Issued 5 pm Friday November 1 2013
Imagine this: It’s February 2nd, 2011 in Milwaukee, WI. All schools are closed, I-43 has been shut down, and Governor Scott Walker has declared a state of emergency in 29 different counties. By 12pm, the snow has dissipated but Milwaukee has received 19.6” in 48 hours. Areas just southwest, such as Janesville, only received 9.5” but just south, in Kenosha; they received around 24” of snow.
Now, although all these snowfall totals are relatively high, why such major differences in a short distance?
The reason for the differences is due to the track of the low pressure system. The track is the path that an area of low pressure takes from the initiation stage to dissipation.
So, why is the track so important in snowfall totals? In the structure of a low pressure system, the area northwest of the center of the low is where we tend to see the most snowfall. In the case of Groundhog Day 2011, the northwest area of the low hit southeast Wisconsin and northeastern Illinois, producing snowfall amounts 18-24”.
In order to have heavy snowfall, a low pressure system needs 3 things: 1. Below freezing temperatures in the clouds and at the ground, 2. Moisture to form clouds and precipitation, and 3. Rising air to turn moisture into clouds/precipitation.
Next question, why does the northwest area of a low pressure system produce the heaviest snowfall? First we need to briefly discuss how lows form to be able to understand the answer to this question.
Low pressure systems can form when a jet streak (wind speed maximum within the jet stream) or wave moves over the Rockies. The air warms as it descends down the right side of the mountains causing low pressure to develop. Low’s that tend to affect our area generally move from the Rockies and continue to develop in the central and southern plains. More specifically, trends have shown that when the center of a low pressure system moves through St. Louis, Missouri and continues through central Illinois, heavy snowfall will generally fall in Milwaukee and southeast Wisconsin. There are not as many examples, but trends show when the low center moves through Chicago, Illinois, heavy snowfall can occur in Madison and south-central Wisconsin.
The structure of a typical low pressure system is usually shaped like a comma, with a cold front extended in a southerly direction and a warm front horizontally (east) from the low center.
A little more in depth: a warm conveyor belt extends in front of the cold front pushing warm, moist air into the atmosphere. A cold conveyor belt moves west in front of the warm front, ascending as it does so.
A zone of rising air known as the “Deformation zone” forms on the northwest edge extending northward from the low center. This zone is an area of rising air where moisture from the warm belt is pushed into the cold belt.
So now we have Cold air + Moisture + Rising Air= Heavy snow northwest of the low center.
Note: There are other meteorological processes which can contribute to heavy snowfall and cause smaller bands of moderate to heavy precipitation to form in the northwest corner in systems that aren’t necessarily strong or cause heavy snow to form in other areas of a system.
The image below depicts the structure of the low pressure system as it moved through the Midwest on February 2nd, 2011
The image below shows the heavy snowfall across the area, with the higher snow totals in southeast Wisconsin.
Another example is from December 1st, 2006 when a blizzard moved through south central and southeast Wisconsin between 3am-3pm. This system dropped 17 inches of snow in Kenosha, WI, 14.6 inches in Racine, WI, and 10.5 inches in Milwaukee, WI. The track of the low pressure system can be seen in the image below. It shows the system moving northeast from Arkansas (after developing in the southern plains) through central Illinois. The northwest quadrant of the system landed in southeast Wisconsin and northern Illinois.
The image below shows snowfall totals from December 1st, 2006, with the snowfall totals of at least 10 inches in Waukesha, Milwaukee, Walworth, Racine, and Kenosha counties.
In conclusion, there are many examples of winter storms dropping massive amounts of snowfall in small regions. The biggest contributor to this process is the low pressure system structure combined with the track of the low—specifically the track of the northwest quadrant of the low.
Keep your eye on the weather maps this winter when the National Weather Service posts about potential heavy snowfall. Where is that northwest quadrant? And where is that system coming from? It may help you better prepare!
Kim Anderson, Meteorology Student Volunteer, University of Wisconsin-MilwaukeeReturn to News Archive