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Big Upper Low Throws Cirrus Our Way

Posted at 9:40 PM, May 05, 2013

WEST MICHIGAN – You can clearly see the big upper level low pressure system on the attached photo over the Southeast United States. This system is bringing huge amounts of rain across the region, but only spinning some high level cirrus clouds back in to Michigan. The red, orange, and yellow around the low indicate the “energy” spinning around this system (what meteorologists call vorticity). Positive vorticity refers to a counterclockwise spin which promotes rising air. Negative vorticity refers to a clockwise spin which promotes sinking air or fair weather.

The snapshot is a computer forecast model from about 18,000 feet above the surface, or about 500 millibars for you pilots. This is a good level to look at upper level lows or disturbances that race across the country in the upper level flow. It also shows the trough/ridge pattern…from which we can derive several things as meteorologists. An upper level low is simply a cold pool of air aloft. Since the air below it is warmer, it’s more buoyant and therefore wants to rise. That creates instability or rising air. That usually means clouds and rain.

Luckily this time, Michigan is far enough north from this low that all we’re seeing is the residual high cloudiness (cirro-form). That said, this system continues to generate an easterly wind at the surface for us. During this time of year on a sunny day, the land heats up while the water stays cool. That creates mini low pressure inland which generates a lake breeze…that is the wind blows from the lake onshore or inland. When the lake breeze hits the low-level easterly surface wind, it creates a convergence zone. The air rises, clouds form, and precipitation may ensue. That scenario is possible on Wednesday with a few isolated showers inland.

While most of this week is expected to be dry, our better chance for widespread steadier rain arrives Friday and Saturday as another low pressure system moves in to the lower Great Lakes. Get the forecast at www.fox17online.com/weather.