Weather Radio Safety program
Updated: 08/04/2011 -
Throughout the winter months, we regularly get strong cold fronts to push through the state, kicking up northwesterly winds and bands of lake effect snow into parts of Iron, Ashland, Vilas and sometimes even Oneida Counties. Every once in a while, the winds are strong enough out of the east or northeast, that bands of snow overspread some of the eastern lakeshore counties in the state. Before I go into the details of how this latest episode of heavy snow on the south side of Milwaukee came to be, let's cover what factors go into producing a lake effect snow event.
The first ingredient needed are strong winds crossing over a large body of water. These can range from brisk breezes across the Great Lakes to the Atlantic or Pacific Ocean. A good example of an ocean effect of rain or snow is in coastal New England from Boston to Cape Cod in Massachusetts. Next you need a strong difference in temperature between the surface temperature of the water and the air about 5,000 feet up off the ground. Ideally, there needs to be a 13 degree C or 23 degree F difference or greater, typically with the temperature of the water being warmer that the air blowing on through. The last and most important aspect is where the snow bands set up. Folks in northwestern Wisconsin as well as those in the lee of Lakes Michigan, Erie and Ontario can tell you that just a minor shift in the direction of the wind can lead to either just a few snow showers or literally a dump load of snow. Forecasters that deal with this winter weather phenomenon literally need to break down the wind direction to the nearest 10 degrees in order to determine which towns may feel the brunt of lake effect snow or rain.
This leads us back to Monday when a long, narrow stretch of snow billowed up over Lake Michigan and drifted into Cudhay, Oak Creek and Caledonia in Milwaukee and Racine Counties . A strong ridge of high pressure was located off to the west, while a major winter storm was sliding northward up the eastern seaboard. Although that there was quite the distance (over 1,000 miles) from the centers of these areas of high/low pressure, the winds along the lake shore were running about 10-15 mph from midnight through about 6pm on Monday out of the north/northeast. How about that temperature difference? Well the water temp in Lake Michigan near Milwaukee was at 36 degrees F, while the air temp at the key 5,000 ft level was at 1 above zero F, meeting the criteria by 12 degrees. The end result was a fetch of heavy snow hitting the south side of Milwaukee, including the airport with over 14 inches, 10 inches in Oak Creek, while just 20 miles to the south about 6 inches fell in Caledonia. However if you ventured 6 miles west to Wautatosa, not more than a few flakes were spotted. Talk about a sharp cut off in snowfall. Typically, there aren't a lot of days in the winter when winds are coming out of the east, expect when a strong storm is pushing by to the south or east of the Badger State, or on the flip side if strong high pressure is stationed to our north. Usually you'll hear us mention how that easterly wind enhances snowfall with a major winter storm along the lakeshore and in the summer how a lake breeze can ignite scattered thunderstorms. Needless to say, the various changes in topography and our proximity to the Great Lakes has an effect on the weather we experience here in the state of Wisconsin.
For more on this lake effect snow event, check out this link to the NWS Milwaukee.
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