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Dave Epstein | Weather

Keep your AC unit out — summer isn’t over yet

The warmest Septembers ever recorded have been overwhelmingly in the past 20 years.NOAA Data

As we close out the first week of September on a beautiful note, you might be thinking about pulling out your air conditioners and wondering if there’s much more summer weather left. I am confident we have more heat and humidity coming this week and if September 2020 is anything like the Septembers of the past couple of decades, it’s going to be warm. Bottom line, better leave those AC’s in the window for now.

September around here used to be more of a transitional month into fall. It often would start on a warm note, but then turn fairly cool. Increasingly, the month is more like a fourth month of summer with many days well into the 70s and 80s. The nights are also a lot warmer than 30 or 40 years ago.


Since 1872, if you look at the top 20 Septembers you will find 10 of them in the past two decades. That’s pretty amazing when you consider how long records have been kept. The fact that so many of the warmest Septembers are the recent ones really does show how the climate has changed.

This week will continue the summer weather with heat and humidity above average right through Thursday. We might see a couple of showers by that time as a cold front gets closer. Depending on your tolerance for heat and humidity, you might need to put the air conditioners on as early as Tuesday afternoon but I think many of us will need them Wednesday night to sleep with the high dew points and temperature staying in the 60s.

Dew points will be back in the oppressive range Thursday afternoon.Tropical Tidbits

Our warmth, while above average, is certainly not the extremes that they’ve seen out west. San Francisco reached 100 degrees yesterday, and Denver reached 101 on Saturday, both establishing new heat records. Eastern Los Angeles County reached a whopping 121 on Sunday, the highest temperature ever recorded in that county. As extreme as that was, the coming cold to parts of the West on the heels of such heat is even more exceptional.


The coming cold to parts of the West on the heels of such heat is pretty exceptional.NWS

Across the Rockies, drops of 30 or 40 degrees in 24 hours are not uncommon, but readings near 100 followed by cold and snow are almost unprecedented in such a short period of time. Back in 1962, Denver reached 100 degrees on Aug. 14 and then it snowed on Sept. 8, 25 days later. If indeed measurable snow occurs on Tuesday, that short three-day gap between such extremes would be unparalleled.

Tuesday afternoon will feature some record cold out west with widespread 90s in the east.WeatherBell

There’s more extreme weather going on in the tropics where Tropical Storm Paulette has just been given a name, and we are likely soon to get Tropical Storm Renee from Tropical Depression 18. This will only leave four more letters to be used before we have to start using the Greek alphabet in order to name tropical systems and we haven’t even hit the peak of the season yet.

Two areas of tropical weather will be tracked by the hurricane center over the coming days.NHC/NOAA

Since August, each new storm that gets named is the earliest that we’ve ever used that letter. Most meteorologists thought it would be an active season, but I’m not sure anyone realized it would be quite this active. Doing some critical thinking about what this might all mean, some the storms that have been named probably wouldn’t have been named a few decades ago. What I find very interesting is that all the energy added up from this unprecedented list of storms in 2020 is only slightly more than average. Further, if you look at the northern hemisphere the amount of accumulated cyclone energy is actually significantly below average for this point in the season. So having all these named storms is interesting, but might not be all that significant.


Northern Hemisphere tropical cyclone activity for 2020Courtesy Dave Epstein

One of the reasons I enjoy studying meteorology so much is that it’s never boring. Whether it’s our changing September climate or short-term weather extremes, there is certainly a lot going on this month.

Follow Dave Epstein on Twitter @growingwisdom.