The dominant analogue winters for December were a good predictor for this December in all temperature projections and two out of three snowfalls calling for cold and snowy. Saginaw;'s snowfall came up lower than the higher December snowfall winters but still appreciably above the average.
What is most striking is how close the temperature averages from the analogues for December came to the actual December 2017 numbers, at all three sites using the colder winter averages added to all the winter averages and dividing by two. I used both numbers; the average of all the analogue Decembers and add that to DOMINANT trend (below normal or coldest Decembers) to get a fair projection for the actual month (see below) which is reflective of the Winter Outlook.
Detroit's temperature analogue data contained a whopping 10 out of 14 below normal Decembers. The average for those 10 winter's came in at 25.6. Even the December temperature average for all 14 winter's came in notably below normal at 27.9 /norm 30.1/. Adding the averages of the coldest December winters and then all the Decembers gives an average of the two /25.6 + 27.9/ of 26.7. The actual temperature average for December 2017 ironically fell between the two average data projections with the actual 26.6 - just one tenth lower than the average of the two. I'd say the December projection for temperatures was very telling and fits nicely into the other dominant 10 winters (71% of the analogue sample)!
Snowfall projections for December at Detroit were also telling with the dominant analogue December snowfalls having an average of the 7 winters at 18.7". The extremes for December snowfall due to the storm track establishing itself was problematic in the outlook from the start with the range being from 4.3" to 25.1"! However, the pattern of dominant snowfalls (7 out of 14; 50% and 1 December being in the normal range and 6 below normal) did revel itself in the study as likely a snowy December. Actually out of the 7 snowy Decembers, there were 4 where that snowfall averaged around 20" or more...a fairly strong showing in itself and right up there with the 22.5" recorded this past December. That 4 out 8 very snowy winters showed that when there was a snowy December; there was a 50% chance that it would be at least more than double the normal amount /9.7/.
All in all; stepping back to all the dominant; prevailing Decembers in the study for Detroit showed an average temperature of 25.6 degrees with 18.7" of snow...and this past December verified the prevailing pattern.
Likewise; Flint's average for the all the sample analogue December temperatures came in at 25.4; 1.6 degree below the 27.4 normal. Using the dominant 7 (out of 12) coldest Decembers at Flint gave a projection of 21.6; this compares to Flint's actual December 2017 average of 22.6. Then again; adding both the 25.4 for all of Decembers analogues to the coldest December analogues at 21.6; gives an average of 23.5 Flint's actual temperature came in between the two at 22.6.
Flint's actual heavy snowfall in December followed right along with the heaviest snowfalls in the study. Using the snowiest but not dominant snowfall trends in the analogues finds: the largest snowfalls projected 28.5" out of three very snowy Decembers and well above the normal of 11.8". Flints projected snowfalls in the analogues were much more variable than even Detroit's in December; partly due to the variable placement of the storm track that sets up. It was noted on the Winter Outlook analogue chart that Flint's snowfall for 1910 -11 was highly suspect since both Detroit and Saginaw measured considerable higher amounts (20.1" and 17.4" respectively) as compared to Flints 11.5" so it was not used. Flint's December 2017 snowfall totaled at 23.5" more than double the norm /11.8"/ and almost double the analogue average at 12.8". There is a huge gap between the analogue average and analogue snowiest Decembers in Flint; ranging from 12.8" to 28.5". Looking at the average between the two (12.8" to 28.5") gives 20.6" and not far off the actual total. Even wider were the analogue December snowfalls all together: from 4.4" to 35.3"! Predicting the December 2017 snowfall for Flint was difficult to say the least but still landed in the heaviest snowfall categories like Detroit.
Up at Saginaw gives the following results; the average December temperature for the analogues came in at 25.5; while the average for the below normal or coldest Decembers came in at 21.6. Using the average of the two numbers leads us to the exact number Saginaw recorded in December 2017 - 23.5! Snowfall at Saginaw was the only lacker in the group. The actual snowfall for December did come in above normal with 13.2" /norm 9.0"/ and close to all the December analogue snowfall average at 14.7". But the snowiest Decembers average number was quite a bit higher at 22.1" This was highly skewed by two Decembers with around 40" not unlike Flint.
The overall precipitation thus far this winter has been lacking however with about half of the normal (1.49", -1.33") falling through January 6th. In Detroit's study there were 3 drier than normal winters, 3 wetter than normal and a dominant 8 normal precipitation winters. If we are to have a normal precipitation winter in alignment with the prevailing winters in the study, we're gonna have to step up the precipitation - and recent models indicate as much. While on the subject of future weather; latest indications are that January will make up some of that below normal departure with more moderate to even milder weather at times between long cold spells. This short thaw ongoing is more than welcome for most after temperatures averaged 20 degrees below normal at around 6 at Detroit through the 6th. The first week or so is reflected in the departure map below...
Next Up: Winter Storm Brewing - But for Who?
After Mid Month - Part Two: A Mid Winter Update - What Do the Best Performing Analogue Winters Thus Far Suggest About the Rest of the Winter and Cold Season?
Making weather fun while we all learn,
Bill Deedler - SEMI_WeatherHistorian
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