What is a “wake low”?

Posted on July 8, 2026 by WeatherGuys Editor

Wake lows are short-lived mesoscale phenomena. Mesoscale weather ranges from about 5 kilometers to 1,000 kilometers in size.

Small arrows indicate surface wind; large arrows relative flow into the wake. Stippling indicates extent of precipitation-cooled air. (After Fujita, T.T., 1955: Results of detailed synoptic studies of squall lines. Tellus, 7, 405-436.)

Wake lows are relatively uncommon. They produce strong winds after a storm moves out. The term “wake low” was defined by Ted Fujita, the same meteorologist who came up with the F-scale ranking of tornadoes. These areas of low-pressure form on the backside of heavy rain, causing winds to surge in at fast speeds.

A mesoscale convective system, or MCS, is a collection of thunderstorms that becomes organized on a scale larger than individual thunderstorms and typically lasts for several hours. MCSs may be accompanied by severe weather hazards: heavy rain, flooding, strong winds, tornadoes and hail.

As a MCS moves out of a region, dry air sinks along the back side of the storm. The sinking air often leads to clearing skies. This descending air warms up rapidly through compression, generating a localized area of low pressure in the storm’s wake. The pressure difference between the rain-cooled air of the MCS and the warm air descending in the storm’s wake, creates a strong pressure gradient that generates strong winds.

Winds speeds can rapidly reach 40 mph to greater than 60 mph. The winds often blow in the direction opposite of the direction the MCS originally traveled, since the wind is rushing toward the storm’s wake.

Wake lows are notoriously difficult to forecast because they rely on the complex decay dynamics of a storm, are of small scale and are not common. The modernization of the National Weather Service improved tracking capability of mesoscale phenomena with Doppler radar, surface observations, satellite observations and finer resolution models. The development of mesoscale weather station networks has improved the ability to locate wake lows.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ ssec.wisc.edu or jemarti1@wisc. edu.

Category: Meteorology, Phenomena, Severe Weather

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Was this spring milder than normal?

Posted on June 29, 2026 by WeatherGuys Editor

With only a few exceptions, it seems as if this year’s run-up to summer, which arrived at 3:24 a.m. Sunday, June 21, was relatively mild. By that we mean very few really oppressively hot and humid days.

Naturally, we wondered if this were actually a true impression and also how this year’s spring stacked up against others that have visited Madison over the years. One way (but not the only way) to make such an assessment is to consider how many days in the interval from April 1 to June 21 have had a daily high temperature greater than or equal to 80 degrees Fahrenheit.

For 2026, that number is 15 or about once every 5.5 days. Using records all the way back to 1869, the average number of such days in that calendar interval is 13.7 (once every six days), so this year is just barely above that long term average. Interesting individual years according to this measure are 1878 when not a single day between April 1 and June 21 was 80 degrees in Madison. On the other extreme, in 1991 there were 36 such days in the same interval — almost every other day!

Because this measure is highly variable from year to year, looking at decade averages can lend more insight into trends that might exist in this data. The coldest decades were the 1900s and the 1870s with 8.4 and 8.9 such days on average each year. The warmest decades were the 1970s and 1950s with 19.5 and 18.9 such days per year on average. The 2010s, averaged 16.7 such days each year.

With 15 days at 80 degrees or warmer, this year is the 94th coldest spring of the last 158 springs, decidedly on the warm side of the distribution.

However, there appears to be no correlation between number of days at or above 80 degrees during spring and the severity of summer heat, so we will have to wait and see on that.

Steve Ackerman and Jonathan Martin, professors in the UWMadison department of atmospheric and oceanic sciences, are guests on Wisconsin Public radio at noon the last Monday of each month. send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Seasons

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What causes a thunderstorm?

Posted on June 22, 2026 by WeatherGuys Editor

Thunderstorm development requires three basic ingredients: moisture, unstable air and upward motion.

The shear environment is important in determining the thunderstorm type. Both the vertical speed shear and directional wind shear have varying magnitudes. To simplify, there are two categories: weak and strong.(Image credit: Weather.gov)

Moisture comes from regions like oceans, lakes and vegetation that provide the water vapor necessary for cloud formation and precipitation.

Thunderstorms are triggered by various mechanisms that lift parcels of air to form clouds. Surface heating by the sun is a common lifting mechanism in summertime thunderstorm development. Lifting also occurs along boundaries of air masses with different temperature and moisture properties, such as fronts.

A parcel of air will not rise unless it is forced upward away from the surface and/or is unstable — warmer than its surrounding environment. Unstable air occurs when warm, moist air is near the ground and colder, dry air is above. This vertical temperature structure creates an instability that allows the warm air to rise.

Rising unstable air will rise and keep rising when given a nudge upward. To interpret how the environment affects thunderstorm potential and severity, meteorologists have invented several stability indices that characterize the atmosphere in a single number.

A factor affecting thunderstorm intensity is the change of atmospheric wind speed and direction from the ground up, known as vertical wind shear.

Small amounts of vertical wind shear lead to upright and majestic but shorter-lived thunderstorms. Moderate amounts of vertical wind shear cause thunderstorm clouds to tilt. If the wind changes direction and increases in speed to a large extent, the thunderstorm itself rotates.

Rotating thunderstorms generally cause the worst severe weather, including large hail and violent tornadoes. A simple rule of thumb is that the greater the vertical wind shear, the more severe the thunderstorm.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Severe Weather

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Are daily weather forecasts affected by climate change?

Posted on June 15, 2026 by WeatherGuys Editor

A friend of ours recently asked whether the accuracy of day-to-day forecasts of weather is affected by climate change. This is a very interesting question whose answer helps to further elucidate the difference between climate and weather.

As it turns out, predictions of the coming weather are nearly exclusively dependent on the observed conditions of the atmosphere in the day (or days) prior to the forecast period. These conditions are known formally as initial conditions.

Those initial conditions are only partially sampled by the many observational platforms (routine surface observations, upper air observations and various satellite platforms, for instance) that have been devised over the years. These initial conditions must be as accurate as possible as they represent the current state of the atmosphere. Once they are acquitted, they are fed into sophisticated computer programs known as numerical weather prediction models, and the ensuing calculations give meteorologists guidance in creating their forecasts.

Even though the climate is changing in the background, accurate measurements of these initial conditions is not substantially affected by that change and so the forecast models are unaffected. Consequently, it is not likely that climate change has any discernible effect on the quality of weather forecasts.

The behavior of the climate system, on the other hand, is governed by a different set of conditions, known as boundary conditions. These include fundamental measures such as the length of the day, the amount of energy coming from the sun, and the chemical composition of the atmosphere. Though these conditions do change, they only do so on long time scales — scales not particularly important to day-to-day weather forecasts.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Meteorology, Uncategorized

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What is the prediction for the 2026 hurricane season?

Posted on June 8, 2026 by WeatherGuys Editor

Tropical cyclones are large, whirling storms that obtain their energy from warm ocean waters.

A summary infographic showing hurricane season probability and numbers of named storms predicted from NOAA’s 2026 Atlantic Hurricane Season Outlook. (Image credit: NOAA/AOML)

Tropical cyclones with maximum sustained surface wind speeds of less than 39 miles per hour are called tropical depressions. Those with maximum sustained winds of 39 mph or higher are called tropical storms. Hurricanes are tropical cyclones that have sustained wind speeds of greater than 74 mph and that originate in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or the eastern North Pacific Ocean. A general rule of thumb is that hurricanes will not form unless the water temperature is at least 80 degrees Fahrenheit.

A tropical cyclone is assigned a name if its sustained wind speeds are 39 mph or higher. Storms are named to reduce confusion and improve communication when two or more storms occur at the same time. The World Meteorological Organization adheres to a strict procedure in assigning names.

The Atlantic hurricane season officially starts June 1 and lasts until Nov. 30. An average hurricane season produces 14 named storms, of which seven become hurricanes, including three major hurricanes (wind speeds greater than 111 mph).

A below-normal Atlantic hurricane season is expected this year.

There are two competing events that make the hurricane forecast challenging and interesting this year.

El Niño is expected to develop and intensify during the hurricane season, while ocean temperatures in the Atlantic are expected to be slightly warmer than normal. El Niño conditions tend to support fewer tropical storms and hurricanes, while warmer ocean temperatures support a more active year. The National Oceanic and Atmospheric Administration prediction for 2026 is a likelihood of 8 to 14 named storms. Three to six of those are likely to become hurricanes, including one to three major hurricanes.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Tropical, Weather Dangers

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