Subsidence Inversion Supresses Sunday Sunshine

Posted at 6:23 PM, Sep 22, 2013
and last updated 2013-09-22 18:24:33-04

WEST MICHIGAN — There’s an old line/joke that says I thought I was wrong once, but I was mistaken. There’s nothing worse than forecasting mostly sunny to partly cloudy skies and seeing low-level, gray, stratocumulus clouds socked in all day across the area. It made for quite the gloomy and cool day as fall was officially welcomed in at 4:44 PM Sunday.

Why so much cloudiness? We had a departing low pressure area over Ontario Canada with low-level clouds/moisture trapped just above the surface. Despite the fact that high pressure continues to build in to the Great Lakes, something called a subsidence inversion is probably at fault. What is it?

Under normal conditions air usually cools with altitude. With an inversion in place, it actually increases. That means the air is a little warmer aloft than here at the surface. That inversion tends to keep a lid on the atmosphere and the movement of air. Despite the fact that high pressure is creating widespread, regional sinking air, it cannot (and certainly didn’t on Sunday) break the inversion and mix out the clouds. At a time when the angle of the sun is lower (closer to the horizon) than it was a few months ago, its weaker influence and rays simply are not strong enough to erode the cloud deck and promote drying and mixing. The result is a temperature inversion that stays in place and literally keeps the clouds locked in at/near the surface.

Inversions are not always bad things. In the possibility of severe weather, an inversion can keep a “lid” on the atmosphere and prevent thunderstorms from developing and building to severe levels. That said, breaking the cap or inversion during these times can also exacerbate explosive thunderstorm development.

The attached visible satellite image was snapped at 5:00 PM on Sunday afternoon. All of the grayish color is low-level cloudiness. Black areas depict clear skies. Note that some areas along/south of I-94 and in the southwest portion of the state did break out in sunshine. We should slowly mix out and erode this cloudiness overnight, leaving us with mostly sunny skies for Monday. (fingers crossed)

Frost advisories have been posted overnight for Newaygo, Montcalm, and Mecosta Counties as lows dip in to the 30s. Metro areas should stop around 40 or so. Get the complete West Michigan forecast at