Weather Blog

Seattle Thunderstorms 101

Seattle Thunderstorms 101

If you've lived in other parts of the country (Miami, New Orleans, Dallas) then you may be part of the school of thought that thunderstorms are born only from hot, humid summer days.

For the most part, you're absolutely right. Warm, muggy air is certainly a catalyst for a late-day light show in the sky.

So what happened yesterday around here? It was perfectly sunny over Seattle at 9:00 a.m., the skies turned dark and ominous by noon, and we had some convective showers and a few isolated thunderstorms before the evening rush-hour.

But the temperature only barely made it into the 50s when all of this happened. That certainly doesn't qualify as warm!

Here's the deal: It's not the "warm and muggy" air that causes the convection, it's the difference between the temperature of the air at the surface and the atmosphere above us.

In yesterday's (and today's) situation, the temperature of the air at, say, 15,000 feet was much colder than normal. Cold air wants to sink. Really cold air really wants to sink. As the sun warmed up the surface, that air started to rise. 

Even at 50 degrees, the "rising" was enough to initiate convection with the air so cold above us. That convection led to heavy showers and a few thunderstorms.

We'll see a similar scenario play out today with a high of 55 degrees. Thursday, however, will actually be warmer (57 degrees) but without any thunderstorms. Why? Because the atmosphere above us will be much warmer, making the difference between the two smaller.

After spending five years forecasting in Miami, it's still strange to say "55 degrees with a chance of thunderstorms."  But I'm back in the Northwest and back dealing with strong upper-level low pressure and super cold air aloft.

Hey, if you want to check out the outlook for the month of May, we have a webcast posted just for you.