Image credit: Minnesota State Climatology Office.
No below-normal temperatures all month in St. Cloud and the Twin Cities
December 2023 went off the charts, literally, showcasing an extraordinary combination of warmth, wetness, and snowlessness that broke or challenged records in all three categories. The magnitude of combined warmth and wetness in particular may make it the most extreme month on record in Minnesota.
In many regards, December was more like November in Minnesota, with bare ground dominating the landscape across the state for much or all of the month, and temperatures remaining mild virtually every day, with a few bouts of very to extremely warm weather.
The month had been running rather warm through the 21st, with roughly normal precipitation, but a rare holiday heat wave arrived with a rare December soaking (producing rain almost exclusively), and propelled the month to the top of both the temperature and precipitation charts.
Setup
Strong El Niño conditions in the Pacific Ocean have kept frigid winter air masses locked up in central and northern Canada, 1,000 to 2,000 miles to our north. Most days have been much warmer than average, and passing cold fronts have struggled to bring in anything other than seasonally-normal air.
Snow cover
The lack of snow cover boosted temperatures further, because bare ground absorbs sunlight and warms the air above it 10-20 times more effectively than fresh snow. Snow cover during winter is a classical climatic "feedback," because snowy ground keeps temperatures lower, making precipitation more likely to fall as snow, which reinforces or deepens the cooling. A lack of snow cover allows temperatures to rise more readily, making rain more likely, diminishing or limiting the snow cover, and enabling further warming.
The final DNR snow depth maps of the month show the toll the warm December has had on the landscape. Only the far northwest corner of the state had a measurable snowpack (of just 1-4 inches), and over 95% of the state was effectively free of snow cover. In parts northern and northeastern Minnesota, this is the only time on record with no snow cover during this week of the year.
Persistent and sometimes extremely warm conditions
December 2023 was not just warm, it was persistently warm, and it was at times extremely warm. Surges of very warm air around the 7th-8th, the 14th, and in the period around Christmas broke daily records across the state. Some of the low temperatures recorded between the 23rd and the 26th were warmer than average high temperatures in early November.
December temperature statistics for Minnesota's first-order climate stations
| Statistic | Duluth | International Falls | Rochester | St. Cloud | Twin Cities |
|---|---|---|---|---|---|
| # Below-normal days | 1 | 1 | 2 | 0 | 0 |
| Highest (F) | 49 | 52 | 54 | 53 | 55 |
| Highest Low (F) | 35 | 41 | 44 | 38 | 45 |
| # Highs 50+ F | 0 | 1 | 5 | 3 | 5 |
| # Record highs | 2 | 3 | 2 | 4 | 2 |
| # Record-high lows | 2 | 5 | 4 | 5 | 4 |
Typically, when a month ends up warmer, or even much warmer than normal, it's because the warm days are more frequent and/or of greater magnitude than the cool days. In December of 2023, however, the cool days were simply nowhere to be found. In the Twin Cities and St. Cloud, no days (zero) had below-normal temperatures. Rochester, Duluth, and International Falls each had just one or two below-normal days, each cooler than normal by just a degree or two. So this historically warm December also was historically persistent. No other Decembers on record were as reliably mild or warm as this one.
Four out of Minnesota's five "first-order" climate stations had their warmest December on record, with each above normal by large margins, including 11.0 F at Rochester, 12.3F in the Twin Cities, 14.3 F at St. Cloud, and 15.8 F at International Falls. In the Twin Cities, December's heat even topped that of December 1877, which had defended its title effortlessly for over 145 years. The extremely warm December in Duluth (13.0 F warmer than normal) was not able to top December of 1877, when official measurements were taken near the lake shore, which produces a noticeable warming effect during winter months.
Many stations set record high temperatures and record-high low temperatures during the month, particularly during the "Holiday Heat Wave." The highest temperature recorded in the state at any time during the month was 68 degrees F at Canby, on December 8.
Minnesota's combined, aggregated record goes back to 1895, and this December will break the statewide-averaged temperature record from 2015 by several degrees (the average values will be available from NOAA during the second week of January). This is an extraordinary margin. Of course, this statewide period of record does not include December of 1877, which we can safely assume would have been neck-and-neck with December 2023. The 1877-78 El Nino event is considered to be stronger, or perhaps even much stronger than this one, accounting for the difficult-to-beat warmth of that December.
Image credit: Minnesota State Climatology Office.
Record precipitation
With many stations in central Minnesota recording over three inches of precipitation for the month and almost all stations finishing above normal, December of 2023 is also highly likely to be the wettest on record, on a statewide basis. In fact, there do not appear to be any Decembers, even back into the 1870s in the Twin Cities and Duluth, that match 2023's values. Initial data from NOAA put the monthly total at 2.06 inch, edging out the previous statewide-averaged record of 2.05 inches from December of 1968.
No month in history back to 1895 has achieved top rankings for both temperature and precipitation. The closest was August of 1900, which was the wettest and 4th-warmest August on record. December of 2015 had been the warmest and fourth-wettest December on record, but obviously has been bumped down to #2 and #5, respectively.
What caused it?
Although it's never possible to know exactly what caused an important weather or climate event, the fingerprints of both the changing climate and El Niño are all over this extraordinary climatic episode.
Winters in the region have warmed rapidly in recent decades, in direct response to increased greenhouse gas concentrations, which lead to increased wintertime heat retention, and also in response to overall warming of the global climate from those enhanced greenhouse gas concentrations. In Minnesota, a typical winter day is now several degrees warmer than in the middle of the 20th century, and average low temperatures during January, our coldest month, have increased by over 10 degrees F in some areas.
The ongoing warming of winter has given us more warm winter days, fewer cold winter nights, and has made it easier to break records for warmth.
December 2023 also occurred near the peak of a strong El Niño, and the warming of the Pacific Ocean near the equator during El Niño events often makes for very warm conditions during winter throughout much of the US. Other major episodes have caused intense warming; the now #2-ranked December of 2015 occurred during the strongest El Niño since 1950. So remarkable winter warmth is a regular feature of strong El Niño events.
In this case, it is likely the two worked together. The trends towards much warmer winters with more warm air available and less cold air around, sets us up to break records, and the El Niño was what got the job done, by producing additional warming on top of the already-large changes.
January 10, 2024