What happens when you combine an unstable airmass, a mountain range, and daytime heating? You get scattered afternoon/evening thunderstorms that bubble up along the Cascade crest! We saw a ton of those today and they made for some really pretty views looking east from Portland to the Cascades.
Credit: alertwildfire.org
Today, a weak upper-level trough moved south along the Oregon/California coast, and the combination of locally cooler air aloft associated with this trough and well-above-average temps at the surface due to clear skies and ridging offshore created an unstable atmosphere (one where the temperature decreases rapidly with height, which is a favorable condition for air parcels to convect).
Credit: University of Washington
Additionally, because warm air is less dense and tends to rise, the Cascades saw “upslope flow” today along their eastern and western slopes. When this air converged over the Cascade crest, it was forced upward, forming clouds and tapping into this enhanced instability in the process.
Credit: UCAR COMET Program
This atmospheric instability is a type of potential energy known as “convective available potential energy,” or “CAPE.” When a rising air parcel taps into this CAPE, the potential energy is transferred to kinetic energy in the form of powerful updrafts, creating taller clouds, heavier precipitation, and often thunder/lightning. There is a direct relationship between CAPE and updraft speed… the updraft speed (in meters/sec) = (CAPE*2)^(1/2).
The satellite loop above shows the development of thunderheads along the Cascade crest and parts of Central/Eastern OR today. The Medford NWS issued their earliest-ever Red Flag Warning today to highlight the threat of lightning-caused wildfires, and Timberline ski resort had to close operations today due to lightning on the mountain. But while all this mayhem was happening in the mountains and over Eastern OR, take a look at the strong onshore flow and persistent low stratus clouds along the coast. That’s the great thing about the Pacific NW… don’t like the weather where you are? Just drive an hour or two away and you’ll be in a completely different climate. 🙂
Saturday and Sunday look warm and dry with highs in the low 80s and bluebird skies. Highs should drop to the upper 70s on Monday with stronger onshore flow and a few more clouds. But we’ll have a significant pattern change to cooler/wetter weather on Tuesday, and cooler/wetter-than-average conditions should persist through at least next Thursday. There’s a lot of variability in precipitation; today’s 12Z Euro showed a measly 0.1″ over Portland by next Friday, while today’s 18Z GFS had just over an inch. An inch of rain for Portland next week would be an absolute godsend; it’s nowhere near enough to erase our drought conditions, but it would substantially increase fuel moisture for smaller branches/grasses, and with snow levels near 4500-6000 feet next week, we could even see a bit of mid-elevation snowbuild in the mountains. Fingers crossed the rainier GFS solution comes to fruition!
Have a great weekend and enjoy the sunshine!
Charlie