Dense fog advisory issued for 5 counties in southeast Wisconsin until noon

Lake Michigan is an effective weather generator

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Lake-effect snow? Sure, we’ve all heard of that. Beginning every year, usually in November, you’ll hear us talk about lake-effect snow showers rolling in off the lakeshore. Lake-effect snow showers can pile up pretty quickly as cold air flows over the relatively warmer waters of the lake surface.

But lake-effect rain showers? Yes, we can get those too, although it sounds a lot stranger than the lake-effect snow showers. But winter or summer, snow or rain showers, for meteorologists it is all about the delta T. That quantity is the temperature difference between the lake surface water temperature and the 850 millibar temperature. 850mb is a constant pressure level that equates to approximately 4,000 feet above the ground. (We know the values of these temperatures aloft thanks to radiosondes attached to weather balloons that are launched across the country twice per day.)

Lake-effect clouds seen on the southern end of Lake Michigan on the afternoon of August 14, 2011. These are generated by cooler winds flowing over the warmer waters of the lake surface.

Today (Sunday, August 14, 2011) our extensive morning cumulus cloud cover was due to cooler north-northeast winds flowing down the length of Lake Michigan. In August the lake water surface temperature tends to be as warm as it gets all year due to the build-up of summer heat. The southern half of Lake Michigan has a surface temperature around 72° to 74° F., which translates to about 22° or 23° C. The 850mb temperature over southeastWisconsinthis morning was approximately 11° C. That is a delta T of 11° or 12° C. (22° or 23° minus 11°).

Temperature of the Great Lakes surface water on August 14, 2011. (Values are in degrees Fahrenheit.)

In a classic lake-effect precipitation scenario, we like to see delta T values of around 14° C. or so. Today it was close to that value and the result was the extensive clouds this morning. They were pushed inland by the north-northeast wind. Radar was detecting a few light showers or sprinkles, but most of those remained just off shore to our east.

It doesn’t happen too often, but lake-effect raindrops can show up in our corner of the state. Most often this will occur in August or September when the lake surface water temperature is the warmest. The lake can really crank out the weather, winter or summer.