Fall: Where Warm Days and Cold Nights Collide

We’re about halfway into October, and it’s evident that we’ve entered the critical transition time between the heat of summer and the blustery and snow of winter. These maps from the High Plains Regional Climate Center show what the hottest maximum temperature and coldest minimum temperature have been since the beginning of October.

The warmest days have ranged from the 50s and 60s in the Pacific Northwest, northern states, and Rocky Mountains, 70s-80s for most of the Plains and Midwest, and into the 90s and even 100s in the southeast. While minimum temperatures haven’t gotten too cold yet, some areas of the Northern Plains have dipped below freezing, and a lot of the interior Rockies have dipped into the teens and single digits.

Some snow did grace the northern Rockies in September, but for many locations, the first major snow event of the season arrived the second week of October. The snowfall map from the Midwestern Regional Climate Center shows that snowfall extended as far south as southern Colorado. The majority of the snow impacted Montana, Wyoming, and the Dakotas, with some areas receiving over 10 inches of snow!  

As some of you finish digging out of that snow, let’s dig into the climate data and see what’s normal for this time of year. Looking at average temperatures for the next couple of months doesn’t tell the whole story. Consider if you have 3 kids, ages 10, 8, and 3. On average, their age is 7. That average is accurate, but doesn’t accurately describe any of the kids individually. Sometimes looking at average temperature can be useful. But fall can be a volatile time of rapidly transitioning conditions, so we may not be getting the whole story. Instead, let’s look at the number of warm days that typically occur vs. the number of cold days.

The first map shows the number of days where the high temperature is 70°F or warmer for October-November. Depending on how much you enjoy warm temperatures, you could be looking at the southern states and desert southwest with some longing. Go higher in latitude and/or altitude, and the number of 70° days decreases. But across the middle of the country, quite a few 70° days still occur – for many stations between 10 and 20. Let’s switch and look at cold days. These are the number of days where the high temperature doesn’t get above freezing (32°F). This is pretty rare for the October-November time period, but it does happen. Regions across northern Colorado, Nebraska, Iowa, and northern Illinois typically experience a handful of these days in the fall. Further north, many stations observe 5-10 cold days, and near the Canadian border at North Dakota and Minnesota, locations typically have 10-15 days where the high doesn’t get above freezing.

How about cold nights? Here, I’m defining a cold night as a minimum temperature that drops below 20°F. This is a bit more common in fall than the cold days I’ve defined. Most of the country north of I-70 are likely to experience at least one night where the low temperature drops to 20° or colder. For the mountains, it’s pretty common and happens almost half the time in October-November. In the Northern Plains, Upper Midwest, and in New England, these cold nights are observed anywhere between 5 and 20 times in the fall.

Where things get really interesting is where the higher numbers of warm days and cold nights collide. These largely varying conditions can be hard on the environment. It would be more difficult for plants and animals to adjust in this type of climate. The region that stands out the most is the High Plains region along and between the I-80 and I-70 corridors. Here, warm days can still occur in the fall (10-20 70° days on average), but cold days and cold nights are also becoming more frequent (with 1-5 cold days and 1-10 cold nights).

Average Number of Warm Days

Average Number of Cold Days

Average Number of Cold Nights

Let’s shift our focus to snow. The first map below on the left shows the average number of days a location will have 1” or more of snowfall in October-November. This really only happens at higher elevations or in the northern states. Most of the Northern Plains, Upper Midwest, and Northeast will observe 1” or greater snowfall at least once and up to 10 times in the fall.

I also wanted to look at the upper bound of the snowfall distribution for the country. The map on the right below shows the 75th (or top 25th) percentile of October-November snowfall for a location. This number tells you that only 25% of the years would snowfall be higher, and 75% of the years, snowfall accumulations in the fall are lower. Focusing in on Montana and the Dakotas, we see that the 75th percentile of fall snowfall ranges between 6 and 20 inches. Recall that a lot of this region received over 10 inches of snow in early October. For these areas, we can definitely say it’s been an anomalously snowy fall. They’re already in the upper category of snowfall for October-November, and we’re only a few weeks in. Is it a sign of things to come? Only time will tell. But one thing I can tell you for sure – be prepared for anything during the craziness of fall!