Post #1315: COVID-19 trend to 11/5/2021: Probable start of the U.S. winter wave.

Posted on November 6, 2021


Let me get right to it.  I think we haven’t had winter wave of COVID-19 yet because we haven’t had much winter yet.  Certainly not when you compare this year to the same months last year.

And now that’s changing.  Both the weather, and the trend in U.S. cases.  Let me do the U.S. COVID-19 case counts first, then talk about the weather.

But just to set the stage:

Last year, the entire first-world Northern Hemisphere had a winter wave.

This year, in Europe, they’re already starting in on what’s shaping up to be a severe winter wave of COVID-19 European Russia‘s already well into its worst wave ever.  (But Asia is a mixed bag, with only South Korea reporting any significant number of cases.)

Given that, it seems unlikely that the U.S. is going to skate by.  By my calculation, we’re in just about the same situation we were last year.  In terms of disease spread, the greater population immunity now (via vaccination and prior infection) just about exactly offsets the greater infectiousness of the Delta variant compared to last year’s prevalent strain.  And whatever problems Europe has, we’ve got them here as well.  Vaccine hesitancy, fading immunity, resistance to COVID-19 hygiene — it’s not as if those have gone away.

I’m hardly the first person to have noted this, and to have said that this means we’re probably going to have a winter wave (e.g., this thoughtful piece in Forbes).

Really, my only value added here is in pointing out that we’ve had a slight weather delay.  I’ll begin to document that in the final section.

Finally, in terms of advice, I’m back to where I was when it seemed clear that the Delta wave was coming (Post #1185).  Panic early and often, that’s my motto.  If you have business to do, in a public space, you might be well served to get that done soon.  Any shots that you haven’t gotten yet — mindful of the lag between injection and boost in immunity — you might want to go for those now.  And so on.

It’s going to take another couple of weeks to confirm or refute the idea that we’re on a path to a 2021 winter wave.  But as of today, I’d say the odds have shifted in favor of it.

U.S. COVID-19 case trend.

U.S. daily new COVID-19 case counts have now clearly leveled off.

Data source for this and other graphs of new case counts:  Calculated from The New York Times. (2021). Coronavirus (Covid-19) Data in the United States. Retrieved 11/6/2021, from”  The NY Times U.S. tracking page may be found at

That flat U.S. average curve (A) is the result of two separate trends.  Growth in the Mountain and Midwest states (B) is offset by continued declines in the South Atlantic and South Central states (C).

Last week, you might have been able to shrug off that leveling of U.S. case growth.  It might have been a blip.  Now, approaching the two-week mark, it’s a lot clearer.  It surely looks as if we passed an inflection point on the curve around 10/25/2021. 

Here’s how this year stacks up against last year (where the “pandemic year” starts on April 1), for the U.S., and then for the Midwest and Mountain regions.

I realize that any curve-reading borders on mysticism, but to me, adding that little flat spot at the end of the orange curve converts this graph from “maybe we’ll skip a winter wave” to “looks like the winter wave is a bit late this year”.  Take that for what it’s worth.  But from the shape of it, I would now expect the U.S. case counts to begin rising.

Well, how late is it going to be?  Beats me.  I can’t even get my mind around what the right measure would be. Now that the Midwest is on a track parallel to last year, it’s about two weeks behind where it was last year (D).  But if we ask when the upward shift started, it’s closer to two month late (E).

I guess the right perspective is to forget about the past, and ask when we expect to retrace last year’s peaks, going forward.  If that’s the right perspective, then the answer is that we appear to be just two weeks behind (D).  As of today, this year’s Midwest (thick orange line) is running parallel to last year’s Midwest (thin blue line), just two weeks later in the year.

I hope you fully appreciate that this is guesswork.  But it’s the best I have to offer.  If you can find something more likely to be accurate, go with that.

The weather

I’m still waiting for the U.S. National Oceanographic and Atmospheric Administration (NOAA) to put out its official summary of October 2021’s weather.  But even as we wait for the official word, we know more than enough to say that this Fall has been much warmer than last Fall throughout most of the U.S.   And, in particular, throughout the Midwest — the region that led the 2020 U.S. COVID-19 winter wave.

The NOAA maps below show the difference from typical (30-year-average) temperatures by month.

Focusing on the upper Midwest and Mountain regions:

September 2020 was average, while September 2021 was significantly warmer than usual.  The difference appears to be perhaps 6 degrees F warmer this year than last year.

Source:  NOAA.

To put that into perspective, let me look at the average monthly temperature in Des Moines, Iowa.  That should be roughly typical of the upper Midwest.  There the difference in average monthly temperature looks like this:

  • August versus September:  10F
  • September versus October: 12F

Source:  Calculated from

And so, that 6F of additional September warmth in 2021 was the equivalent of around half a month’s worth of Fall cooling.   Compared to the typical year, September 2021 ran about half-a-month late in the upper Midwest, while September 2020 was right on time.  On net, then, temperature-wise, September 2021 ran two weeks late (warmer), compared to last year.

For October, by contrast, the weather turned cold last year.  October 2020 was about 6F colder than normal.  Or, on that same scale, October 2020 ran half-a-month early (colder).

We don’t yet have the NOAA map for October 2021, but by all accounts, October 2021 was warm.  It set numerous daily records for warmth in the eastern half of the U.S.   Here’s a crude map of the temperature anomaly for October 2021, showing that much of the Midwest averaged about 6F warmer than normal.  Or, on that same scale, October 2021 ran about half-a-month late (warmer).

Source:  Washington Post.

On net, then, September and October 2021 were somewhere between half-a-month and a month later (that is, warmer) than the corresponding months in 2020.  (In the U.S. Midwest).

In other words, up to now, the onset of winter is running two to four weeks late in the U.S. Midwest, compared to last year.

And so, if air temperature really is the critical variable for winter spread of COVID-19 (through its influence on indoor relative humidity (Post #894), we ought to expect the 2021 winter wave to run about half-a-month to a month later than the 2020 wave.

Add in a backdrop of continuing declines in case counts in the the South, from the late-summer Delta wave, and things seem to be pretty much on track.  I don’t think we’ve skipped a winter wave this year.  I just think it’s running a few weeks later than last year, owing to the difference in the weather.

Right or wrong, give it another couple of weeks, and we should know one way or the other.