Did someone predict the recent Wisconsin tornado?

Ron Bloomberg, who witnessed victim and neighbor Eric Gavin’s body being recovered, embraces his girlfriend as he returns to his home in Chetek, Wis., after a tornado flattened a trailer park and nearby trees. (Richard Tsong-Taatarii/Star Tribune via AP)

We cannot yet forecast tornado occurrence with any accuracy. One problem is the small size of a tornado, which is a narrow column of strong winds that rotate around a center of low pressure.

Over the last 60 years, forecasts of the development of large-scale low-pressure systems, which often organize the ingredients needed to form a tornado, have steadily improved. Because of these advances, meteorologists are better able to predict those conditions a few days in advance, enabling forecasters to identify counties where there is a threat of severe weather sometimes as many as three days in advance. Two days in advance of the recent EF-2 tornado (later upgraded to EF-3) that hit southeastern Polk County, the National Weather Service’s Storm Prediction Center’s convective outlook issued a slight-risk for the area.

For a thunderstorm to produce a tornado requires warm humid air near the surface with cold dry air above. These conditions make the atmosphere very unstable, in the sense that once air near the ground is forced upward, it ascends freely and quickly (like a helium balloon), cools as it expands and forms a storm. Severe thunderstorm conditions also include a layer of hot, dry air between the warm, humid air near the ground and the cool dry air aloft. This hot layer acts as a lid that allows the sun to further heat the warm, humid air — making the atmosphere even more unstable. In the central U.S., such air is created over the plateau of Mexico and sent northeastward over the Great Plains.

To form a tornado, the host thunderstorm must also rotate. This happens in a storm when wind at the ground is moving in a different direction and speed than the air above. The change in wind speed and direction with height is known as wind shear. Both wind shear and atmospheric instability are needed for tornado formation.

Recent advanced models have been able to simulate development of a tornado, a first step to better predictions. Advances in radar technologies have helped to identify storms that are producing a tornado, or about to produce one. Based on observations from such advanced technologies, the Storm Predication Center (SPC) issued a warning about one hour before the Polk County tornado, and the NWS issued a tornado warning about 10 minutes before the first sighting of a funnel cloud.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

Category: Meteorology, Severe Weather

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Is the weather in Madison more extreme than other locations in the U.S.?

Spend an entire year in Madison, and you’ll likely experience a huge swing in temperatures and humidity, thanks to the area’s continental climate. (Photo credit: John Hart, State Journal archives)

Sitting nearly in the middle of the vast North American continent, Madison has what is known as a continental climate. Continental climates are characterized by large annual extremes in temperature and humidity as well as very distinct seasons.

The continental nature of Madison’s climate is what makes a year’s worth of weather in Madison usually a lot more varied than a year’s worth in, say, Seattle.

There is an astounding 144-degree difference between the all-time highest temperature in Madison (107 degrees on July 13, 1936) and the all-time lowest (37 below zero on Jan. 31, 1951). In addition, the amount of water vapor in the air can range from the barely detectable level in the midst of a deep winter cold spell to as much as 3.5 percent of every breath you take during a severe July heat wave.

No matter what the season, the vast majority of the invisible water vapor in the atmosphere is contained in the lowest mile or two from its source at the surface.

At any given time, the Earth’s atmosphere contains 37.5 million billion gallons of water vapor – enough to cover the entire surface of the planet with 1 inch of rain if condensed. This amount is recycled, through evaporation powered by the sun, 40 times each year in what is known as the hydrologic cycle.

In each of these 40 cycles, enough energy is expended to power the United States – the largest consumer of energy in the world – for 3,441 years, a truly astounding amount of energy.

Category: Climate, Seasons

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Is the California drought over?

Drought in the United States has been assessed by a number of different measures over the last several decades.

One of the older such measures is the Palmer Drought Index introduced in 1965. The Palmer Index is based on a supply-and-demand model of soil moisture. Of the two, supply is easier to calculate, as it is tied so obviously to precipitation. The demand side of the model is more complicated as it depends on processes such as evapotranspiration — the loss of water from the soil through direct evaporation and transpiration from plants — and the recharge of soil moisture by a variety of processes.

The Palmer Index approximates this complicated nest of processes based on a formula that is based on recent temperature and precipitation. It is quite effective in assessing long-term drought but suffers in making statements on shorter time scales.

In 1999, the National Oceanic and Atmospheric Administration and the U.S. Department of Agriculture developed the weekly U.S. Drought Monitor. By this measure, drought in the U.S. fell to a record low this past week with only 6.1 percent of the contiguous 48 states experiencing drought conditions. The previous record low according to this measure was 7.7 percent in July 2010.

Conversely, the most extensive drought in the 17-plus-year history of the Drought Monitor occurred in September 2012 when drought conditions of one degree or another were spread over 65.5 percent of the lower 48 states.

Most notable currently is the official end of the years-long drought in California that was announced last week. The past winter was quite wet and has filled reservoirs in much of California to capacity. In fact, there are now fears in certain parts of California that the spring snowmelt will induce flooding.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

Category: Climate, Meteorology

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Why is the grass wet in the morning, when it didn’t rain?

Covered with dew, the wind-driven seeds of milkweed rest in a nest of dried leaves. Dew is made of liquid water that has condensed from some of the water vapor in the air. (Photo credit: La Crosse Tribune archives)

Recent spring mornings have produced wet lawns. To explain this, we start with the fact that air contains water in the gas phase, called water vapor. Dew is made of liquid water that has condensed from some of the water vapor in the air.

Dew occurs when objects cool. A common example of dew formation is when drops of liquid form on the glass of an ice-cold drink. Dew forms when the object, such as the glass, cools down to the dew point temperature.

Water molecules in the air continually bombard surfaces, like blades of grass. Some of the molecules stick, forming a very thin film of water. This film may not last long, as the water evaporates. The evaporation rate depends on the temperature of the water, which is the same temperature as the blades of grass.

So, condensation depends on the state of the atmosphere, such as its temperature and moisture, while evaporation depends on the temperature of the object.

If the object gets cold enough, and there is enough moisture in the air, condensation is much greater than evaporation and the film grows into dew drops.

Each night the weather report includes the temperature and the dew point temperature.

If the two temperatures are close, it is likely that dew will form during the night.

Dew also tends to form on calm nights that come with clear skies. Windy conditions and cloudy skies keep the ground from cooling.

Category: Meteorology, Phenomena

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What are the plant hardy zones and how do they relate to climate?

Signe Knudsen, right, talks with Ia Xiong, in Troy Community Gardens in Madison. Hardiness zones let gardeners know the type of plants capable of surviving their climatic conditions, including the ability to survive a minimum temperature range. (Photo credit: Michelle Stocker, The Capital Times Archives)

If you are involved with gardening, you probably are aware of the hardy zones listed on seed packets.

Southern Wisconsin is largely in zones 5a and 4b, while northern Wisconsin lies in zones 3a to 4a.

The U.S. Department of Agriculture developed the zones and first published them in 1960.

A hardiness zone provides information on the type of plants capable of surviving certain climatic conditions. These conditions include the ability of a plant to survive a minimum temperature range.

Regions with hardiness zone 4b have an average annual minimum temperature range between minus-25 and minus-20 degrees, while zone 5a has a range between minus-20 to minus-15 degrees.

The climate zones are determined from temperature records kept by National Atmospheric and Oceanic Administration (NOAA).

Recognizing that climate varies over long time periods, every 10 years NOAA computes a revised 30-year average temperature and extreme temperatures for the U.S. Between 1961-1990 and 1976-2005, the 30-year average minimum winter temperatures increased at nearly all locations in the continental U.S. This suggests appropriate changes in plant hardiness zones.

The USDA Plant Hardiness Zone Map developed in 1990 was updated in 2012. Previously, south-central Wisconsin was in zone 4b, in the revised map we are zone 5a. Given the continued warming of our minimum wintertime temperatures, when the USDA next revises the hardy zone map, the zones in Wisconsin may again shift northward.

While the hardiness zones are useful, they cannot account for all climate and weather conditions such as snowfall, which can insulate the plants during a cold winter, or severe summer heat. In addition to knowing your plant hardiness zone, it is also very useful to talk with local master gardeners and nurseries as you plan your plantings this season.

Category: Climate, Meteorology, Seasons

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