I’m committing one of the cardinal sins of weather blogging here: putting a high-impact, but uncertain, forecast in the title of a weather blog. I truly believe that this is an unethical practice, and that those who do it are trading a little part of their journalistic soul in exchange for a few more clicks.
But at this same time, this isn’t just a weather blog. This is my meteorological diary, the digital tapestry of my atmospheric ruminations. So let’s just see where this blog goes this evening.
One of the truly beautiful things about the weather – during these days in particular – is that it is one of the few inevitabilities in a time where the future seems so uncertain. At the risk of sounding like those blowhard Republicans who sidestep global warming concerns under the guise of ‘the climate is always changing,’ I actually do find solace in the fact that our weather is predictably unpredictable, and that each day presents a familiar new challenge for a weather forecaster such as myself.
My brother hates it when I pontificate and I’m sure you’re pretty sick of it too by now, so let’s cut to the chase and get straight to the forecast. I don’t want to be like one of those food blog people that feels obligated to share their entire life story before the chocolate chip cookie recipe we all came there for! Ok ok, enough complaining…
Credit: University of Washington
We currently have weak ‘zonal flow’ over the Pacific Northwest, which is where upper-level winds travel in a roughly west-east direction with relatively little displacement to the north or south due to ridging or troughing. But despite the zonal flow over the Pacific Northwest, there is a strong ridge over Southern California and the Desert Southwest that is currently keeping those areas much warmer-than-average. Over the course of the week, this ridge will build northward, and while the primo heat will stay over the Desert Southwest, Portland should rise into the mid-upper 80s Thursday and Friday as the heat spreads north. Eastern Washington and Oregon will be even hotter; Tri Cities will approach the triple digits Friday under bright, sunny skies.
Credit: University of Washington
But the main feature of interest – and my stipulation for writing this blog – arrives Friday night into Saturday. A weak upper-level trough off the coast of California is expected to turn upper-level flow southerly and direct warm, moist air into the area at mid and low levels of the atmosphere while temperatures aloft stay relatively cool. This is a classic thunderstorm pattern for the Pacific Northwest, and while it’s too early to tell if Portland and Seattle will see any flashes of lightning, I have pretty high confidence in some thunderstorms over the Cascade Crest and potentially into Eastern WA/OR.
The below images show the 500mb temperature and “total precipitable water” at 5pm Friday 5/29/2020. At this point, the upper-level trough is off Southern California Coast and is beginning to pump warm, moist southerly flow into the Pacific NW.
The large decrease in temperature with height, or ‘lapse rate,’ coupled with the additional low and mid-level moisture, destabilizes the atmosphere and increases a measure known as “convective available potential energy” (CAPE), which, as the name suggests, is the amount of potential energy an air parcel can tap into via convection. Due to the Pacific Northwest’s relatively mild climate, we rarely see CAPE values exceeding 500 J/kg west of the Cascade Crest, but this pattern can give us values in excess of 1000-2000 J/kg, with the highest values usually over the Cascade Crest.
CAPE alone doesn’t cause storms – remember, it is only potential energy. In order for air parcels to tap into all that juicy CAPE, they need to rise to the level of free convection (LFC), which is the point at which a rising air parcel becomes less dense than its surrounding environment and can freely rise without any additional forcing. During Friday night, the upper-level trough is expected to be far enough south that we’ll have relatively little large-scale lift over the Pacific NW, and even regions that see additional lift due to topographic effects – such as upslope flow on the Oregon Cascades and Blue Mountains – may not see enough vertical forcing to bring air parcels to the LFC. To read more about CAPE, LFC, and a whole bunch of other fun acronyms, see my tutorial on Skew-T charts here.
While Friday should be warm and humid with a chance of thunderstorms late, Saturday will be much cooler and wetter as the upper-level trough approaches Western Washington and Oregon. With cooler, more stable air west of the Cascades, most of the CAPE will likely be east of the crest and Western Washington/Oregon could see plain ol’ rain on Saturday while thunderstorms stay over and east of the Cascades. However, it wouldn’t take a large shift in the location of this upper-level low to direct the area of maximum CAPE up through the I-5 corridor, and if that happens, Western Washington and Oregon could be in for an exciting Saturday night. So consider our Saturday lightning chances slim and don’t count any electrical storms to spice up your weekend if you live west of the Cascades, but don’t be shocked if the forecast changes a bit.
Lightning or no lightning this weekend, next week looks seasonable, with weak onshore flow and a few weak systems clipping the area later in the week as a broad trough forms in the Gulf of Alaska and weak systems rotate around the base of this trough into the Pacific Northwest.
It’s not quite ‘Junuary’ weather, but it’ll be a while before we see temperatures as high as they’ll get later this week!
– Charlie
