WeatherWatcher: About last week's thunderstorms

Tuesday, May 9, 2017

A clear line between normal weather and thunder and lightning clouds
Photo by Jan Hansen

On Thursday May 4, the dark side of the force must have heard everyone's "May the 4th be with you" chants. In the photo above you can see the first of several lines of thunderstorms moving in from the south at about 35 mph.

The setup for Thursday started Wednesday morning with a marine air flow bringing dense fog in places, clearing to mostly sunny skies and our highest temperature since September 26, 2016 in at 74.5°F. The marine air brought high dew points, a measure of how much moisture content is in the air.

Thursday brought continued sunny skies into the early afternoon when cold air aloft started making its way in. This, with the high dew points near the surface, sent the CAPE (Convective Available Potential Energy) levels well above normal for the Pacific Northwest, rivaling levels typical of the midwest.

These elevated levels of CAPE caused strong convection over the region in the afternoon. Cumulonimbus clouds (the ones that look like cotton balls) started growing at rapid rates. A normal thunderstorm cloud in this region reaches an average of 20,000 feet in height. Thursday's storms were exceeding the 40,000 ft level, which is very rare for this area.

Over 2,500 cloud to ground lightning strikes were recorded in Western Washington in a 24 hour period.

I always watch dew points as they are a good indicator on a warm sunny day if bad weather is on the horizon. Typically any dew point over 55°F is considered muggy here, and means at least one of the major components for thunderstorm making is present. Dew points are also a good indicator on other weather events, especially in the winter season.

Daily average dew point.
As you can see in the graph above, the dew point soared above 55°F both Wednesday and Thursday last week. Normal dew points typically stay in the 40's.

It was an amazing test of modern forecast models as many of them were predicting this event several days out. 10 years ago this type of system would have gone unnoticed until the afternoon of the event. The intensity of this system, however, was a bit underestimated.

I'd like to point out that though this was a rare event, similar events like this have happened in this region in the past. They are just few and far between.

You can view current dew point temperatures, and other weather conditions live at


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