Hard to believe that September and “meteorological” fall have arrived. Summer 2020 was a surprisingly boring one, as far as climatology goes:  Temperatures were very near normal for the month of June, and just slightly on the warm side for July and August.  Overall it was very slightly warmer than normal…or if you prefer, almost exactly in line with the expected “new normal” for our changing climate.

The first 10-12 days of September are going to be anything but normal, and anything but autumnal.  Indeed, this Sunday-Wednesday might test the limits of our climate’s potential for late-season heat.

 

For nearly a week now, weather models have shown a prolonged “death ridge” or “rex block” in our upper atmospheric pattern.  It’s essentially the longest stretch of hot weather for the season; not just a couple days in the 90s followed by a quick marine surge.  We don’t get one of these every summer; sometimes they have the bad habit of setting up during winter inversion season instead!  Here are the 500mb anomalies in the upper atmosphere, which roughly show us where the warm and cool air are hanging out.

These are yesterday’s 12z maps, for yesterday afternoon. Notice the strong area of “high” anomalies over the West Coast.  Warm air tends to expand upward further, causing the 500mb barometric elevation to bulge upward with it.  That pocket of cool upper air in the central Pacific?  That’s a cutoff low.  It helps to anchor the northern ridge in place for long periods of time. This particular pattern is called a “Rex block,” named after a meteorologist with the same name.

By Saturday afternoon the ridge is slightly stronger but still not budging.  This week we are experiencing “normal” September heat; by that I mean temps in the upper 80s/low 90s under light onshore or northerly flow.  The relatively weak sun angle doesn’t allow for the airmass to heat up much more than that.

 

Monday’s upper level pattern shows a stronger ridge moving further north. We are widely expected to be even hotter in Portland by then, though the ridge itself doesn’t really explain why.  That, ironically, has something to do with the cold anomalies found east of the Rocky Mountain Divide.  Read on to find out the details.

And finally by next Thursday, the ridge begins to weaken, possibly marking the beginning of the end of our warm-hot pattern:

 

Now let’s look at the Tropical Tidbits maps for the 850mb airmass.  First for today Thursday

Just a slightly hotter version of the last two days, maybe 93 or 94 late this afternoon for PDX?  It also looks closer to an offshore pattern. Heat should back off slightly Friday-Saturday before (likely) resurging Sunday.

This is when things start to get interesting. Cool air east of the Rockies will drag polar high pressure down with it. When this happens in early fall, it often pushes the hot air back toward the West Coast, causing scorching east winds and Santa Anas.  This is the pattern that might threaten our September temperature records.

By Tuesday afternoon the 00z GFS is showing slightly cooler air slipping into the Columbia Basin, forcing more hot air out over Western Oregon and the coastline.  This is one of those days where Astoria and Tillamook could be in the 90s.

This also would mean very hot temps for the Willamette Valley, likely 95-100 degrees for two or three days. This map ,called the “operational, ” is actually a little hotter than the ensemble average. I don’t think we’re actually going to see 102-105 in Salem and Eugene!

 

To get a better picture of the lower levels and the surface wind, here is a map from the WRF-GFS models.  Yesterday’s 12z shows the hot air and easterly flow (thermal trough) over us Monday afternoon:

 

Now for the caveat.  There has actually been some disagreement in the models concerning the fine details Sunday-Tuesday.  Some maps show less chilly air to the east, which would mean a ‘flatter’ ridge and less heat for us.  Other models are pushing in the opposite direction, bringing more cool air into the Columbia Basin. That would still mean a hot coast, but less heat and more wind for Portland and the Columbia River Gorge.

Same goes for last night’s 00z ECMWF model.  Notice the strong and cooler east wind sweeping over NW Oregon/ SW Washington Monday:

Even if modified Canadian autumn air despoils our record heat, it will still be very dry, warm and windy on the west side. That means extreme, California-style fire danger.  (Suggestion: skip the campfires this holiday weekend!)

Finally, one last image of the WeatherTogether ensembles for yesterday’s 12z. Notice disagreement beginning Sunday and especially Tuesday; this is likely a signal of ridge retrogression and a stronger, cooler east wind:

So….it’s a dry and warm early September pattern, but the models are still deliberating over the first half of next calendar week.  If the east wind is nonexistent we stay in the 90-95 range.  If a moderate east wind, then hotter with possible records. If the cold air comes too close, then less heat but more wind.  Confused yet???

One way or another, it appears this pattern will be over by the end of next week.  We don’t know if it will return to mid-September norms (70s), or if a serious fall chill is in the cards.

In the meantime stay cool and hydrated, avoid unmasked crowds of recreations, and refrain from setting any fires.  We don’t want our skies clogged with smoke and falling ash, like September 2017!

-Karl