Another Cold and Wet Weekend Expected

I ran through some quick climate stats for Portland Int’l Airport, and since Saturday of Memorial Day Weekend (5/23), only 2 of 10 weekends (this includes Memorial Day) have had temperatures at or above-average, while 15 of 20 weekdays have seen at or above-average temperatures. And unfortunately for weekend adventurers, this week will bring more of the same: warm, beautiful early summer weekdays followed by another bout of cooler and wetter-than-average weather for the weekend.

Today was awesome. We had bluebird skies and above-average temperatures with awesome air quality. Take a look at the US Forest Service ‘Air Quality’ webcam looking south from Timberline Lodge to Mt. Jefferson… it doesn’t get much better than that! And with the sunset at 9:02 pm, you can expect some light in the sky by 10:00 pm, and even as late as 11:00 if you have a clear view to the north.

Why so clear? We have a huge ridge of high pressure over the Western US but a strong jet stream over Vancouver Island, and being in the Pacific Northwest, we are just close enough to get the best of both worlds; clear skies due to the large-scale, cloud-killing subsidence associated with high pressure, but light onshore flow due to southwesterly flow aloft as this jet glides over the top of this ridge and onshore pressure gradients at the surface due to the thermal gradient between the cool surface air over the Pacific and hot air (and corresponding lower pressure) over the Inland Northwest.

Forecast:

Summary

• This week will be warmer, calmer, and sunnier-than-average. Tuesday will be the hottest day, temperatures will moderate on Wednesday as onshore flow increases with morning clouds and afternoon sun, and Thursday and Friday should again be much warmer-than-average with mostly sunny skies
• This weekend should – once again – be cooler than average. I think Saturday will be dry while Sunday could see some light rain with temperatures a few degrees cooler. We will NOT see the same type of heavy showers/hail/tornadoes we saw two weekends ago!
• The first half of next week should remain cooler-than-average with a slow moderating trend, and the second half of next week onward looks seasonable with near or slightly above-avg temps (avg this time of year is upper-70s for highs, mid-50s for lows) and dry.
• Inland areas, especially over the Rockies, should see convective precipitation in the afternoons/evenings over the next 10 days that will continue to moisten fuels, improve our short-term drought situation, and lessen fire danger for July. It’s still a bit premature to reduce fire danger forecasts for August and September.

We reached a high of 84 degrees today, 9 degrees above average. But tomorrow will even toastier as ridge over the West strengthens and onshore flow weakens. The Portland NWS has temperatures reaching either side of 90 degrees tomorrow for the Willamette Valley, which seems reasonable given the ridging, warm (but not hot) temperatures aloft, and weak onshore flow. With the sun at its highest angles of the entire year, do yourself a favor and wear sunscreen if you are out and about tomorrow! Your skin will thank you.

Temperatures fall substantially on Wednesday as the ridge over our the Western US weakens and the jet stream over Vancouver Island sags south, but ‘substantial’ is a relative term here; we should still rise close to 80 degrees and continue our theme of warmer-than-average weekdays. Both the Willamette Valley and the Puget Sound lowlands should see morning clouds but afternoon sun, with the Valley seeing sunshine a few hours earlier than the Sound. Ridging rebuilds Thursday and Friday for another round of crystal-clear, sunscreen-worthy skies, with highs in the mid-upper 80s – slightly cooler than Tuesday but still a good 10-15 degrees above-average.

But just like clockwork, our ridge will move east Friday night, making way for a strong onshore push Friday night and a thick blanket of morning clouds Saturday that will stick around through the afternoon for the Willamette Valley and the evening for Puget Sound lowlands. It’ll be close, but I think Saturday will fail to make it to 76 degrees, the average high for 6/27.

But it gets worse. An upper-level trough arriving from the northwest will spread high and mid-level clouds over the Pacific NW Saturday evening, and by Sunday morning, much of Western Washington and Oregon may light rain. This won’t be ‘Juneuary’ weather by any means, and we certainly won’t see any lightning or – god forbid – tornadoes out of this setup, but it may prove trying for those just hoping for their weekend weather to match the weekdays.

Cool, northwesterly flow persists for the first half of next week before we moderate to a more seasonable pattern with morning clouds, afternoon sun, and highs a few degrees above-average. They say summer starts on July 5th; will Sunday the 5th prove to be a scorcher? We’ll see.

Inland Precipitation

Inland areas will see quite a bit of precipitation over the next 10 days. Most of this precipitation will be convective in nature and will fall over Wednesday/Thursday and then again Saturday-Monday, mostly in the afternoon and evening. It’s too early to pin down exact amounts/timing, but the highest amounts look to be over the Rockies.

The recent rains have done wonders for our fire danger, and these will continue to help. When I wrote my fire weather outlook at the end of May, I gave a pretty sobering view of our fire danger for this summer based on low snowpack, long and short-term drought, and above-average ‘fine-fuels’ (grasses and shrubs) due to previous wet winters/springs (notably 2015-2016 and 2016-2017). While our wet June certainly hasn’t put much of a dent in our long-term drought, it has at least ameliorated our short term drought and substantially moistened fuels.

If there’s one forecast I’d love to be wrong about, it’s a threatening fire season forecast. 2020 hasn’t been kind to us so far, but let’s keep our fingers crossed that the fire season won’t be as bad as originally feared.

Charlie

Featured image: Rainy day in Minneapolis on 6/16/2018. Credit: Sharon Mollerus