I have a somewhat bad habit of overusing alliteration in my titles and posts, and this post marks the second consecutive post I’ve made a tongue-twisting title revolving around spring. But when you realize how many weather words begin with the letter s… supercell, sleet/snow/slush, all the stratus clouds, and even Sharknados 1-4 (rumor has it there’s a 5th slated for summer ’17), it’s hard not to get a little carried away.
The Pacific Northwest is famous for its seemingly never-ending supply of showers and sunbreaks, and we see more showers and sunbreaks during spring than we do during any other time of the year. The summer is our dry season, and during the autumn and early winter, our precipitation primarily comes in the form of powerful extratropical cyclones and atmospheric rivers. But in the late winter and early spring, when the higher sun angles warm the Earth and the lower atmosphere but not the upper atmosphere, we get a significant proportion of our precipitation from convection. Convection is the process by which warmer (and therefore less dense) air rises, forming convective clouds called cumulus clouds. If these clouds become tall enough, they can grow into cumulonimbus clouds, meaning they are cumulus clouds with precipitation falling from them. Or, as they are more commonly called in our neck of the woods, “showers.”
The past few days have been particularly showery. For a meteorologist, I’m not very adept at checking the radar before I bike to work, and I got absolutely soaked as I rode in Wednesday evening to work the night shift for my wind energy forecasting job in Portland. The showers were lighter and more widely scattered on Thursday evening, leading to a much more pleasant ride into downtown Portland. Alas, I got hit by a heavy shower riding in tonight, but the sun shined through right behind this shower, giving us a stunning double rainbow right outside our office.
The satellite pictures from NASA’s MODIS sensor over the past two days look very similar, with an unequal distribution of showers and sunbreaks over the different regions of the Pacific Northwest. Yesterday’s shot is on the left, and today’s shot is on the right. In particular, you can see how shower activity increases dramatically over land. This is because the daytime sun heats the land up more than the ocean, making the air near the surface less dense and causing it to rise, forming showers in the process. Shower activity is also enhanced over the mountains due to the terrain forcing the onshore flow to rise.
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Showers should continue for Saturday, but Sunday should actually feature a nice break from the unsettled weather as a ridge of high pressure builds over the area, giving us our warmest day of the year! Highs should reach the upper 60s in the Portland metro area, and there is a good chance some people will touch 70, especially if you are further south.
Our unsettled weather returns for next week as a large upper-level low spins a series of disturbances into our area. Enjoy Sunday’s warmth while it’s there, because I don’t us flirting with the 70s any time next week.
Credit: USDA/NRCS
On a positive note… all these cool showers have done wonders for our mountain snowpack. Nearly everywhere west of the Continental Divide has above-average snowpack. Some places in the Sierra Nevada have almost 300% of normal! I’m hoping to do some spring skiing up at Alpental when I have time… I have no doubt they will have enough snow to stay open for Cinco de Mayo this year.
Enjoy your weekend!
Charlie