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Purple Rain

While severe weather tears through the Midwest, Seattle enjoys uneventful spring weather

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purple rain

One minute the sun can be shining, the next the sky opens up above and unleashes a heavy downpour in that exact same spot.

Although Seattle’s weather is hardly dull, the changing conditions in the Pacific Northwest can not be compared to the severity of storms in the Midwest. Springtime weather over much of the United States is characterized by electric storms, large hail, and frequent tornadoes.

But why do these conditions occur so infrequently in the Pacific Northwest?

“A simplified, but decent acronym to kind of think of is M.I.S.T: moisture, instability, shear, and a trigger,” assistant professor in atmospheric sciences Alex Anderson-Frey said. “Generally speaking, that's kind of what we're looking for, for any sort of thunderstorm activity.”

Anderson-Frey, who is an expert in severe weather, spent her recent years with the national weather service at the storm prediction center in Oklahoma before arriving at the UW as an assistant professor this fall. M.I.S.T., the acronym she refers to, includes the four main aspects of severe weather forecasting. 

Moisture, the first letter in the acronym, is not hard to come by in the Pacific Northwest. With the Pacific Ocean in close proximity to the urban corridor, any Seattleite will tell you that there is plenty of moisture in the region. The problem with the Pacific Ocean is that its water is relatively cold compared to the surrounding air. In places such as the Southeast — located near the Gulf of Mexico — the water is much warmer, which produces favorable conditions for thunderstorm development.

The second word, instability, is a bit harder to come by in the Pacific Northwest.

“For thunderstorms to develop you need instability,” atmospheric sciences Ph.D. candidate Nick Weber said. “And instability just means, if you were to lift a parcel of air, it's like an imaginary balloon. If you were to let go, would it keep going up or would it come back down? If it would keep going up, then that air column is unstable.”

A favorable setup for instability is warm air near the surface and cold air aloft. Due to the science of warm air rising, if there is heating near the surface and a colder pattern in the upper atmosphere, that air will be forced upward, which creates those big, puffy, thunderstorm clouds.

In Western Washington, the problem with instability is that there are already so many clouds around that it is a challenge to get the sunlight to the surface.

“In Seattle, if there's a chance of thunderstorms, that usually means there's not a whole lot of instability,” Weber said. “What we're really counting on is for the sun to warm up the surface quite a bit. If you look outside [in] the morning and it doesn't look like it's clearing up, you could probably count on not getting thunderstorms that day.”

The third letter in the acronym, shear, is a little bit more complex.

“For really organized, severe thunderstorms, you need what's called shear, which is the change in direction or the intensity of wind speed with height,” Weber said. “You basically want to relocate the downdraft, the raining part of the storm, from the updraft so that it doesn't tear itself apart.”

With Seattle located so close to the ocean, most of the wind is coming from the west, right off the Pacific Ocean, leading to very little shear. The lack of shear is part of the reason that Seattle rarely experiences tornadoes. Even when thunderstorms do form, according to Weber, they are often very short-lived, quickly losing energy.

In the Midwest, though, spring is the peak season for severe weather, with the most tornadoes occurring from April through June and peaking in May. With the wind from the jet stream headed from west to east and warm and moist air being pulled up from the south near the Gulf of Mexico, the shifting wind directions create a lot of shear. 

Lastly, the word trigger is much more common in the Pacific Northwest. A lot of different mechanisms can be defined as triggers, whether it be a cold front, changing wind directions, or a big mountain. The trigger is what allows all the other ingredients in the thunderstorm development to pop. 

“Most importantly, probably in days where we have sort of a marginal chance of thunderstorms, we need some sort of trigger,” Anderson-Frey said. “So that's why we often talk about convergence zones and just sort of triggering mechanisms.”

While in Western Washington, storms aren’t quite as common as those in the Midwest, they are still possible. Most recently, a large thunderstorm passed over Husky Stadium in September during the Huskies’ game against the California Golden Bears.

As for Seattle, while the weather may seem less than thrilling compared to that of the Midwest, there is always a chance for showers and sun breaks.

Reach reporter Anthony Edwards at Twitter: @edwardsanthonyb

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