How Unusual Is the Recent Dry Period?

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From Cliff Mass Weather Blog

Cliff Mass

I have gotten several nervous emails from folks concerned with the relatively dry period of the past month over the Pacific Northwest.

Are such dry conditions unprecedented?

Are late springs getting drier?

Is global warming behind it?  Or El Nino?

I will try to answer the questions below.

Let’s check the numbers, starting with Seattle.  Below I have plotted the total precipitation for May 1 through June 5 for SeaTac Airport and added a best-fit trend line as well.

You will note that our recent period was dry, but not the driest by far.

In fact, a list of the driest May 1-June 5 periods shows that this last month was the 11th driest since the late 1940s.

Now check out the trend line.  There is NO indication that the May 1-June 5 period is getting drier.  In fact, the long-term trend is towards wetter conditions.  This suggests that progressive global warming was not the origin of our lack of rain.

What about the other side of the Cascades?  Let’s consider the long-period observation location at Kennewick in the Tri-Cities.

Similar story to Seattle.  We have had a dry period, but it has happened before many times, and the trend is towards a WETTER late spring.

According to the records, this recent period was the 10th driest on record at Kenniwick.

The bottom line is that we had a dry period, but such dryness in late spring is not that unusual and there is no trend towards drier conditions in May and earlier June.   No suggestion that global warming/climate change is the cause.

What about El Nino?   

El Nino conditions in the tropical Pacific have only really developed during the past month, but let’s check out the NOAA Climate Prediction Center historical precipitation anomaly from normal for April to June during El Nino years (below).  A very mixed bag over Washington State.  Wetter than normal over the Southwest.

Now if I blog about the lack of precipitation,  you KNOW what is going to happen in the future.  

Below is the latest total precipitation forecast through Friday at 5 PM.  Lots of precipitation (many from thunderstorms) east of the Cascade crest, with perhaps a few errant showers getting into western Washington.  Bountiful precipitation in northern California.