James Bay low puts lid on July Heat

Posted at 9:02 PM, Jul 29, 2014
and last updated 2014-07-29 21:02:48-04

GRAND RAPIDS, Mich. — Ever wonder why at a time in July when we should be seeing temperatures in the 80s and 90s with plenty of humidity we’re only in the 70s? There’s a simple explanation!

July is typically the hottest month of the summer for us in Michigan, but Mother Nature has taken a bit of a break this week. The snapshot attached to this story is from one of our computer forecast models. It actually shows several things. First, the colors correspond to the temperatures we see at the surface. So that means the yellows are warm (out west), while the green and light green are the cooler temperatures in the east. Note these temperatures also correspond to the location of our jetstream. The jetstream is a river of fast moving upper level winds around 30,000 feet that moves our weather across the country from west to east. When the jetstream pushes far to the north in to Canada over the Rockies, we refer to that as a ridge. Areas under the ridge usually see warmer temps and dry, quiet weather. That said, note how the lines on the map (500 millibar geopotential height lines for you weather geeks) dip well south over the eastern half of the nation. We refer to that as a trough and areas under the trough typically see cooler-than-average temperatures and unsettled weather.

If that weren’t enough, a strong upper level low pressure system, or cold pool of air aloft in Canada over James Bay is almost stationary with everything spinning counter-clockwise around the center. Michigan is caught in this “cyclonic flow” under the low and seeing temps only in the 70s with the chance of a shower popping up the next several days. So there is some rhyme and reason as to why our temps are below normal in July and why we have the chance of a shower over the next few days. Blame it on the position of the jetstream (the trough) and the low pressure system in Canada over James Bay.

Once the trough and low begin to lift out somewhat this weekend, we’ll see our shower chances diminish and our temperatures increase back to 80 or better which is where we should be for this time of year.

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