How normal was this summer’s weather?

Overnight Low Temperatures by County

In southern Wisconsin, the average temperature for the summer months — defined as June, July and August — was near normal.

Rainfall was a different story. All of southern Wisconsin had summer accumulated precipitation of more than 16 inches, or 125 percent of normal. For much of the region, accumulated precipitation in August was more than 7 inches, which is more than 175 percent of normal.

August saw very heavy rainfall on areas of Wisconsin. Exceptionally heavy rain fell in south-central Wisconsin between Aug. 20 and Aug. 22. Storms formed and reformed over the same areas, resulting in 13.02 inches of rain in Middleton and 11.14 inches at Charmany Farm, southwest of Madison.

Unofficial reports had totals of more than 14 inches near Cross Plains. This led to flash flooding in many areas and high lake levels. Additional rains resulted in historic flooding that devastated parts of southern Wisconsin the following week.

Seventeen tornadoes were reported in southeastern Wisconsin on Aug. 28. Only one of these tornadoes was rated EF-2, while the rest were rated either EF-1 or EF-0. No injuries or fatalities were reported. Multiple weak tornadoes were also reported in southern Wisconsin on June 26, and on June 16 a brief EF-0 tornado touched down near Poynette.

As for the rest of the nation, most experienced a summer of above-average temperatures. The nationally averaged overnight lows were exceptionally warm this summer, 2.5 degrees above average and 0.1 degrees warmer than the previous record set in 2016.

In general, the U.S. summer overnight low temperatures are warming at a rate nearly twice as fast as afternoon high temperatures. The 10 warmest summer minimum temperatures have all occurred since 2002.

Above-average precipitation was observed from the Great Plains to the East Coast, while below-average precipitation was observed for much of the West and parts of the South.

Category: Climate, Meteorology, Seasons

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What caused last month’s flooding?

Tyler Statz takes a break from cleaning out his flooded home in Black Earth. The village and nearby towns were swamped when nearly 15 inches of rain fell in the area. (Photo credit: Steve Apps, State Journal)

The nearly unprecedented flooding that has plagued the southern half of the state over most of the last two weeks began with the incredibly heavy rains that fell on Dane County and surrounding areas in the afternoon and evening of Aug. 20.

Western parts of Middleton recorded over 12 inches of rain, and a staggering total of 14.7 inches fell in Cross Plains in well less than 24 hours.

The Cross Plains total now represents the single greatest 24-hour rainfall event in the state’s history, eclipsing the 11.72 inches that fell near Mellen on June 24, 1946.

A curious aspect of the heavy rain that night, at least in Madison (where 3.92 inches fell during the event), was that it was relatively lightning free as such summer downpours go. That observational characteristic is an important clue about the mechanisms that forced this state-record rain event.

Like the substantial rain that preceded it on Aug. 17, this precipitation was tied to passage of a mid-latitude cyclone that ingested extremely vapor-rich air into its circulation.

A week later, the devastating precipitation amounts were shifted northwest toward the La Crosse, Sparta and Viroqua area where, again, widespread heavy rain (near 12 inches in some locations) resulted in record flooding of the Kickapoo River.

The storms responsible for that flooding were more like the heavily precipitating convective storms we are used to here in the summer.

Category: Meteorology, Severe Weather

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What is a 100-year storm?

Flooding in Mazomanie WI on August 21st.
Photo credit: Casey Heron

The phrase “100-year storm” refers to the estimated probability of a storm event happening in any given year.

A 100-year event has a 1 percent chance (or 1-in-a-100 chance) of occurring in any given year. The term “100-year flood” allows us to place a particular weather event in context with other similar events.

It is wrong to think that a 100-year storm happens only once every 100 years. While not likely, two 100-year storms can occur within a week of each other.

These 100-year storms are often considered in terms of the precipitation that comes with the storm and the flooding that results.

A flood occurs when water flows into a region faster than it can be stored in a lake or reservoir, absorbed into the soil, or removed by runoff into a drainage basin. There are several conditions that can result in flooding: a long-lasting rainfall over a watershed, intense rainfall, or rainfall that causes rapid snow melt.

The recent flooding in the Madison area (which will be addressed in next week’s column once certain potentially record-breaking measurements can be verified) resulted from record-breaking rainfall over one watershed and heavy rainfall over another. Because floods result under different circumstances and in different places, their impacts vary.

Scientists collect data on how frequently different sizes of floods occur and the time between these floods. They use this data to calculate the probability that a flood of a particular size will be equaled or exceeded during any year.

The term “100-year flood” is a statistical designation of an unlikely event. Statistically, a 100-year flood has about a 63 percent chance of occurring in any 100-year period, not a 100 percent chance of occurring. Extreme and unlikely values are important for assessing the risk of unusual events.

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: Uncategorized

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What forces a mid-latitude cyclone to develop?

Mid-level cyclone from August 18 via GOES-16.

Our rainy Friday was arguably the first storm, or cyclone, of the autumn/winter season. Though it will surely be followed by more powerful examples, you may well have wondered how do such storms come to be?

That has been the central motivating question in meteorological science for most of the past 100 years. During that time, meteorologists have learned a great deal about how these mid-latitude cyclones are formed.

In most instances, two or more days before the storm is noticed at the surface of the Earth, processes are at work in the upper troposphere. Specifically, at the height of the jet stream (about 6 miles above the surface), weak downward vertical motions begin to drag the tropopause downward into the middle troposphere. This process eventually results in the creation of a mid-level vortex, a region of counterclockwise-rotating winds, about 3 miles above the ground.

Once generated, this vortex is then moved around by the atmospheric winds in its vicinity. At the forward side of this moving vortex, the air is forced to rise. Such upward vertical motion evacuates air from the lower troposphere, lowering the pressure at the surface.

Simultaneously, the upward vertical motion produces clouds and precipitation. So long as the mid-level vortex continues to intensify and move, so too does the surface cyclone. In many cases, the mid-level vortex eventually becomes quasi-stationary and positioned directly above the surface cyclone. This usually marks the end of the intensification for the storm though it can still deliver high-impact weather at this stage.

Category: Meteorology, Seasons

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When is tornado season?

A tornado that touched down Thursday in Deerfield, though relatively weak, tore a roof off International Machinery Exchange, 214 N. Main St. (Photo credit: John Hart, State Journal)

Tornadoes form in regions of the atmosphere that have abundant warm and moist air near the surface with drier air above, a change in wind speed and direction with height, and weather systems such as fronts that force air upward.

The United States provides these three ingredients in abundance, so it is not surprising that the majority of the world’s reported tornadoes occur in the U.S.

Within the United States, tornadoes can occur in nearly every state and in every month of the year. Wisconsin has experienced tornadoes in every month except February.

Tornado season is based on when the ingredients for severe weather come together in a particular place. Because a change in wind with height is closely related to the presence of a jet stream, tornado season moves north and south during the year with a jet stream.

Tornado season peaks in March and April in the Southeast but not until July in the upper Midwest and Northeast. The deep South has a secondary peak in tornado occurrence in November.

Tornadoes can also happen at any time of day or night. However, the most likely times for tornadoes are late afternoon or early evening. More than half of all U.S. tornadoes occur during the hours of 3 p.m. to 7 p.m. local time.

In Wisconsin, June is typically the month when the most tornadoes occur, followed by July. Wisconsin averages 24 tornadoes per year.

So far this year, Wisconsin has experienced only eight tornadoes. The most recent was Thursday in Deerfield. The National Weather Service determined that tornado had 80-mph winds. The storms were relatively weak, in comparison to other tornadoes, but they still caused damage and posed a danger.

Category: Meteorology, Severe Weather

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