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December 2014
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Three Slices of Pineapple

Posted: December 8th, 2014

 

Pineapples. Pineapples everywhere.

Pineapples. Pineapples everywhere.

 

 

A long, meandering atmospheric river is poised to give the west coast a wet and windy week. The stream of subtropical moisture is expected to lash the northwest three times as successive low pressure systems swing through the region.

 

While forecasts have not suggested particularly intense rainfall outside of the Olympics and North Cascades, the past few weeks have been wet everywhere and some impacts to SPU’s urban drainage and water supply systems can be expected. And there is also a fair amount of uncertainty, so the next few forecast models runs must be watched closely.

 

The first frontal system attached to a big storm up in the Aleutians will feature two waves, first late tonight, then midday tomorrow. After a quiet evening, the next round arrives early Wednesday morning when a warm front passes through. The most intense rainfall of the week, as currently forecast, will be associated with a sweeping cold front late on Wednesday. At the moment its orientation should spare the City and Cascades, but as we witnessed on Thanksgiving, orientation is everything.

 

The final pulse is expected to occur on Thursday. At the moment, forecast models develop a significant windstorm and take it through Washington. Depending on the track and strength of the system it may also bring a brief reprise of the same atmospheric river and extra rainfall with it.

 

Once we get through these three systems, the weekend is looking mostly dry, if not even nice. Longer range outlooks are also showing splitting fronts broad low pressure troughs, i.e. relatively warm and dry conditions or signs of El Niño.

 


 

The latest National Weather Service forecast for Seattle shows 0.47″ tonight, 0.78″ tomorrow, 0.78″ Wednesday, 0.34″ Thursday; then dry conditions Friday through Sunday. The latest UW WRF-GFS has 0.85″ through tomorrow afternoon, and 0.91″ on Wednesday. The ECMWF is slightly drier overall, particularly in the long range.

 

Over SPU’s mountain reservoirs, this afternoon’s NWS forecast shows 3-4″ tonight through Thursday with more falling over the Tolt than the Cedar. High snow levels should drop below 5000′ on Wednesday, then below 4000′ by Friday.

 

– JRH

 

NWS Area Forecast Discussion
NWS Forecast Table Interface—Seattle
NWS Forecast Table Interface—Tolt Reservoir
NWS Forecast Table Interface—Chester Morse Lake