The Transition of Seasons

It seems like every year, around November 1st,
the heavens open up and our stormy season begins. We cool off dramatically, and
by mid-late November, we’ve historically seen our first significant lowland
snowstorms. We are at our stormiest by Thanksgiving, and we slowly calm down from
there, nevertheless remaining pretty stormy right through January. However, by
the time mid-February rolls around, we’ve calmed down significantly, and after
Valentine’s Day, it’s much harder to get snow in the lowlands. All of this begs
the question: why do we have a storm season, and why does it approximately
start and end on these dates?

To understand why we get our storms, you have to understand
the mechanisms that form them. Our storms are midlatitude cyclones, and get
their energy from north-south temperature differences in the Westerlies. Generally, the larger the temperature difference, the stronger the cyclone. As the graphic below shows, mid-latitude cyclones need, among other things, a meridional temperature difference and some wind shear to get started. Once there is a bit of rotation, a cold and warm front begin to appear, with the cold front eventually catching up to the warm front and forming an occluded front before the storm finally dissipates.

The “jet stream”, the band of high-altitude winds in the mid-latitudes, is determined by temperature differences throughout the atmosphere which then result in the pressure gradients that drive the jet stream. The stronger the temperature difference, the stronger the jet stream that feeds energy into the storm. The jet stream is typically at its strongest in the Western Pacific where there is a stark difference in temperatures from Siberia to the north and the warm water of the equatorial Western Pacific to the south. Some of the most intense extratropical storms in the world form in the Northwest Pacific. Thankfully, we don’t have to deal with these, but Alaska does on occasion!

How temperature affects pressure gradients, which in turn affect the strength of the jet stream. The jet stream is the geostrophic wind (parallel to isobars, or lines of constant pressure) around 200-300 mb. Credit: University of Illinois

During the autumn, the northern hemisphere starts to tilt away from the sun, causing the polar regions to cool off dramatically while the subtropical regions are still relatively warm. This is why November is such a stormy month for us; there is a large temperature difference from north-to-south and thus a strong jet stream that is more often than not pointed directly at the Pacific Northwest. Moreover, some of our biggest storms, such as the Columbus Day Storm of 1962, occur when tropical systems from the Western Pacific actually get sucked up into the westerlies and transition to extratropical storms. As the graph below shows, there are still a fair number of cyclones that occur during the months of November and December in the Western Pacific, but very few occur from January to April.

As we go into December and January, the polar regions continue to cool, but the subtropics cool off as well. By late February, the polar regions are starting to warm dramatically while warming is much more modest in the subtropics, leading to a smaller north-south temperature difference and thus a weaker jet stream, with weaker and less frequent storms affecting us here in the Pacific Northwest. This trend continues throughout the summer, with the smallest north-south change in temperature in the northern Hemisphere occurring around late July/early August. After that, the temperature discrepancies start to pick up, and we start to see progressively more and more powerful storms.

Jet stream in the summer and winter. As you can see, the jet stream is stronger and further south during the winter due to increased north-south temperature gradients, giving stronger storms and heavier precipitation to the West Coast. Credit:UCAR MetEd COMET Program

Even though we are relatively calm in the spring, many other places in the midlatitudes are actually wetter in the spring than the winter. For example, Oklahoma City’s dry season is from November to February, with precipitation sharply increasing in the spring and decreasing slightly for the summer months. This is because unlike us, Oklahoma City gets the majority of their precipitation from thunderstorms and squalls. These storms have a tough time forming in winter, but once spring comes around, look out!

While the north/south temperature gradients are not as strong in the spring, this is when the difference in temperature between the surface and aloft is the highest. When there is a large decrease in temperature with height, the atmosphere is very unstable, and there is a ton of potential energy available for thunderstorm formation. Typically, warm, moist air from the Gulf of Mexico comes in at low levels, while hot, dry air from the desert southwest comes in at mid-levels, creating an inversion that initially prevents any clouds from forming. However, this inversion acts as a pressure-cooker of sorts, and as the lower atmosphere heats up during the day, the inversion becomes weaker and weaker until it can no longer contain the hot, moist air at the surface. When the inversion finally buckles and the air starts to rise, it rises incredibly fast and violently and to great heights due to the cold air surrounding it and the fact that it is less dense than the surrounding air. 

Our storm season may be coming to an end, but it won’t be long before we’ll be talking about severe springtime thunderstorms. Although not every place is blessed with fascinating weather all the time, you’d be hard-pressed to find a time when there’s no interesting weather anywhere in the world.

Charlie