Extreme Temperature Diary- Sunday August 30th, 2019/ Main Topic: U.S. Average Temperature Fall Forecast

Sunday August 30th… Dear Diary. The main purpose of this ongoing blog will be to track United States extreme or record temperatures related to climate change. Any reports I see of ETs will be listed below the main topic of the day. I’ll refer to extreme or record temperatures as ETs (not extraterrestrials).😉

Main Topic: U.S. Average Temperature Fall Forecast

Welcome to boreal summer everyone on Tuesday, or the three months that are September through November. Meteorologically and on climate summaries fall is considered to start on September 1st instead of when the equinox occurs on the 22nd. So how will boreal Fall 2020 stack up compared to long term temperature averages across the United States? Will we continue to see the climate crisis signature of warmer than average conditions? Let’s try to make a forecast as usual at the start of a new season.

So how did the forecast work out for Summer 2020? Here is a link to the post for that forecast:

By September 7th the National Center for Environmental Information will finish their assessment for Summer 2020, so our verification is not complete as of August 30th. Let’s do fill in ranking numbers with 1 being the coldest and 126 warmest for a verification for June and July, which have already been assessed:

I’ll complete the Summer 2020 verification for this post around September 7th when NCEI processes their numbers (As promised I have done so.):

Here was my bottom line forecast for Summer 2020:

“I think that this summer will be ranked slightly cooler that of the last four. Carbon pollution is definitely making below average seasons more rare. I’m going to guess that the Summer 2020 ranking will be around 90 + or – 10,  with near average confidence given all of the factors in this post.” 

This forecast was probably a bust looking at records and average temperatures, especially from August, which were very hot. We had two long heat waves over Summer 2010, one in July, and one in the West, which I named “Beelzebub.”

Summer is the most important season for temperature forecasting because heat during this time can turn deadly, particularly in this day and age of global warming. It’s important to give local authorities as much lead time as possible to make preparations for deadly heat that also, more often than not, is accompanied by drought for agricultural interests. Granted though, the state of long range forecasting for regional areas is not great, so only general forecasts can be made. And we had a lot of anomalously high heat over the course of Summer 2020.

It’s time for me to make an attempt at another seasonal forecast for temperatures for the lower 48 states. Here is my two cents for a broad, rough forecast for the U.S. for Fall 2020, which I guarantee to be cooler than this past summer, of course, as many get heat relief as the amount of daylight decreases in the Northern Hemisphere. First, I like to look at water temperature anomalies surrounding North America just before the start of a season to get a sense of how much potential added heat can be added to the atmosphere across the continent. Here is what we see:

Around most of North America we do see above average sea surface temperatures, which is one factor that would lead me to believe that Fall 2020 will see above average temperatures.

In the Pacific we have a developing La Nina, so ENSO should be a factor for some chillier than average conditions in the U.S. this fall. During stronger La Nina’s the northern polar jet is more active, bringing cold, usually dry, air masses southward from Canada.

Second, I like to look at the strength of the Hudson Bay low or polar vortex at the start of any season:

For late August we have fairly strong northern branch systems moving through Canada and the northern U.S. that initially in September will lead to colder than average conditions in about half of the U.S. along the northern tier of states. A solid polar vortex or Hudson Bay low, has not formed yet. This factor should produce cooler than average conditions for most of September, but we won’t know if it will be a very cold factor until October if the parade of strong northern branch systems persist.

Here is the National Weather Service forecast for Fall 2020:

Unlike in my previous seasonal forecasting posts here I have some disagreements. I think that the NWS forecast is way too warm across the northern tier of states, but about right across the rest of the country.

Overall, Fall 2020 will probably verify above average looking at trends from the rest of the planet, but only slightly so.

Last, we can get another clue looking at prior ranking and temperature record count data. For this I like to drag out that “Record Scoreboard” (updated through 6/28/2019):

Here we see clues that Summer 2020 started out relatively hot in June, and just got hotter as the summer progressed..

All three months were above average or “red” with many record reports coming into the National Center for Environmental Information system, especially in August. 

I expect a slight cooling trend through September since usually for the last ten years we don’t see back to back months of +10 or more ratios of daily record maxes to daily record minimums. September could be the coolest of the three fall months, relative average temperature wise. September may be the only month of fall below a ranking of more than 62.5. (avg. rankings per year for the lower 48 states since 1895):


Not all seasons in the near future will see above average temperatures, but seasonal forecasters are beginning to”chuck it,” discounting colder than average scenarios due to carbon pollution.

Again, here are all seasons ranked for the last decade:

Here is my bottom line forecast for Fall 2020:

“I think that this fall will be ranked slightly warmer that that of the last two. Carbon pollution is definitely making below average seasons more rare. I’m going to guess that the Fall 2020 ranking will be around 90 + or – 10,  with near average confidence given all of the factors in this post.” 

Notice that the past five falls had a ranking at or above 70, but that of 2018 was the coldest relative to averages. The big El Nino of 2015/16 really spiked global temperatures as well as those for the U.S. From Fall 2018 through most of 2019 U.S. averages have gotten considerably lower than the torrid levels of 2016 up until the torrid Summer 2020. Will increased global heat once again be a catalyst, helping to produce another “heat spell” or two during fall for the United States, causing my relatively cool forecast of a ranking below 100 to bust? We will see.

As of 2020 the top ranking for any month or season would be 126 since climatological rankings for the United States started in the year 1895.  I think that this fall will be ranked at least as high as 90. Carbon pollution is definitely making below average seasons more rare. As stated, I’m going to guess that the Fall of 2020 ranking will be around 90 + or – 10,  but with average confidence given all of the factors in this post.

We will see how this forecast pans out around December 7th.

Here is more climate and weather news from Sunday:

(As usual, this will be a fluid post in which more information gets added during the day as it crosses my radar, crediting all who have put it on-line. Items will be archived on this site for posterity. In most instances click on the pictures of each tweet to see each article. The most noteworthy items will be listed first.)

Here are more notes from Sunday on the western fires and the heatwave, which I’ve dubbed Beelzebub:

Now here are some of today’s articles and notes on the horrid COVID-19 pandemic:

(If you like these posts and my work please contribute via the PayPal widget, which has recently been added to this site. Thanks in advance for any support.) 

Guy Walton “The Climate Guy”

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