Updraft: December 10, 2009 Archive

Lake effect snow machine kicks in

Posted at 8:28 AM on December 10, 2009 by Paul Huttner (2 Comments)

Lake effect snow plumes coming ashore near Grand Rapids, Michigan.

The lake effect snow bank is open for business on the eastern shore of the Great Lakes today.

Cold air rushing in behind this week's storm is causing the snow. Lake effect snow warnings are out for parts of several states today. Muskegon, Michigan reported 7 inches on the ground early Thursday morning, and heavy snow will continue to pile up all day. In Michigan's Upper Peninsula, snowfall totals of between 17 and 32 inches are forecast by Saturday morning.

Lake effect snow is a common occurrence in early winter. Cold air rushes over the still relatively warm waters of the Great Lakes. As it does so, it draws heat and moisture from the lake water, then rings it our as snow on the leeward shores. Topography can enhance snowfall totals.

From a forecast standpoint, you generally need a temperature difference between the lake surface and 5,000 feet of about 13 degrees Celsius to create good lake effect snow. This is easy to come by early in winter when the lake waters are still relatively warm.

As winter goes on, lakes can freeze up, and shut down the lake effect snow machine alter in winter.

Lake effect greatly enhances the average annual snowfall in cities to the lee of the Great Lakes. Marquette, Michigan averages 141 inches of snow annually. Surrounding areas can pick up as much as 300 inches in a single season!

We can be thankful we don't live on the lee side of the lakes in Minnesota. Even so, residents in northeast Minnesota are well aware that northeast winds can cause heavy lake effect snow events in Duluth and along the North Shore on occasion.

PH

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Instant Replay: Storm radar loop

Posted at 5:14 PM on December 10, 2009 by Paul Huttner (1 Comments)

Twin Cities radar loop 1:30 am to 7:30 am December 9th. Note widespread moderate snow, and heavy snow band in the east metro. Also note snow rotating around the vorticy or "pivot point" near Austin in southern Minnesota.

Check out this cool radar replay courtesy of the Twin Cities NWS and their excellent storm recap. There are several interesting features evident on the loop. It's a great illustration of why it's difficult to pinpoint snow amounts in a storm, and how snow totals can vary over even short distances.

-Notice how the individual snow bands seem to have a life cycle of anywhere from a few minutes to a few hours. They also are in constant motion, even though they may linger over an area for a period of minutes or hours.

-Notice the vorticity or "spin" center in southeast Minnesota just east of Austin. The circulation around this feature helped throw moisture back toward the Twin Cities.

-Watch the heavy snow band set up in the eastern metro, then slide southeast. It's a great example of how dynamic these storm systems are. The band is also pretty narrow, that's why one community can get several inches more than another spot just miles away.

-Note the ragged western edge of the snowfall west of the metro. This is drier air eating away at the west edge of the storm. That's why snowfall totals were much lower in places like St. Cloud and Willmar.

GOES 14 visible image shows snow covering the Midwest landscape today. Note the rivers clearly visible and the sharp cut off on the southeast edge of the snowfall. Image courtey OSEI Image of the Day.

The computer forecast models we look at have improved dramatically in the past 10 years. Still, there is nothing that even comes close to portraying the detail and intricacy of what you can see on the radar loop. That's why there's still so much art and forecaster interpretation in weather forecasting

Pretty cool stuff for a weather geek like me!

PH

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