Some folks think of eastern Washington as a dry desert with lots of sun. For the warm portion of the year that is generally true, but during midwinter (roughly November to February), much of eastern Washington is cloudy and cool, with the region from Yakima though the Tri-Cities to Spokane stuck in a persistent murk of low clouds and near-freezing temperatures.
In fact, much of eastern Washington was stuck in this unpleasant situation for much of the last two weeks, with a welcome break occurring last night. To illustrate, here are the temperatures at Pasco and Yakima for the past 2 weeks. Murkville. Very little daily temperature variation --temperatures were stuck around 30F forever.
You can look at the observations at Pasco for the 24-h ending last night to see how this episode ended. For much of yesterday the surface temperatures were near freezing and fog, freezing rain, rain and ice pellets were observed. Warming was occurring aloft but it wasn't until late in the day that strong southerly flow mixed down to the surface and temperatures soared in the 50s.
So what is going on? Much of eastern WA is a topographic bowl, with higher terrain (Cascades, Okanagan highlands, Rockies, Blue Mts, plateau of eastern WA surrounding it. During the winter, cool air tends to settle into this bowl, producing what is known as a cold pool. This cold pool can be quite shallow....a few hundred meters to a kilometer in depth--- with an inversion capping it. Inversions are very stable and it is hard for air to mix down. The cool air becomes saturated and thus fog and low clouds are often present.
Let me show you how shallow the cold air can get by presenting the temperature data from a 400 ft high meteorological tower at Hanford, WA around 9 AM last night. 22F warmer at 400 ft than near the surface!
Here is a satellite picture from January 3--you can see the low clouds, which were banking up on the eastern side of the Cascades. This often happens when there is lower pressure over western WA compared to the east.
What causes the strong winds that helps mix out the cool pool? Generally, this is associated with passage of a vigorous front or low pressure center after a period dominated by high pressure.