A Monster Moisture Plume is About to Hit the Pacific Northwest

 The latest satellite imagery is stunning....and concerning.  A huge plume of water vapor, extending thousands of miles to the southwest, is about to hit our region, resulting in massive rainfall and localized flooding.  

Consider the water vapor imagery late this afternoon (below). Green and yellow are the highest values,

Look at a wider view, and you can see this atmospheric river of moisture extends many thousands of miles to the southwest.

We are now close enough in time to the event, and forecast models are similar enough, that we can have great confidence in the forecasts.  

The forecast totals through Wednesday morning are stunning, with over ten inches over the Cascades and coastal mountains. Some locations in SW Washington and NW Oregon will receive 15 inches.

Very concerning.

But it doesn't end there.  The next 24 hours will bring even more, particularly over the southern Cascades.  Major rivers in SW Washington (e.g., the Cowlitz) are going to flood.

Don't take my word for it:  here are the latest predictions from the NOAA NW River Forecast Center:

TEN Rivers are predicted to go into major flood, up from two predicted a few days ago.

Some rivers, like the Collitz, are predicted to approach all-time record levels (black line below).

Stunning.

I am more than surprised that the National Weather Service does not have flood warnings out for drainages in the central and southern Cascades (see below).  They should.

Flooding Ahead

It is now clear that we will have a major precipitation event on Monday and Tuesday, with flooding over vulnerable areas of western Washington, such as the Snoqualmie Valley, on Tuesday and Wednesday.  Later in this event, substantial snowfall is expected in the mountains.

Overall, this will be a very positive event for water resources in our region.  

The NWS has now released a flood watch over much of western Washington, which is good (see below).  Soon they should upgrade this to a warning. 

Models, such as the UW WRF forecast system, are still going for a big event, although the totals have backed off slightly.  Still predicting ten inches or more over large sections of the Cascades and Olympics (see total precipitation through 4 PM Wednesday below). 

Perhaps what is just as impressive is where it is not precipitating.  Look more closely at the predicted totals, and you will see a profound rain shadow northeast of the Olympics.

We are talking about less than a tenth of an inch.  So ONE HUNDRED times more rainfall in the mountains than in Sequim.  Much less rain in north Seattle than at SeaTac.

Back to flooding potential, it is now highly probable for vulnerable locations such as the Snoqualmie Valley.   

The situation near Carnation is stunning, with the river reaching major flood level for THREE days (see below).    You don't see that very often.

Please...if you live in local flood plains, avoid driving through water.  And complete your preparations for flooding by early Monday.

Finally, there is a lot of talk on social media and online about the importance of rain versus snow and snowpack versus reservoir storage.   

A lot of it is not correct.  Some of the AI information is not accurate either.     I will discuss the water situation more in a future blog.