When somebody asks me the forecast and I want to give them a (good-spirited) sarcastic response, I’ll usually give them a forecast of “showers and sunbreaks.” I’ll say something like “we’ll see various types of weather tomorrow, with some locations seeing mainly sun, other locations seeing mainly rain, and others seeing a combination of showers and sunbreaks.” To which the other person usually replies, “you’re probably the worst weatherman that ever existed.” And then we both laugh and I tell them the actual forecast.
But in spring, the best forecast often is “showers and sunbreaks.” During spring, the upper and middle levels of the atmosphere are still very cold, but the increasing amount of solar radiation is absorbed by the surface, warming both the surface and the lower atmosphere. This creates an “unstable” atmosphere, with a sharp decrease in temperature with height. This type of temperature profile is unstable because warm air is less dense than cold air, so it will tend to rise or convect, forming clouds and precipitation. One of the indices used to measure the amount of instability in the atmosphere is called Convective Available Potential Energy, or CAPE. CAPE values are generally pretty low here in the Pacific Northwest due to the relatively cool & dry air at the surface, but over the Great Plains, which see the combination of warm southerly winds from the Gulf of Mexico and much colder, westerly or northwesterly winds aloft, enormous amounts of CAPE can build up, leading to a ton of convection and violent thunderstorms with strong winds, damaging hail, and in some instances, violent tornadoes.
Today didn’t feature tornadoes, but we saw hail, lightning, and extremely heavy rain! There were several showers that came through my neighborhood today, but one from around 3:30 to 4 was the heaviest and featured both hail and lightning.
Crazy hail in the Hollywood District of Portland around 3:45 this afternoon! #orwx pic.twitter.com/aumfSPxJsx
— Charlie Phillips (@GeoduckChuck) March 8, 2021
The “showers and sunbreaks” pattern was very visible on satellite as well!
Awesome satellite imagery showing the instability and showers/sunbreaks we saw today. #orwx #wawx pic.twitter.com/X3NVSK0ybo
— Charlie Phillips (@GeoduckChuck) March 8, 2021
The Monday morning commute should be dry, but more “showers and sunbreaks” will arrive Monday afternoon and intensify Monday evening as an upper-level-low moves towards Southern Oregon. These showers will persist through most of Tuesday before dying off on Wednesday, though I can’t rule out a stray shower Wednesday afternoon. Unlike today, I am not expecting any thundershowers for Portland Monday or Tuesday, as instability will be less and the heaviest precipitation/showers will be over Southern Oregon and Northern California. However, any showers could still be quite sudden and heavy, so don’t get fooled by any sunbreaks! I’ve gotten soaked during these “showers and sunbreaks” patterns far more than I care to admit… and it’s especially embarrassing considering that it is my job to predict these things!
Thursday and Friday should be quite nice as a ridge of high pressure builds over the area. A front arrives Friday night/Saturday, bringing periods of rain and snow above 2,500 – 3,000 feet. On Sunday, we’ll experience yet another round of post-frontal showers and sunbreaks. But whether they will be accompanied by hail and lightning remains to be seen.
Have a great week!
Charlie
