Local Forecasts

Dry Streaks To End Friday?

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As of 8/2, Portland and Seattle have seen 48 and 49 consecutive days without measurable precipitation, respectively. Such dry spells aren’t unheard of for the summer – Portland’s record for consecutive dry days is 71 and was set in 1967, and Seattle’s is 55 and was set just back in 2017. But this anomalously long stretch of dry weather, coupled with well-above-average temperatures, has resulted in sharply deteriorating drought conditions since mid June. I was walking around Fort Vancouver Friday evening and saw significant “flagging” on many of the cedar trees there due to the drier-than-average conditions.

Western Red Cedar undergoing “flagging.” Cedar flagging peaks in the autumn, but can occur in summer if the summer has been particularly hot/dry. Flagging occurs for 2 primary reasons: to help the tree conserve water, and to allow the tree to shed old limbs and focus on new growth, resulting in faster growth rates. See my blog about cedar flagging for more info.
Image taken 10/15/2015 in the Madrona neighborhood of Seattle, WA.

Both of these dry streaks may come to an end on Friday. It’s not a slam dunk, and any precipitation we will get should be fairly light. But it’s certainly a welcome change from the bone-dry weather of late.

Highlights:  

  • Expect warm/sunny weather Tuesday/Wednesday, but Tuesday in particular will be quite hazy/brown due to smoke from California fires.
  • An upper-level trough will approach the region on Thursday, resulting in stronger onshore flow and cooler weather. A cold front associated with this trough should bring some light precipitation to Western WA and NW Oregon on Friday, giving the first widespread rain to Western Washington and NW Oregon since mid June.
  • A gradual moderating trend will occur over the weekend as this trough moves inland and a ridge begins to build offshore.
  • This ridge will strengthen the following week, bringing continued warmer/drier-than-average weather. Some models show this ridge moving inland and bringing a true heat wave with offshore flow to the Willamette Valley, but most keep it off the coast and give us weak onshore flow.

Current Conditions 

There is currently a broad ridge of high pressure over the Western US and a weak trough of low pressure off the California Coast. This is giving the Pacific Northwest moist SSW flow aloft, allowing monsoonal moisture from the Desert SW and smoke from California fires to move into our area.

Precipitation over the Inland NW from these monsoonal storms has been quite heavy, with several inches falling over portions of the Idaho Rockies. Unfortunately, the thunderstorms were much drier over Central Oregon & the Oregon Cascades, and Red Flag Warnings have been a fixture there since Friday. These strikes resulted in over 10 confirmed fire starts yesterday and today in Oregon and likely far more.

Forecast: 

Tuesday should be the smokiest day thus far of 2021 for Portland as warm, southerly flow continues to direct smoke from California fires northward. Thankfully, most of this smoke will be at the mid and upper levels of the atmosphere and air quality at the surface should still be pretty good. The sky will definitely look more brown than blue though. The thickest smoke should move to the east on Wednesday as flow turns more southwesterly. Both Tuesday and Wednesday will be warm, with highs likely reaching the low 90s both days.

Temperatures will cool and onshore flow will increase on Thursday as an upper-level trough approaches from the WNW. On Friday, this trough will move inland, bringing light precipitation to Western Washington & Western Oregon in the process.

500mb heights at 5am PDT Fri 8/6/2021, showing the upper-level trough moving inland.
Total precip from 5am Thu – 5am Sat. Most precip for Western WA/OR will occur on Friday.
Credit: University of Washington

Because Friday’s system will arrive from the WNW, flow will be nearly perpendicular to the Cascades and they will see very effective orographic enhancement on their windward slopes. Unfortunately, we’ll need far more rain than this to substantially decrease the wildfire threat, but this rain will still temporarily increase fuel moisture before warmer and drier weather returns for the weekend and the following week.

100-hour fuel moisture in NW02
Credit: Northwest Interagency Coordination Center

Models are in good agreement on a ridge building somewhere between the Northeast Pacific and British Columbia the following week, which would give the Pacific NW another extended period of warmer and drier-than-average weather. We’ll have to watch for the exact location of this ridge – if it ends up being further inland, there’s the chance of offshore flow for the Pacific NW, which would result in heightened fire danger and temperatures potentially hitting the low 100s for the Willamette Valley.

Most models (and the ensemble mean) keep this ridge just offshore, which would give us light onshore flow and temperatures closer to the low/mid 90s. Let’s hope this cooler, onshore flow solution comes true.

Have a wonderful week and do some rain dances!
Charlie

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