“Will it be a harsh or mild winter this year?” The science behind what makes winters extreme, mild, or just average

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Around mid-October to early November a lot of people start to wonder what the next winter will hold, but summing up winter as harsh or mild usually depends on a lot of factors. In some cases, it can completely depend on the person asking. One question that may differ person to person is, “Do you enjoy snow or hate it?” Some people also genuinely like the colder temperatures that come with winter.  

There is actually a scientific way to measure how harsh winter is using factors like temperature, snowfall, and snow depth. It’s called the Accumulated Winter Season Severity Index (AWSSI). It was developed nearly 10 years ago by Barb Mayes Boustead and Steve Hilberg, two meteorologists/climatologists in the Midwest. 

How it works
On a daily basis, different point values are given based on how cold the high and low temperature were, how much snow fell, and how much snow is on the ground. Points are added up each day to give the daily AWSSI. As the season progresses, points continue to add up and the total value is ranked based on percentiles. The percentiles are based on the historical AWSSI values. Since the data for this index begins with the 1950-1951 winter season, there are approximately 70 years worth of data incorporated into the percentiles, and as the season goes on the percentiles follow along with the historical values. For example, a season can sometimes start off extreme, but level out to mild if the rest of the season experiences low enough AWSSI values day to day. (more on these categories below)
Note: Because winter weather still tends to happen sometimes in May, the season ends June 1st. 

Let’s break down the points, percentiles, and categories
Points
The point system is fairly complicated, but can be followed with this table provided in the paper written by Boustead et al. and published by the Journalism of Applied Meteorology and Climatology in 2015. To simplify, more cold and snow=more points. 

Percentiles
As stated above, percentiles are based on historical values. For example, in certain locations, the total AWSSI value that is in 81st percentile on January 1 may not even be in the 20th percentile by February 1 (that would assume that zero points were gained between Jan 1 and Feb 1. In Des Moines that would be unheard of). During an average January, Des Moines’ AWSSI value gains approximately 200 points. Each site has their own percentile ranges depending on the highest and lowest AWSSI values day to day.

Categories
Categories are associated with a range of percentiles. So the lowest AWSSI value recorded up to the 20th percentile is considered “Mild”, while the 81st percentile to the highest AWSSI value on record is deemed “Extreme”. There are 3 middle categories: Moderate (21st to 40th percentile), Average (41st to 60th percentile), and Severe (61st to 80th percentile)
Remember, the percentiles and corresponding categories shift throughout the season, so the highest AWSSI value on January 1 in Des Moines is 550 (extreme), but on February 1, 550 points would fall between the 61st to 80th percentile (severe).

Past Winters in Des Moines
Here’s the fun part: Let’s analyze some of the previous winters in Des Moines starting with the 2020-2021 winter season which was considered Severe by June 1.
Below you’ll see the image comparison. Both images are the same, but the image on the right shows the AWSSI value for the 2020-2021 winter season (866) along with the highest and lowest values and corresponding percentiles for June 1. Now look at the image on the left (brighter colors). Notice how from 01/01 (Jan 1) to 02/01 (Feb 1) the black line (2020-2021 season) is in the yellow or average category. Then look at how the black line makes a big jump into the Extreme category throughout February. Remember the two week look cold snap we had in mid-February? The AWSSI gained 373 points in February thanks to extreme cold, 11.9″ of snow, and large snow depths.

1961-1962 Winter in Des Moines: the most extreme winter (from 1950-2021)
The 1961-1962 winter seems to have started similarly to this past winter in Des Moines. However, rather than the uptick in cold and snow occurring in February like it did this year, December 10, 1961 is when the weather started to turn ugly that year and it didn’t stop for 4 months.
What contributed to such an extreme winter? Based on archived data, Des Moines saw A LOT of cold and snow from December 1961-March 1962.
A few highlights include
Dec 1961: monthly average temperature: 9.2° below average, 23.9″ of snowfall
Jan 1962: monthly average temperature: 9.4° below average, snow depth of 6-13″ all month long
Feb 1962: monthly average temperature: 6.4° below average, 21.3″ of snowfall
Mar 1962: monthly average temperature: 9.6° below average, 11.1″ of snowfall

Last 6 seasons (includes the mildest winter season)
In the past six seasons, Des Moines has experienced 2 mild winters, 2 moderate winters, and 2 severe winters. If you swipe the bar to the left you’ll reveal the final numbers for each of these seasons. The 2016-2017 winter season sticks out, in fact, it finished as the mildest winter on record since 1950. Remember the 70° highs we saw in February of 2017? That was a big contributor to the little point gain.

Limitations
The AWSSI does not include wind or mixed precipitation like freezing rain or sleet.

Try it out!
Have a season you remember as being extreme or mild? Play around with the maps on your own by visiting the Midwestern Regional Climate Center’s website. There are 10 sites in Iowa where the AWSSI is available and more than 100 sites across the United States!

2020-2021 Winter Season across the lower 48 states

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