be. So if you live north of Seattle, you’ll start to feel the effects of the rainshadow, and if you live by Port Townsend, you’ll probably be wondering what all this fuss about flooding is about. Because the winds in the middle atmosphere are traditionally very strong with Pineapple Expresses, they drop a ton of rain over the mountains via orographic enhancement, and then sink rapidly on the leeward sides, preventing much more precipitation from falling (at least until they hit the Cascades again).
We will have to be careful of urban flooding as well. While we won’t have any convective downpours like we did that one morning last week, we will see a lot of rain in a fairly short period of time, and with all the leaves down, expect storm drains to get clogged up and large puddles to result.
Latest river forecasts can be found HERE: http://water.weather.gov/ahps2/forecasts.php?wfo=sew&view=1,1,1,1,1,1,1,1&toggles=10,7,8,2,9,15,6

Stay dry, and stay safe 🙂
Charlie

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