Significant Heat Wave To Impact Pacific Northwest This Week

7am Sunday

As you’ve probably heard by now, the Pacific Northwest will experience a significant, prolonged heat wave this week. Excessive Heat Watches are currently in effect for the Willamette Valley, Northeast Oregon, and nearly all of Central/Eastern Washington, and the hot weather is expected to persist throughout the week before a very gradual cooling trend occurs over the weekend and next week.

Before I delve into the details below, here are a few main points:

1. This will be a prolonged event. Portland will likely be above 95 degrees Monday through Friday and has a good chance of seeing multiple days near 100 degrees. Seattle should be cooler due to the influence of Puget Sound, but should still be in the upper 80s/lower 90s from Tuesday – Friday.
2. This heatwave will be centered over the Inland Northwest. The hotter spots in the Columbia Basin, such as the Tri-Cities and Hermiston, may see multiple 110+ degree days towards the end of the week and could even touch 115 degrees.
3. Very little overnight relief is expected, particularly Tuesday night through Friday morning. Current forecasts have Portland remaining in the upper 70s by midnight Tue/Wed/Thu. This is due to the prolonged nature of the heatwave, as well as increasing (but not oppressive) humidity throughout the week.
4. Flow will be weakly onshore for the entire heat wave. This will help put a lid on high temperatures west of the Cascades and will mitigate fire danger. However, as mentioned above, it will also result in higher humidity and warmer overnight lows.
5. If you want to escape the heat, go to the coast! Due to onshore flow, the beaches will be in the 60s to low 70s throughout this event.

Upper-Level Pattern

We currently have a large, relatively “flat” ridge building in the Northeast Pacific. In this context, “flat” means that the ridge doesn’t significantly perturb the direction of the jet stream, and it contrasts with an “amplified” ridge, which is typically stronger and has a much greater influence on the location and strength of the jet.

Over the next 24 hours, this ridge will amplify considerably and approach the Pacific Northwest.

Over the workweek, the ridge will very slowly move inland and stall over the Inland Northwest. Western WA/OR should have nearly steady temperatures over the week, while the Inland Northwest will see gradually increasing temperatures that will peak late this week.

After Friday, models show a very gradual cooling trend and return to average summertime conditions by the first week of August, with a few morning clouds, plenty of afternoon sunshine, and highs in the mid/upper 70s for the Seattle area. As the “spaghetti” plot below shows, there’s still quite a bit of spread in the model solutions after August 3rd, but such uncertainty is the norm for forecasts more than 10 days out.

Surface Pattern

The heat wave will begin in earnest on Monday when this ridge is firmly over the region. However, the hottest weather is expected to be east of the Cascades for the entire event, and because hotter air is less dense and is associated lower surface pressure, flow is expected to be weakly onshore the entire event.

The chart below shows the temperature, pressure, and wind forecast at 5pm Monday from the University of Washington WRF model. Note how the brunt of the heat wave is over the Inland Northwest and Western Washington/Oregon are experiencing light-to-moderate onshore flow.

By 5pm Wednesday, the heat wave has strengthened and moved further north, but flow is still clearly onshore throughout Western Washington and Western Oregon.

Due to this onshore flow, temperatures are “only” expected to top out in the low 90s for Seattle and near 100 for Portland. If flow was instead offshore, Portland could easily see 105 degrees in this setup – but climbing much above 100-102 will be difficult with any sort of onshore influence.

The chart below shows the ECMWF wind and temperature forecast at 6pm Wednesday, with the shaded areas representing the strength of winds (purple is weaker, green is stronger), the streamlines showing the direction of winds, and the labels showing the modeled temperatures at certain cities. This model is overdoing the cooling effect right along the coast, but it still shows how much of an impact onshore flow has for cooling coastal communities.

Many of these communities, particularly those west of the Olympic Mountains and Oregon Coast Range, experience strong downslope warming during offshore flow. For example, Tillamook, Oregon reached 95 degrees during our first heat wave of the year on 6/25 and 6/26, setting record highs both days. Because this heat wave will feature weak onshore flow, Tillamook will struggle to exceed 80 degrees, and places along the immediate coastline that are exposed to onshore flow will be in the 70s or even the mid-60s. So if you want to escape the heat, head to the coast.

Humidity and Warm Overnight Lows

Although onshore flow off the Pacific helps moderate temperatures, it also increases dewpoints. Because summertime water temperatures along the West Coast are usually in the 60s well offshore and the 50s immediately off the coast, we do not see the same sort of oppressive humidity that other parts of the country see in the summer. However, dewpoints can still rise into the low 60s during heat waves with typical onshore flow, and if the flow has a more southerly component or is associated with an atmospheric river, dewpoints can reach the upper 60s or even exceed 70 in exceptional cases.

With persistent heat and weak onshore flow, dewpoints should gradually rise from the upper 50s on Monday to the low/mid 60s Thursday, with a slow decrease back to the mid-50s (average) next week. The below chart shows the dewpoints predicted Thursday evening… note the widespread dewpoints in the low 60s across Puget Sound and the Willamette Valley, while dewpoints east of the Cascade crest are just slightly above freezing.

Water vapor is an extremely potent greenhouse gas and reduces the amount of heat radiated to space. As a result, Western Washington/Oregon – and the Willamette Valley in particular – will see very little overnight relief this week, particularly towards the end of the week. Downtown Portland will still likely be in the upper 70s by midnight on Tuesday, Wednesday, and Thursday.

As an energy trader, I’m curious to see how overnight power demand compares to the “Heat Dome” last year. I’m sure it will be lower since temperatures are much cooler this time around, but more people also have AC than they did during last year’s event, and AC usage tends to increase the longer a heat event goes on as infrastructure heats up and people get worn down by the continuing heat. So it’ll be an interesting comparison.

That’s all I have for now. Stay cool and stay hydrated this week. For those east of the Cascades, I highly recommend reading some of Mark Ingalls’ recent posts and his Tri Cities Weather Facebook Page. The Tri-Cities could touch 115 by the end of this week, and he is all over this event to prepare people for the dangerous heat ahead.

Charlie