A Rather Cool and Unsettled Pattern

Friday, May 27, 2011

10:16 A.M.

First off, as an addition to my last blog post, I thought I'd mention that Seattle gets tornadoes too, but they are very weak and are not part of supercell thunderstorms. Instead, they are called “cold-core” funnel clouds and come from localized areas of circulation within a thunderstorm. I remember seeing one form on the coast near Ocean Shores once (the atmosphere was extremely unstable that day and I nearly got struck by lightning) but it didn’t touch down.

Seattle funnel cloud (photo credit: Bruce Sussman)

South Dakota wedge tornado (photo credit: Weatherpix Stock Images)

Since I gave a huge weather discussion yesterday about all the tornadoes this year, I’ll talk about the forecast today. There is another entry I’ll write about shortly about a weather station that survived a direct hit from a EF-3 tornado, recording gusts over 150 miles per hour, but I’ll save that for some other day.

We have been in a rather cool and unsettled pattern as of late, and the diagram of the current jet stream above shows you why. The jet stream is in a zonal pattern and is very strong for this time of year. Although we aren’t seeing any big storms, we are seeing a lot of unsettled weather due to weak systems being continuously directed towards the Pacific Northwest by this jet. Additionally, you can see how it has its origins at the Aleutian chain and drops down to our area thanks to a long trough of low pressure. This helps shuffle in cooler than average air into our area since the high altitude winds have northerly origins.

Today will start out fairly dry, but showers will increase throughout the day as the sun warms up the atmosphere. These convective showers occur because when the sun heats up the Earth as the day progresses, there is more potential heat energy stored to support the formation of showers, and the atmosphere is destabilized due to larger temperature changes at different levels of the atmosphere. This phenomenon is not just native to the Pacific Northwest; the greatest time for all convective activity is in the afternoon. Tornadoes occur most often from 4 P.M. to 7 P.M., when large supercells tend to explode after the atmosphere is destabilized in the morning.

The following diagrams for the forecast at 11 A.M. today and the forecast at 5 P.M. today nicely illustrate the convective showers that are expected to pop up this afternoon, and the graphic at 2 A.M. shows how many of the showers, with a weak convergence zone being an exception, have died away.

11 A.M.

 4 P.M.

2 A.M. Saturday

There is a possibility that we could see some weak thundershowers today, but nothing major.

Our nicest day will be on Sunday, with partly cloudy conditions and highs returning to the mid 60s. However, wetter weather returns next week, and I am keeping my eye on Monday, where a moist and unstable air mass will move into our area from the south and create a risk of thunderstorms. There is a possibility we could get rather sunny next weekend with highs in the 70s, but that is a long ways off. However, I am forecasting that we will have some additional days in the 70s this summer. You might just have to wait a bit. :)

Charlie