Stepping into September: the ups & downs as we transition from summer to fall

Weather Blog

Now that we’re officially diving into the month of September, I thought it would be fun to look at what’s ahead, some wild weather in parts of the U.S., and explain why we start to notice these subtle changes this time of year. 

It’s getting to be that time of year now where we’re making the gradual transition from summer to fall. How can we tell? Besides the fact that we’ve already entered the month of September, meteorologically speaking we’re already starting to notice hints of the cooler weather as brief rounds of cooler air have been making appearances from the north. Slowly but surely this air is occasionally engulfing the region in what we’ve been calling, “Fall-like” air. Whether you like it or not, fall is coming. For some, it’s a somber sign that summer is over, and for others, it’s a welcoming sign to the cooler weather and changing of the year. 

Here’s where we stand:

As of September 1st we’ve already started meteorological fall, which means climatologically we’re already stepping into a cycle of cooler temperatures during the months of September, October, and November. Astronomical fall begins on September, 22nd this year which means that on that day we’ll be receiving roughly equal amounts of sunlight, and an equal 12 hours of day and night.

MORE on the difference between astronomical and meteorological seasons HERE.

According to the National Weather Service, autumn in Rochester is usually pleasant and at times mild and dry, but it’s not long before even colder air moves in come late October… and well, we all know what that means. Don’t worry, I won’t get into that just yet.

Let’s look at the data from the three most recent Septembers [2017, 2018, 2019]:

Data from NWS

The main takeaways here, are that in all three years temperatures were able to reach 90 degrees if not close as a high, and get as cool as the 40s.

Just for fun, here are the record high and low temperatures that have been seen in the month of September:

 HIGH              98   09/03/1953
 LOW               32   09/30/1991

All in all, we can get some pretty drastic temperature differences in the month of September, which makes it all that more interesting not only to experience, but to forecast for as well.

As you’ve already seen above, heading into fall doesn’t always mean we are guaranteed to see “Fall-like” temperatures every single day. For example, check out the temperatures the European model is throwing out for us here in Rochester:

ECMWF Daily/High Low Temperatures courtesy of WeatherBell

While we’ve been in the 70s now and through tomorrow, more summer-like temperatures are still headed our way. Check out Tuesday through Thursday of next week where we could see temperatures close to 90!

It’s the time of year where not only us here in Rochester, NY can see large jumps in temperature, but those across the entire U.S. are fair game too… some more than others.

So if you think that’s a decent jump, check out Denver, Colorado:

ECMWF Daily/High Low Temperatures courtesy of WeatherBell

All from an intense building ridge in the western U.S., Denver, Colorado will go from temperatures in the 90s, to temperatures in the 30s and 40s… and seeing SNOW! By the way, this is all in a span of less than 24 hours. Could you imagine that? Our folks out west won’t have to, because they’ll be living that reality first hand come next week.

So why is this time of year so prone to such drastic changes in temperatures? Besides the astronomical change in season itself where our northern hemisphere becomes less tilted towards our sun, another big player that’s connected in all of that is the jetstream. 

It’s essentially a relatively narrow band of strong winds that lies way up in the atmosphere where the winds blow from west to east, and the band itself will shift from north to south as it follows boundaries of colder and hotter air.

The jet stream is stronger and more pronounced during the winter because the temperature differences are the greatest. The Polar jet stream is the one where we notice more of as it makes its shift southward across the U.S. As we approach the colder seasons of fall and winter, colder air moves in and the warmer air is pushed towards the south.

Image courtesy:

Once we start to lose that prime, summer-time heating from the sun, that’s when we notice the polar jet stream begin its journey south by giving us quick bursts of cooler weather.

So before you go putting away your shorts and t-shirts, remember that temperatures can heat back up just as quick as they cool down during this transition from summer to fall.

For a look at your local weather forecast at any time to keep up with how temperatures will change, click HERE.

~Meteorologist Christine Gregory

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