Our First “Heat Wave” Late This Week

May 4, 2020

9pm Monday…

May has arrived after the 4th driest April on record in Portland.  Less than 1″ of rain through the entire month!  That’s mid-summer dry.  Some areas west of the Cascades had a bit more…around 1.00 to 1.50″.  Still a dry month.  Warmer than average as well.

What’s ahead?

May in Portland Stats

May is typically the month where we turn the corner, leaving behind the really chilly spring weather.  We also tend to see some “very warm” weather at some point during the month.  In fact each of the past 8 years we’ve hit at least 85 at some point during the month.

May Warmest Temp

Most interesting is that the 89 last year occurred on May 10th.  That would be this coming weekend.  It appears we’ll be very close to that number again this Saturday.

We have our first warm season “heat wave” pattern setting up Thursday through Sunday.  The weather setup for these events is always similar.  Strong upper-level high pressure builds along the West Coast.  Then the sinking/drying of the airmass leads to surface high pressure east of the Cascades.  At the same time easterly wind around 5,000′ develops across the region.  That produces a “lee side trough” in western Oregon & SW Washington.  We also call that a “thermal trough”; an area of low pressure that develops on the “back side” of the Cascades due to the easterly flow.  That leads to a surface easterly wind as well since pressures are lower west of the Cascades.  This all adds up to the warmest possible weather for us anytime between April and September.  In the case of late this week, the atmosphere overhead won’t be crazy warm, so no 90s.   But we’ll be close.

You can see the progression of events on the Canadian model.  Strong surface high pressure building just north of us Wednesday afternoon.


Quick clearing that afternoon after some morning showers…ignore the colors.  Then by Thursday afternoon the “thermal trough” is getting established west of the Cascades, although it’s more of an northeast wind instead of straight easterly at the surface.


Friday is the big warm up, a very sharp trough is sitting right along the coastline.  Strong easterly wind through the Gorge and down the west slopes of the Cascade/Coast ranges


This is the one day the beaches may hit the low-mid 80s, especially north of Lincoln City to the Long Beach peninsula.

7 Day North Coast

The thermal trough is still west of the Cascades by late Saturday afternoon, but easterly wind goes calm.  This is typically the warmest day of the episode in the metro area; early east wind going calm or very light westerly late in the day.


By this time flow is back to onshore along the coast and maybe into the southern Willamette Valley too.  Could be a few degrees cooler from Salem south

How warm could we get Friday and Saturday?   It is possible we make it to 90 degrees. otherwise it’ll be close.

Models are in very good agreement laying down +15 to +17 (celsius) 850mb temperature both afternoons over Salem.  Based on past cases in early-mid May with offshore flow and 100% sunshine, PDX should see a high temp between 86 and 91!  Last year a +16 produced that 89 degree day on the 10th.  So our temperature forecast of 84 & 86 those two days might actually be a bit low.

Data Driven Forecast Highs Next 7 Day Meteogram

Regardless, we have several days of summer weather coming up Thursday through Mother’s Day.  Get your BBQs and kiddie pools ready…

Since it’s been so dry you should be watering everything starting this week, including your lawns.  Dry easterly wind really dries out potted plants quickly.  That’s too bad since it’ll be the third consecutive year we’ve seen very dry weather start early in the season.

Chief Meteorologist Mark Nelsen