Lake Effect Snow Wrap-up

A band of lake effect snow brought 10 inches of snow to parts of Kenosha County on the evening of Christmas Day.  The band moved onshore around 5 pm just south of the Milwaukee airport southward into Racine and Kenosha counties.  Southern Milwaukee County was skirted by the northern edge of this band, sparing the county of the heaviest snow. 

 The radar loop below from the terminal doppler near Milwaukee Mitchell Field shows the snow from 5pm Saturday evening through 5am Sunday morning (frames are approximately one-hour apart).  You can clearly see the band of snow that affected this area.  And if you watch carefully you can see that sections of the band will drift east and west throughout the loop.  This drifting is what makes lake effect events very difficult to forecast.

The graphic below shows an analysis of snowfall totals received from cooperative observers, trained spotters and the general public.  The highest snowfall received at this time has been 10.3”. This total was reported northwest of Kenosha and also near Pleasant Prairie.  You can see that the difference between 6” of snow and 1” of snow is a very short distance….only 5 or 10 miles.

We were pretty certain that a lake effect band was going to develop over Lake Michigan and move onshore in southeast Wisconsin or Northern Illinois, but pinpointing exactly where that band comes ashore and drops 10 inches of snow can be a daunting task.

Very minor changes in a few ingredients in the atmosphere that create lake effect snow can lead to big differences in where the snow falls.

We don’t see the big lake effect events frequently on western shore of Lake Michigan that they see in Michigan or northern Indiana, simply because more often than not, wind is flowing from west to east or northwest to southeast.  For lake effect snow in Wisconsin, cold air must flow over the warm lake waters to pick up moisture and create instability in the atmosphere. Therefore, we generally need to see a north or northeast wind to see the snow. 

That said, there are still additional factors to consider.  The graphics below show two types of lake effect set-ups we can see in Wisconsin.  The first scenario has a general wind flow from the northeast across the entire lake.  The distance the air has to travel across the lake, or the “fetch”, here is relatively small, and in a single direction.  This will usually result in lighter snowfall totals, but over a larger area along the lakeshore.  We would expect to see multiple weak bands of snow coming in off the lake.

The second scenario is what we saw on Christmas Day.  The high and low pressure centers are shifted slightly in this image, allowing for a more north-northeasterly flow across a longer distance of the lake.  In addition, the influence of the high pressure center gives a slightly west of north trajectory to winds right along the shore.   Where these winds come together is where a lake effect band will set up, and if all other conditions are right, can produce significant snowfall totals where it moves inland.  This is also the setup we saw on March 2, 2009 when the Milwaukee airport recieved 14 inches of snow, and just a few miles north and south very little snow fell.

Other factors that are considered when forecasting lake effect snow are the difference in temperature between the lake surface and the air in the first few thousand feet vertically.  With the case this week, lake surface temperatures were near 40F or around 4C and temperatures at 850mb, or around 5000 ft were -11 degrees.  This gives a temperature spread of 15C degrees in 5000ft, providing the instability and lift needed to produce convective snow showers.  In generally we're looking for this temerature difference to be at least 12C, with the best snow production between 17 and 20C.There are plenty more factors to consider in forecasting lake effect snows, but those are for another day. 



Penny Zabel
NWS Milawukee/Sullivan

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