The jet stream flow across North America changed dramatically during mid-October, with the transition of the highly amplified western troughing/eastern ridging pattern of the past few weeks to a western ridging/ eastern troughing pattern. With northwest- erly flow across Michigan and the Great Lakes region, the change led to much cold- er temperatures and to at least a temporary end to the active, wet weather pattern of late September and early October.
Mean temperatures for the mid-Septem- ber through mid-October reporting period varied widely across the state and closely reflect the jet stream pattern that persisted for much of the same period, ranging from more than 5oF below normal across western sections of Upper Michigan to 4oF above normal in the far southern Lower Peninsula.
The rapid onset of colder weather result- ed in some of the first accumulating snow- fall of the season across northern sections of the state, although totals were generally light. The weather change also led to the first freezing temperatures of the season in many central and southern sections of the state on the 16th and 18th of October.
Total precipitation for the reporting period ranged from less than 2” across sections
of the southeastern Lower Peninsula to more than 6” across western and northern sections of Lower Michigan and almost all of Upper Michigan with more than 10” at a few spots in the Upper Peninsula. The heavy rainfall led to extended fall harvest and fieldwork delays in many areas. This weather scenario was also common across large sections of the Corn Belt region to our west and south.
In view of the recent stretch of challeng- ing weather conditions, and assuming that the upper air pattern with the recent jet stream change continues into late October, there is good and not so good news. The good news is that while occasional frontal passages will bring the threat of rain and snow, it should remain on the light side, with below normal totals in most areas.
On the less positive side, temperatures will likely remain at below normal levels, which will slow drying of crops and soils. Still, conditions for fieldwork should gradu- ally improve over the next couple of weeks. Medium range forecast guidance suggests a very fall-like pattern into early November, with some moderation in temperatures.
NOAA’s Climate Prediction Center calls for a return of warmer-than-normal mean temperatures for the late fall and early win- ter time frame, with no forecast direction on precipitation totals. In the equatorial Pacific, sea surface temperatures have warmed in recent weeks, suggesting the development of an El Nino event during within the next 1-2 months. As a result, outlooks for the middle and late winter call for milder than normal mean temperatures and for normal to below normal precipitation totals.
Andresen is a professor of Meteorology/Climatology with Michigan State University’s Dept. of Geography, Environment, and Spatial Sciences; MSUE specialist and the state climatologist for Michigan.