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Thundersnow

**Winter Storm Forecast Discussion**

December 19, 2012

Last week, it was Winter Storm Caesar, which paralyzed Central Minnesota and the Twin Cities Metropolitan Area. This week, we have yet another large, dynamic winter storm taking aim on regions to the south, including Nebraska, Iowa, Kansas, Wisconsin, and Michigan. Blizzard Warnings, Winter Storm Warnings, and Winter Weather Advisories have been posted for these states as this storm begins to take shape.

Regional radar currently shows a large area of snow centered over Nebraska, and Iowa, with rain being reported to the south in eastern Kansas and Missouri. The surface low, as of 7:00PM EDT, is centered over extreme south-central Kansas, with a pressure of 997mb. This low is forecast to strengthen to 988mb and deepen as it lifts to the east northeast overnight, tracking through Chicago by early Thursday evening. This system is well supported in the upper levels, with a strong, 500mb trough, going from a neutral tilt, to a more negative tilt as it lives east northeast Thursday. Aided by a strong jet streak of 100 knots and a closed low of 528dm at 500mb. Additionally, this storm will be aided by strong warm air advection and deep moisture transport from the Gulf of Mexico, wrapping into the storm. Visible satellite imagery depicts a large, baroclinic leaf setup with this storm, which indicates the presence of this strong jet at 500mb. This is often the initial stage of a developing low pressure system. Overnight, the system will begin to form along this boundary and what was once a wave of low pressure, will strengthen into a powerful winter storm over the Great Lakes region. With a negative tilt trough, both cold air advection and WAA will further strengthen, as the storm continues to deepen as it moves through Chicago Thursday evening.

By the time this system makes its way through Chicago, blizzard conditions will be seen through much of Iowa and Wisconsin. Very heavy snowfall will be seen in areas such as La Crosse, Wisconsin Dells, and Ames. The dynamics of this storm are impressive, as snow is being generated, not through colder air moving in behind this storm, but through the strength of the warm air being ingested into the storm. As this warm air rises, we see pressure falls in response, and the column of air cools to the north of the low, causing rain to change over to heavy, wet snow. Additionally, this storm has a great deal of moisture to work with, setting the stage for very heavy, wet snow. Precipitable water values (the depth of water in a column of the atmosphere if all the water in that column were precipitated as rain. As a depth, the precipitable water is measured in inches) are high with this storm, between 0.75 and 1.25 inches. This will lead to widespread snowfall totals of 6-18″ with isolated amounts of up to 2 feet, where heavier snow bands set up. In the heaviest snow bands, thundersnow is possible, with rates of 2-3″ per hour likely. Strong winds, gusting to 45-55 mph will be seen to the northwest of the low and north, causing whiteout conditions. This is a life-threatening storm and all precautions need to be taken to protect life and property.

With regard to the safety of life and property, it is very important that you have a winter preparedness kit in your car, in case you get stranded. I would advise not to travel unless it is absolutely necessary. Road conditions will quickly deteriorate and travel would be unwise. It is best to hunker down and ride this storm out. This storm is very similar to the Blizzard of 2011 in Chicago, that paralyzed the region for days. Thundersnow occurred with that storm as well, as parts of Chicago and Milwaukee saw upwards of 30″ inches of snow. This storm won’t produce quite as much snow, but the winds will be just as strong, creating very dangerous conditions.

I will bring updates on this storm as it continues to develop tomorrow, and until then, stay safe and hunker down for this one.

Meteorologist Mack

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