When we look at a snow season (October to April) to judge how bad it was overall for winter weather in this period, we use AWSSI or the Accumulated Winter Season Severity Index.
This is how it works according to Midwest Climate Center:
The AWSSI is not limited to meteorological winter (December ‐ February) but is intended to capture winter weather from its earliest occurrence to its last. The winter season begins when the first of any one of the following instances occurs:
First measurable snowfall (>= 0.1 inch)
• Maximum temperature at or below 32°F
• December 1
The winter season ends at the last occurrence of any of the following:
• Last measurable snowfall (>= 0.1 inch)
• Last day with 1 inch of snow on the ground
• Last day with a maximum temperature of 32°F or lower
• February 28/29AWSSI Point Thresholds
Daily scores are calculated based on scores assigned to temperature, snowfall, and snow depth thresholds. The daily scores are accumulated through the winter season, allowing a running total of winter severity in the midst of a season as well as a final, cumulative value characterizing the full season. Accumulations of the temperature and snow components of the index are computed separately and then added together for the total index. This allows comparison of the relative contribution of each to the total score.
The AWSSI has been processed for 52 locations across the continental U.S. to provide a variety of locations in different climate regimes for analysis. The AWSSI is calculated for each season from 1950‐1951 to 2012‐2013. The seasonal data is then subject to quality control, and seasons missing data that would contribute 5% or more of the seasons AWSSI are removed . Averages and standard deviations are calculated for running accumulations of daily temperature and snowscores as well as the total AWSSI. The AWSSI data is gathered every hour throughout the day.
Example of a score page:
Midwest Climate Center has already done the work for the 1950-present period & the result confirm that the Winter of 1977-78 was the most extreme winter of that period for Indianapolis, Fort Wayne, South Bend, Evansville, Indiana & over a large area of the Ohio Valley, Midwest & Upper South to even Dallas & as far to the northeast as Boston, Massachusetts.
In calculating AWSSI for Greater Lafayette the winter seasons that reached the "EXTREME" category since 1950 are (data from Purdue University 1879-1944 & the Purdue Airport 1944-present):
16 WINTERS FOUND TO BE IN "EXTREME" CATEGORY 1950-51 TO 2019-2020
Calculated AWSSI 1879-1949.....these winter season considered "EXTREME in the index (the worst 1879-1949 was 1884-85):
21 WINTERS FOUND TO BE "EXTREME" 1879-80 TO 1949-50
Scattered snow showers & flurries mean coatings/dustings this AM. Cat track & tire track snow. Watch for new slick spots.
Sun/clouds today, wave of snow showers evening-tonight mainly after 7-8 pm. 0.5" snow or less.
Blowing & drifting from south winds to 27 mph Saturday with 27-32 & sun/clouds, then blowing/drifting Saturday night with gusts to 30 mph with lows 24-28, then rising to 26-32. Freezing rain,sleet, snow Sunday AM with some very minor to minor ice/snow accumulation.
Potential is there for 1-3" of snow in the north with ice. 1" or less of snow & ice southward before going to rain. We will monitor.
Impacts to travel expected.
Tweaks to timing are possible, too.
0.40-0.60" liquid is possible.
Rainfall is possible:
Thursday (February 25)
Saturday & Sunday (February 27-28)
The following Tuesday-Thursday (March 2-4)
..........with warmer, windy weather & big melting & thawing.
We will need to monitor flood risk with snow melt & potential of 2-4" total rainfall/liquid February 21-March 11.
March: warmer & much wetter than normal with higher risk of severe weather (including tornadoes).
Bit of lingering effects of SSWE & Polar Vortex & snow pack may lead to a random brief, intense cold flirtation in the first half of March after a strong storm system.
However, thinking is still early start to spring.
April: warmer & wetter than normal with higher than normal risk of severe weather, including tornadoes.
May: early start to summer heat........warmer & wetter than normal (with higher than normal risk of severe weather, including tornadoes) turning warmer & drier than normal with time.
June-July-August: Hotter & drier than norma.
Active tropics & "Ring of Fire" pattern would help ease any drought.
Higher risk of "Ring of Fire" Serial Derecho.
Fall looks much warmer & drier than normal.
Increased potential of double La Nina with traditional La Nina next winter as well. We are near strong La Nina right now.
This moderate to strong La Nina double dip would be similar to pattern 2010-11 & 2011-12 years, as well as 1998-99 & 1999-2000. Had near back to back La Ninas in 1973-74 & 1975-76.
Had a lot of neutral years in the 60s except solid El Nino in 1965-66.
1954-55 & 1955-56 saw solid double La Ninas.
Trend is for La Nina again 2021-22 winter then El Nino evolving for 2022-23.