August 25, 2019

Near 100F in Portland But Low to Mid 80s in Seattle. Why the Difference?

The contrast in weather between Portland and Seattle, only 145 miles apart, will be substantial the next few days, with Portland approaching the century mark, while Seattle only warms into the lower 80sF.

Why does this happen?   To understand, one has to know about a crucial local weather feature, the thermal trough, and the impacts of terrain and land/water contrasts.  And it will also explain why it will get quite breezy in Seattle on Tuesday, with strong winds from the north.

Let's start with the latest forecast from the excellent system:

On Tuesday, Seattle is predicted to get to 80F, while Portland hits 97F.   Quite a contrast.  Byt why?

Let's start by looking at the latest surface air temperature forecast from the UW high-resolution WRF simulations.   At 5 PM Monday, it is warmer in the Willamette Valley  (80s), then Puget Sound (70s)

But then the heater is turned on within the Willamette Valley with temperatures rising into the upper 90s on Tuesday, even above 100F around Medford.   But way cooler over Puget Sound and almost chilly in NW Washington.

The key large scale feature that develops on Tuesday is an upper level ridge building over the region from the Southwest (see 500hPa--about 18,000 ft-- map at 5 PM Tuesday).

This feature is associated with increasing sea level pressure to the east of the Oregon/Washington Cascades and easterly (and thus offshore) flow across the Cascades. 

The sea level pressure map for 5 PM Monday below shows high pressure offshore that is canting into eastern Washington.  Temperature at 2 meters is indicated by the colors, with brown and red being the warmest air. You can see an area of low pressure over northern California that is associated with warm temperatures...this is the thermal trough and is associated with easterly flow descending down he Sierra Nevada and Cascade ranges.  Notice how this feature produces a large north-south pressure difference over the Willamette Valley and southwest Washington, with higher pressure to the north.  This pressure difference will accelerates the air towards the south (northerly flow).

 By Tuesday afternoon, the easterly flow across the Cascades will strength the thermal trough in the Willamette Valley causing the northerly flow to accelerate over western Washington and warming the Puget Sound region. This northerly flow will bring in cooler air from off the Georgia Strait and Strait of Juan de Fuca into western Washington, preventing Puget Sound from heating up like the Willamette Valley.

Expect the winds to get gusty in Puget Sound on Tuesday.  The wind gust forecast for 2 PM on Tuesday shows wind gusts hitting 25-30 knots from Seattle to Tacoma.   Get your sailboat ready!

So expect blustery/warm (low 80s) condition around Puget Sound on should wonderful to experience--while Portland and the Willamette Valley will be toasty. 

This heat wave won't last.  Wednesday will remain warm, but then marine air will push in on Thursday and seasonal temperatures will return.  And there is no expectation of lightning during the next few days, so I would not expect any high-terrain fires to accompany this short heat wave.  But human caused fires could occur in western Oregon if folks are not careful.

Announcement:  I will be giving a talk "The Great Storms of the Pacific Coast" in Ocean Shores at 6:30 PM on September 7th at the Shilo Inn as part of the Coastal Interpretative Center's summer lecture series.  More information is found here:   Shilo Inn is offering special room rates for those wishing to stay overnight, as well as a special buffet.   


  1. We're getting into that time of year, especially in September when the interior valleys west of the mountains are prone to hotter heat waves than in Eastern Washington.

    Off topic, but have you noticed how non existent the monsoon season and hurricane season has been so far. There's not a single thunderstorm in the desert southwest this afternoon. Its eerily odd. Wonder how these seasonal circulations could effect our weather in the fall and early winter.

  2. 48.6F this morning in NW Bellingham. Coolest temp of the month so far and the first time it's been in the 40s since July.

  3. Although fuels are just seasonally dry right now, tomorrow could be interesting as far as wildfires go. Even though the Willamette Valley will be much hotter, 25-30 mph winds with 80 degree temps should have local fire departments on edge. In my experience, if fuels are seasonally dry (normal for summer), wind is a much bigger contributing factor than temperature or humidity in making fires resistant to control.

  4. Where are you seeing 25-30 kts for Tues? NWC says 5-15 for Puget Sound in the coastal waters forecast, and ECMWF 9km says 10-12 kts, gust to 21 (which I interpret as poor confidence interval in this case). We're sailors so pay close attention to wind models. 25-30 is a huge divergence (400%+ force on sail area) from any model I can find, so I'm wondering if there's some other data you have.

  5. What was the setup of the 1981 heatwave which Portland had 107F? I wonder if ANY unofficial temp hit 110?

  6. I'm noticing lots of haze in the low levels in the Willamette Valley today, but I don't know of any fires around. What's going on?


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