How unusual has the recent dry period been?

I have received a number of anxious emails from people worried about the relatively dry period of the past month in the Pacific Northwest.

Are such dry conditions unprecedented?

Is late spring becoming 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 from May 1 to June 5 for SeaTac Airport and have also added a trendline of best fit.

You will note that our recent period has been arid, but not the driest to date.

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

Now check the trend line. There is NO indication that the period from May 1 to June 5 has become drier. In fact, the long-term trend is towards wetter conditions. This suggests that increasing global warming is not the source of our lack of rain.

What about the other side of the Cascades? Consider the long-term observation site at Kennewick in the Tri-Cities.

Same story with Seattle. We’ve been through a dry period, but it’s happened many times before and the trend is wetter in late spring.

According to records, the recent period is the 10th dry spell on record in Kenniwick.

The bottom line is we’ve had a dry period, but such aridity in late spring is not unusual and does not tend to lead to drier conditions in May and early June. . There is 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 over the past month, but let’s take a look at the NOAA Climate Prediction Center’s historic rainfall anomalies from April to normal. June during El Nino years (below). A very mixed bag on Washington State. Wetter than usual in the Southwest.

Now if I blog about the lack of rainfall, you KNOW what will happen in the future.

Here is the latest total rainfall forecast through Friday at 5pm. Lots of rain (many due to thunderstorms) east of the Cascades, perhaps a few unusual showers west of Washington. Abundant rainfall in northern California.