Arid Spring

I have been doing a lot of gardening the past week, and I am startled by how dry the soil is.

What is particularly surprising is the aridity of the soil following the wettest winter in Northwest history.  The lack of rain was accompanied by very warm temperatures (April was the warmest on record at several western WA location, which has led to an earlier start of the fire season, with several wildfires west of the Cascades.  The picture below shows a fire in Snohomish County.

So what happened?  Let's start by looking at the percentage of normal precipitation for the past two weeks (below). Western WA and Oregon are very dry, with some locations at less than 5% of normal.

But percentages can be deceiving--here is the departure of observed precipitation from normal for the same period.  Less scary....but you will notice that some areas of western Washington are down a bit over two inches.  Much better situation east of the Cascades and over California/Nevada.

What about the entire spring?  Here are similar figures for the last 60 days.   For the percent of normal precipitation, much of the Northwest is well below normal--some locations 25-50% of normal.

Departure of accumulated precipitation?  2-6 inches in western Washington, 0-2 inches in eastern WA.  

So why have we been so dry and warm here in western Washington?   The key cause is persistent high pressure (ridging ) aloft.    Here is the difference from normal (anomaly) over the past 30 days for upper level  (500 hPa) heights (like pressure).  Above normal heights (yellow colors) over SW Canada and the northern portions of the Northwest).    Such ridging brings sinking air and dry/warm conditions.

This persistent pattern is rapidly evolving and next week an area of low pressure will move towards our coast.  The result will be clouds and rain.     Take a look at the latest 6-10 day precipitation forecast by the Climate Prediction Center.  Above normal precipitation over our region.  I suspect this will end the fire threat for a while.

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KPLU Event!

Meet an astronaut (and me) tomorrow (Saturday, May 14) at the Museum of Flight for a KPLU fundraiser (music at 6 PM) and presentation (7PM).  Information here:

Major Cooling Ahead

We have all gotten used to the warm, sunny weather, which followed the warmest April on record for many Northwest cities.

But it is all going to end this weekend, with cool, showery weather settling in for an extended period.  

Real meteorologists prefer to look at the output of ensemble forecast systems that present the results of many predictions.  Here is the graphic from the North American Ensemble Forecasting System.   Warming through Friday...and then bang... an extended cool off, with increased chances of precipitation and clouds (the yellow boxes show the range of the middle 50% of the ensembles, the "whiskers" indicating the range of the predictions.

The latest 6-10 day predictions of the National Weather Service Climate Prediction Center?  Cool and wet over the region.

The precipitation total for the 72h ending 5 PM on Tuesday?  Wet!

Why the change?   The high pressure area over the West Coast is going to ridge heaven.  For example, at 5 PM Saturday, an area of low pressure is over us, with the ridge pushed inland (at 500 hPa, about 18,000 ft).  Cool.

 But look at Wed. at 11 AM!  Wow.  A strong low off of our coast, with moist southwesterly flow invading the Northwest.

So if you want to enjoy a sun-bathed walk in the park, do it during the next few days.   Thinking about planting some grass or a plant in your garden.  Do it NOW!  You won't have to water much.