The 2007 to 2008 winter season over southern Wisconsin has been filled with frequent snow storms. In fact, Madison WI has broken the all time seasonal snowfall record with over 80 inches so far. Milwaukee will likely end up in the top 5 snowiest winters ever since record keeping began in the late 1800s.
There have been 4 days this year with 6 inches or more of snow in Milwaukee, and 3 such days in Madison. In a typical year, we only have 1 day (from midnight to midnight) with 6 inches or more.
So why is this happening? It appears that the moderate La Nina event may have something to do with it. However, not all La Nina (or El Nino) events are created equal. Each one is unique and so it is impossible to predict exactly how warm or cold, wet or dry an individual location will be months ahead of time. However, you can get a general idea by looking at sea surface temperatures in the tropical Pacific.
For more information on El Nino and La Nina, go to the Climate Prediction Center site:
Let’s begin by reviewing what La Nina is, and how it usually affects the weather over the lower United States during the winter months of December, January, and February. The image below shows the most recent sea surface temperatures. Notice the blue area along the Equator in the Tropical Pacific. That is below normal water temperatures. When these unusally cold waters persist for many months, it is considered a La Nina event. The opposite is El Nino, when the water temperatures in this same area are warmer than normal. Even though these waters may only be a few degrees colder than normal, it affects the weather patterns and storm tracks across the entire planet.
As you can see below, there have been several El Nino and La Nina episodes over the past 18 years. You can see that these patterns take months or even years to evolve.
The following two graphics show how a typical La Nina affects the storm track over the eastern Pacific Ocean and the United States. Normally, the Pacific Northwest coast is much wetter than normal during La Nina winters. Wetter weather is also experienced in the Ohio Valley and Tennessee Valley. It tends to be drier than normal in the Gulf Coast states and warmer than normal across the southern states. Colder than normal La Nina winters are common in the northern Rockies into western Canada.
Thus, history suggests that most Moderate to Strong La Nina events result in near normal temperatures and near normal precipitation across southern Wisconsin. However, notice that it tends to be quite wet (and potentially snowy) just south and east of us from Michigan to the Ohio Valley.
The following series of graphics will summarize some key features of the winter so far across the United States. These graphs show the average or departure from average from December 1st 2007 to February 13th 2008. Averaging these weather maps over a long period of time shows some interesting features. Many of these resemble a typical La Nina winter. Several show clues to why so much snow has fallen over the winter in southern Wisconsin.
The first image below shows the flow pattern about 18,000 feet above sea level (500 millibars). This helps us visualize the jetstream, which helps steer the storm track. Notice how a trough of low pressure has been common over the Rocky Mountains, with a ridge of high pressure over the eastern states. This favors a storm track from the southern Plains into the Ohio Valley and Great Lakes. Storms tracking along this path favor frequent heavy snows in southern Wisconsin.
Next, we’ll look at the average jet stream winds at that level. Notice how they are well above normal from the Pacific Ocean into Oregon and northern California. Also, they are stronger than normal from Texas and Oklahoma into the Ohio River Valley. This extra jet energy helps to track and intensify storm systems into the Ohio River Valley.
Now we’ll look at the surface barometric pressure. As you know, most active weather including snow storms are associated with strong low pressure areas. You can see that pressures have been lower than normal from the central and southern Plains into the Great Lakes. Low pressure systems that track between Chicago and the Ohio River Valley tend to produce the heaviest snows in southern Wisconsin.
We’ve had numerous storms take this favorable track during the winter.
Let’s see how the temperatures have been behaving. As is typical in La Nina winters, it has been quite warm from the southern Plains into the Ohio Valley. However, it has been close to normal from Kansas into southern Wisconsin. So, as storms track into the middle Mississippi and Ohio River Valleys, they have brought warm and moist Gulf of Mexico air close to us but not quite into Wisconsin. Since we are normally cold enough to snow in most storms over a typical winter, we’ve had snow from all but a few of the low pressure systems this winter.
As you would expect, having more warm Gulf of Mexico air drawn up toward us can be a bad thing in the winter. The image below shows how there has been higher than normal moisture in the lowest 3 miles of the atmosphere (1000-500mb Precipitable Water) centered over Indiana, with high values over much of the Great Lakes region.
So, as you can see below, it has been much wetter than normal over the Ohio Valley and the Pacific Northwest states this winter. Although this is pretty typical for La Nina winters, there are some subtle differences. You’ll notice that the wettest area is northern Indiana, rather than western Kentucky if you average ALL of the moderate to strong La Nina winters. This wetter than normal area extends into southern Wisconsin. Although that is a small difference if you consider the size of the United States, it is obviously a huge impact on our winter snow season.
There are a few more images that are a bit more technical that describe why our snow production has been so impressive. In order to get heavy snow, you need lots of moisture and rapidly rising air. Research has shown that when the fastest rising air occurs between -12 and -18 C (or zero F and 10 above F), you get the most efficient formation of dendrite snowflakes, which are those pretty six sided crystals. This temperature zone is often located about 3 miles up in the atmosphere (about 14,000 feet, or 600 millibars).
The two images below show you that at this level, there has been above average rising motion and it has been located within this favorable temperature layer. Thus, we’ve had numerous storms that had ideal conditions to produce heavy snowfall.
Also, it has been known for years that the heaviest snows occur close to the -5 C (23F) line about a mile up in the atmosphere (about 5,000 feet or 850 millibars). There are many reasons for this including the fact that the heaviest snows often occur just north of the rain/snow line. As you can see below, this zone was also near the WI/IL border over the past few months.
So, you can see that the overall pattern of La Nina is certainly visible in this data. However, you can see the huge impact that shifting the area of heavier than normal precipitation just a state northward can have. Much research has been compiled and will continue to be done to determine how we can better anticipated which states will be most impacted by the El Nino and La Nina weather patterns.
Most of this data is Courtesy of NOAA’s Climate Prediction Center, Climate Diagnostic Center, and the Earth System Research Laboratory.
For more information about specific Milwaukee and Madison statistics for this winter, please refer to:
Jeff Craven, Science and Operations Officer, National Weather Service Milwaukee/Sullivan WI
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