Sunday, December 6, 2015

Flooding Ahead

With the grounds saturated from prolific precipitation during the past week, we are about to be hit hard by several wet systems, including a potent atmospheric river.

Let's start with the predicted precipitation over the next 72 hr from the UW WRF model.  Virtually all the mountains get 5-10 inches, with some hit by 10-20 inches.  Oregon gets hit particularly hard (in sharp contrast to last year).   Even northern CA gets a piece of it.


The heaviest precipitation over Washington State will occur on Tuesday as a strong atmospheric river heads right into us.  To illustrate, here is a map of the forecast column-integrated wave vapor content (adding all the water vapor in a vertical column of air) at 10 AM on Tuesday.  Red and white are high values.  You can see the plume of moisture coming from north of Hawaii, straight for us.  

As this moisture is forced to rise by our mountains, they will release torrents of rain, something clearly shown by the forecast 24-h precipitation totals for the period ending 10 AM Wednesday.  1-2 inches around Seattle, 5-10 inches in the mountains.


Unfortunately for those interested in skiing,  the air mass will be warm, and rain will fall over most of the high terrain.  Sorry.

Such heavy rain on saturated ground suggests the potential for flooding, and the NW River Forecast Center is predicting widespread flooding over most of the regional rivers west of the Cascade crest (red dots).   So be ready.


The cynical and perceptive among you will be asking....after we messed up on the Thursday windstorm, how can you trust the precipitation forecast?  Good question.

Let's turn to the NWS SREF ensembles to get an understanding of the uncertainty of the forecasts.  Below are a collections of forecasts for Seattle over the next few days for cumulative precipitation.  The black is the average of all the forecasts (the ensemble mean).  Although there is some variability, all are going for substantial precipitation (click on figure to enlarge).


Furthermore, atmospheric rivers are large scale features that are easier to forecast than winds from tight, small, and rapidly moving low centers.

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Don't forget to express your feelings about the proposed termination of KPLU, in the ill-advised sale to UW.  More information here and here.  An excellent source of information is savekplu.org.
 This sale is marked by secrecy and disinformation by PLU and UW administrators.  KPLU can be saved if listeners tell the UW Board of Regents and the PLU administration to back off.

If any of you are interested in attending a strategy meeting for saving KPLU on Sunday, Dec. 6th at 2 PM, please let me know (you can email me for more information--search on "cliff mass email" to my email address)

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