Upper trough stays locked in over the Great Lakes

Posted at 5:18 PM, Oct 04, 2014
and last updated 2014-10-04 17:18:29-04

WEST MICHIGAN — In a post last week I mentioned how our pattern would change and be dominated by sharply colder temperatures, showers, and general cloudiness and unsettled weather. A week later this pattern change that occurred will remain locked in place at least through Tuesday.

The attached snapshot is from the GFS (Global Forecast System) computer forecast model valid for Tuesday morning October 7. It’s about 18,000 feet above the surface (500 mb level for you pilots) and gives us a good overview of the trough/ridge pattern. As I’ve mentioned in many other posts, when there is a ridge over the region (where the jet stream pushes to the north) the weather is usually tranquil, uneventful, and sunny. A trough means the jet stream is overhead or dipping south, which allows all the cold air to spill in from Canada. Weather in these areas are characterized by cooler-than-normal temperatures, clouds, precipitation, and generally unsettled.

Note the huge upper level low pressure system to our north over Canada. Michigan will remain under its influence the next several days with a term we call cyclonic flow. That is…the flow around these lows is the opposite the direction in which the clock moves. Cyclonic flow promotes rising air, clouds, and precipitation. In addition to the cyclonic flow, small waves or spokes of energy will pivot around the main upper level low from time to time. This will enhance precipitation chances, especially Saturday evening/early overnight, Sunday evening/Monday morning, and again on Tuesday.

The trough and associated upper low should gradually lift out on Wednesday but temperatures will be slow to warm (if it happens at all) since the cooler Canadian airmass will already be in place with nothing to knock it out of the region. Weather changes, so stay tuned to later forecasts!

It’s also possible during this time frame to see waterspouts on Lake Michigan with the high degree of instability…that is the difference between the colder air coming across the lake and the relatively warmer surface waters of the lake. It promotes lake-effect clouds, precipitation, a turning motion, and rising air motion of instability.

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