WeatherWatcher: Thunderstorms

• Thunderstorms
• Convergence zone explained
• Shift to a cooler weather pattern

Thunderstorms

Tuesday and Wednesday we have had a couple of lightning flashes and thunderclaps, weather we are not very used to anymore after such a mild winter.

On Tuesday we had the more consistent kind of thunderstorm. For the first time in, I believe, the entire 2014-2015 season, we had a strong, solid and long-lasting convergence band form over Snohomish county and slowly drift into northern King county before dissipating after nightfall. As the band moved through it sparked off a few lightning strikes. This is fairly common in a strong convergence band. Hail was reported with some of the heavier precipitation today, and yesterday as well. The bulk of the accumulating hail missed our area for the most part, and got Seattle proper a lot more.

Wednesday was more of a hit and miss showery unstable air type of thunderstorm. These are short lived and move out of the area fairly quickly.

Convergence zone explained

Convergence bands form usually in a west-east line between the Cascade and Olympic mountain ranges. Sometimes they are slightly southeast, but typically oriented in a thin east-west band.

Typically after a cold front from a storm passes, a westerly flow develops, which is then split by the Olympic mountains, directing winds to flow around the mountain range on the north and south sides. On the opposite side of the Olympics, the winds flow back to the middle, guided by the boundary of the Cascade Mountains. This is where the winds out of the north, and the winds out of the south collide. This is called convergence.

The converging winds laden with moisture are forced upward at this point, and as the air rises, the temperatures drop within that air mass causing condensation. Keep that going and you get large towering cumulonimbus clouds oriented in an east west band. Clouds tall and heavy start to produce precipitation. Heavy precipitation mixed with upward winds causes friction which induces a static charge, thus lightning.

This convergence band's favorite spot to develop east of the Olympics is usually between Marysville and Sea-Tac. The Shoreline and Lake Forest Park area are actually in the very center of that range, and we typically get it more often than any of the other areas inbetween.  Many of our snow events were actually the result of a narrow convergence band. Most famous of all convergence storms is the December 18, 1990 storm that dumped 12 inches of snow in about 30 minutes, complete with thunder snow.

On a typical winter we usually get persistent convergence bands lasting a good 12-15 hours after a cold front passes, however never strong enough to generate thunderstorms. In the springtime it is more common, only because the difference in temperature from the surface to the upper atmosphere is much larger. So the higher dew points of the air create much larger clouds, for much larger precipitation.

Shift to a cooler weather pattern

The photo below was taken at the Snoqualmie pass ski area at the beginning of March. We've had normal precipitation this year, however almost all of it has fallen as rain in the mountains. That is about to change.

A snowless Snoqualmie Summit, March 5, 2015.
Photo by Troy Wolford

A change to a cooler, near normal weather pattern is happening right now, so we will for once, have near normal temperatures. Even the mountains are getting some snow now. We are now looking at high temperatures in the upper 40's and very low 50's with lows near 40. Showery weather is expected to last into the weekend. Next week right now is looking cool and wet as well.

Longer range, I think it's still possible through April to see a brief snow accumulation of a couple of inches out of a convergence band. I have seen it happen here before. April 18, 2008 is a good example of a convergence zone snow. Chances of this kind of event happening this year are slim, but I think more likely than the past two winters.

Snow on April 18, 2008, Shoreline, Washington.
Photo by Carl Dinse

For current weather conditions check out my weather station at http://www.shorelineweather.com