Remember the annoying cool & gloomy spring weather we had throughout most of March & the first 3 weeks of April?  It is now but a memory.  Our “La Niña Lag” has finally ended, and we are shifting into a warm springtime pattern instead.  The past two days have been particularly glorious with nearly full sunshine and afternoon temps in the mid/upper 70s.  Flowers and trees have finally caught up after a very slow start this spring!  Here is a photo of the lilac in my backyard that I took today:

In fact, the next 7-10 days are slated to feel more like summer than spring:  this is looking to be one of the warmest and most prolonged May patterns the Pacific Northwest has seen in many years.

SUMMARY

1.  Beginning tomorrow Friday, we should see an extended period of very warm to hot weather, as in 7 days or more above 80F in the western valleys of Oregon & SW Washington.
2. As of right now, the heat doesn’t look unprecedented in terms of absolute temperature.  This is more of a “duration” event.
3. Offshore (easterly) winds should begin some time Saturday evening and continue into Monday morning; this could bring very warm temperatures to the Northern Oregon coastline.
4. There is a chance of thunderstorms in our area Monday afternoon and evening as a weak low pressure pulls moisture up from the south/southeast.

A couple days ago, this event looked very similar to the June 2021 “heat dome” that brought historic 110+ temps to the western valleys.  Of course in mid-May it would never get that hot, but the forecast was still showing highs of 95-97 degrees for Sunday and Monday in Portland.  But now models have changed slightly, pushing the heat dome a bit further north into SW Canada and allowing more moisture & instability to undercut us from the south.  It will still get hot (in May, anything above 85 counts as “hot”), but not quite as extreme.

Let’s run through a bunch of upper-level maps, showing the height anomalies at the 500mb level (approximately 18,000 ft. above the surface).  Redder colors = higher heights = warmer atmosphere.  I’m going to use the 12z ECMWF maps from Pivotal Weather this time, as their resolution is somewhat better than Tropical Tidbits.

First, 5am tomorrow morning.  The ridging is rapidly building over us, which will help push our temperatures at the surface into the mid/upper 80s:

Then by Saturday morning, even stronger:

This should still be good for temps near 90-92 at the surface.  There is a deep trough out over the Pacific, which allows the ridge to build like crazy.  But look carefully at Nebraska and South Dakota in the map above?  That little whitish spot is a weak upper-level low in the middle of all the warm anomalies.  Pay close attention on the next two maps.

5am Sunday, Mother’s Day:

The Canadian heat dome looks absolutely insane now; this would definitely be a record heat event to our north.  But that weak Plains low has actually migrated westward into Nevada!  This is very unusual, and will be a player in our weather early next week.  Now 5am Monday:

It’s hard to see the cutoff low here, but it’s centered over southwest Oregon.  This is going to pull in moisture from the south and destabilize the atmosphere on Monday, as shown on the WRF-GFS maps:

We have a widespread region of 800-1200 CAPE over Western Oregon, which is quite impressive for our climate.  If this is correct at 5pm Monday, we would be looking at a strong chance of T-storm activity across much of the Willamette Valley.

Beyond that…temperatures cool slightly, back into the low/mid 80s for a couple of days.  But there is NO “return to normal” in the long range as the ridging remains over the West Coast, albeit in a somewhat weaker form.

The 12z GFS model looks very similar, the only difference being slightly stronger ridging late next week.  That would most likely mean the difference between 80-85 temps and 85-90.

The CPC’s 6-10 day outlook (next Wednesday through Sunday the 21st) agrees with the idea of very warm and dry conditions remaining over our region:

We have never before seen more than 3 days 90+ at PDX during the month of May.  If the ridge rebuilds late next week this could very well happen.  In addition, the record for 80+ is 10 days; there is a very good chance we could be flirting with that number by the end of next weekend.

Two final notes:  one, the very warm airmass Sunday will be coupled with quite a bit of east wind.  If the breeze continues overnight Saturday and Sunday nights, temperatures in the Portland Metro could remain very warm, possibly in the low-mid 60s at night.  It’s pretty unusual to see a 60+ night in May here.

Second:  since the heat this weekend is associated with offshore flow, places to our east will not get the benefits of downslope warming.  So the Columbia Gorge & Columbia Basin will not be exceptionally hot, at least not during the initial stages of the heat wave.  In particular, areas like Bend and Redmond should remain in the low/mid 80s Saturday and Sunday.

Okay…a third note, this as a warning.  Rivers and lakes are still very cold after the chilly spring.  Don’t be foolish and jump in trying to cool off!  These early-season heat waves are notorious for drowning fatalities.  Your ability to swim in 50-degree water is greatly reduced by the shock, and the fast flow from spring runoff makes things more dangerous anyway.  Instead, pack lots of ice water to stay cool and hydrated if you’re going to be outdoors.

Enjoy the heat, and you can safely plant tomatoes & peppers outdoors now!

Karl