Atmospheric Rivers and Rainshadows

In past blogs I have talked about atmospheric rivers---plumes of moisture approaching our region that generally start in the tropic and subtropics. These moisture rivers are generally associated with warm air (since warm air can contain more water vapor than cold air) and they can dump huge amounts of precipitation when they interact with the substantial terrain of our region.

You know the most famous of these atmospheric rivers well...the Pineapple Express, which begins somewhere near Hawaii.

Not all atmospheric rivers approach us from the southwest, and our region is about to be influenced by one from a more westerly direction. The direction of the flow is very important. A nice video produced by NOAA is found below:



Here is an image showing you the total water vapor in a column predicted for 8 PM on Wednesday. A nice plume of moisture heading to us from the west-southwest. Not the equivalent of the real primo atmospheric rivers....but good enough to supply plenty of moisture.

As we will see the direction of the flow will be important.

In many a blog I have talked about local rainshadows and their brethren, the windward precipitation enhancement. Flow approaching a barrier (particularly moist flow) will produce copious precipitation, while descending air dries rapidly. Normally, our major precipitation events and primo atmospheric rivers are associated with southwesterly or south-southwesterly flow and the rainshadow is over Sequim and Port Townsend, home to many retirees and golfers. But these folks don't always get the shadow...when coastal winds turn more westerly the rainshadow shifts southward over Puget Sound and the lavender farms NE of the Olympics get wet.

That is what will happen tomorrow. Here is the 24h rainfall predicted for the period ending 5 AM on Thursday. The rainshadow will move southward and north Puget Sound will be virtually rain free, while the Cascades and the Olympics get pounded with as much as 5-10 inches of rain. Rivers will rise rapidly and some flooding is possible (see the NWS web site for flood watches and warnings). There is also a substantial avalanche threat with all the snow we have had lately gets hit by heavy rain and warm temperatures. Sequim looks soggy too!

Here is the 72-h rainfall ending 5 AM on Friday. The north Cascades will really get drenched with almost the whole area predicted to get 5-10 inches. Add to this all the snow that will melt implies very serious flooding potential.

So if you want to stay dry tomorrow...head to southern Snohomish County or the northern Kitsap....or drive over to Vantage on the Columbia River....