After an early start of winter across much of the Great Lakes region in November, weather conditions generally turned milder and drier than normal in December. During early December, the configuration of the jet stream across North America transitioned to a progressive, split-flow pattern.
A northern branch running along the U.S./Canada border and the second through the southern USA supported periodic west to east passages of troughing features across the country and to a storm track generally to our north and west (leaving most of Michigan on the relatively warmer side of the passing storms).
This pattern limited the number and duration of arctic-origin air masses passing through the Great Lakes region and led to several prolonged periods of milder-than-normal temperatures. Mean observed temperatures for December generally ranged from 1 to 5 degrees above normal.
With Michigan geographically between the two branches of the jet stream, precipitation totals fell back to below-normal levels in most areas of the state, ranging from less than 1 inch across western sections of the state to more than 2 inches across east central sections of the Lower Peninsula.
Given the relatively mild conditions, seasonal snowfall totals through the end of December are now running below normal in most areas, with greatest negative departures from normal across western sections of Lower Michigan. An exception to this pattern is western Upper Michigan, where snowfall totals are running slightly above normal.
Despite the recent drier weather, long-term moisture indices continue to be strongly positive statewide and across much of the Upper Midwest due to heavy rainfall during the late summer and fall seasons. The latest Palmer Drought Severity Index categorizes all of Michigan with wetter-than-normal conditions, ranging from “unusually moist” or “very moist” over most sections of the state to “extremely moist” across the central and northeastern Lower Peninsula.
Looking ahead, the most important issue is how long the recent upper air pattern (mentioned above) will persist. Latest medium-range guidance suggests that it may continue into mid-January, but there are hints of a change after that time that would at least temporarily result in colder-than-normal temperatures across central and eastern North America.
In the central and eastern equatorial Pacific region, warmer-than-normal sea surface temperatures continue to signify weak El Nino conditions, and while this still plays a role in outlooks through the upcoming spring season, the El Nino influence is generally less than in previous outlooks.
The latest NOAA Climate Prediction Center outlooks for January and for the January through March both call for milder-than-normal mean temperatures across northwestern sections of the state and for equal odds below-, near-, and above-normal mean temperatures elsewhere (earlier outlooks had called for milder-than-normal mean temperatures statewide).
Near-normal precipitation totals are forecast for both time frames across far northern sections of the state with below-normal totals across central and southern sections.