WeatherWatcher: Thunder in the night sky

Photographed from Log Boom Park in Kenmore on July 3, 2008.
Photo by Carl Dinse

It's not uncommon to have a summer night around here to bloom into a thunderstorm in the early morning hours, typically between Midnight and 3am.   They're usually confined to the mountains, and Eastern Washington, but occasionally they drift over the Puget Sound as what happened last night, and could possibly happen tonight.

First I'd like to explain the difference between an on shore flow, and an off shore flow.  On shore flows is when the high pressure is off of the coast of Washington, this causes wind to blow in off the Pacific Ocean.  Typically when this happens we have cooler summer days with morning low clouds or fog, and afternoon sun with temperatures in the upper 60's or lower 70's.  Then there's off shore flow, where the high pressure is east of the coast, or inland, typically east of the Cascade range.  In this case Air flows west from over the Cascade mountains and west out towards the Pacific ocean. 

On our hotter summer days, typically when we reach or exceed high temperatures into the 80's with the humidity at or above 40% and an offshore flow we have that possibility of having a lightning show at night.  This is because as the moist air cools overnight, the air condenses into Cumulus clouds (the little puffy popcorn shaped clouds) then to Cumulonimbus clouds, which start to appear in the shape of the anvil shape that everyone describes as the thunderstorm cloud.   Long story short, these clouds are perfect lightning generators and usually form in a long row, in the case of the Puget Sound region, they typically grow big enough to cover the entire Puget Sound merging as one solid cloud generating lightning throughout the whole area.

In the case of last night, we actually only saw the western edge of a massive thunderstorm cell that covered almost all of Eastern Washington as well.  We had an off shore flow from the south east, which pushed the storm mostly north, to north west as it traveled over us during the 2-3 hour window over night.  By dawn the storm had gone into British Columbia.

By comparison typical Stratocumulus clouds that usually generate the majority of our rain and cloudy days are very low hanging clouds, where the cloud tops don't even clear the elevation of the Cascade or Olympic mountain ranges at times, thus generating rain shadows.  The Cumulonimbus cloud that covered most of the state last night at one time had a cloud base that was above the majority of the Cascade mountain range.  By Midwest standards, this was a small thunderstorm, however for our state, it is very rare that we get one of these systems that covers the majority of the state with lightning.

I often can predict on the day of, if we will have lightening in the evening or overnight.  Thunderstorms are difficult to predict, as any winds too strong can break up the clouds before they develop.  Stay up to date with my twitter feed for day by day lightening predictions for this summer.  Twitter: @SWeatherWatcher

Carl Dinse's Shoreline Weather Station