What is an atmospheric river?

Posted on June 20, 2022 by WeatherGuys Editor
An atmospheric river system that drenched parts of California on 14 February 2019— dropping more than 25 centimeters of rain at Palomar Observatory northeast of San Diego, for example, in the observatory’s wettest day on record—is seen in this image from the National Oceanic and Atmospheric Administration’s GOES West satellite. (Image credit: NOAA)

Atmospheric rivers are relatively narrow regions in the atmosphere — typically 250 to 375 miles wide and well over 1,000 miles long.

These sky rivers transport water vapor outside of the tropics to mid-latitude and polar regions. We estimate that 90% of Earth’s north to south water vapor transport is done through atmospheric rivers.

An atmospheric river is identifiable in imagery from weather satellites, appearing as elongated tendrils of moisture stretching from the tropics to the mid-latitudes. This moisture laden air moves away from the Equator and can carry enough water vapor, that if condensed to liquid form, is approximately equivalent to the amount of water carried by 25 Mississippi Rivers.

When an atmospheric river flows over land, extreme flooding events can occur, often through interactions with mid-latitude weather systems. Think of an atmospheric river as a conveyor belt that provides huge amounts of tropical moisture into mid-latitude weather systems, intensifying their rainfall. Extreme, or prolonged, precipitation can create floods, induce mud slides, and cause catastrophic damage to life and property.

Most rivers in the sky, however, are weak and simply provide beneficial rain or snow that is crucial to the water supply of a region. Atmospheric rivers are responsible for a large percentage of the rain and snow in the western United States during winter. When these rivers are forced up the sides of the Sierra Nevada mountains in California and Nevada, the water vapor is condensed into liquid and solid form, bringing much needed rains to the area.

Categorizing the flow of moisture by an atmospheric river can be a useful way to gage the likely impact of these features on precipitation. For example, an Atmospheric River Category 1 (AR Cat 1) is the weakest category and is considered primarily beneficial in terms of rainfall. An AR Cat 2 is moderate and in most situations beneficial, but can be hazardous if the resulting precipitation lasts a long time. The strongest category is AR Cat 5 and that is an exceptional and primarily hazardous event.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena, Weather Dangers

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Did weather forecasting play a role in D-Day?

Posted on June 13, 2022 by WeatherGuys Editor
American assault troops move onto a beachhead code-named Omaha Beach, on the northern coast of France, on June 6, 1944, during the Allied invasion of the Normandy coast. The success of the invasion was extraordinarily dependent on weather conditions. (Photo credit: Associated Press)

Last week was the 78th anniversary of the Allied invasion of Europe that began with the landings on the beaches at Normandy. The combined land, air and sea assault of June 6, 1944, remains the largest such event in history.

The success of the invasion was extraordinarily dependent on weather conditions. More than three months before the invasion, a combined British and American forecasting team began rigorous forecast exercises designed to iron out the physical and logistical kinks of such a coordinated effort.

As June drew near, the nature of this collaboration was still problematic as the two groups employed vastly different methods in fashioning the requisite three- to five-day forecasts. The British were attempting to make such forecasts based upon the understanding of atmospheric dynamics that had grown substantially during the war. The Americans were employing a method based on a statistically based search through old weather data for historical analogues that could be used to guide the forecast.

To maintain secrecy, a large portion of the Allied fleet was squirreled far away in northern Scotland. Consequently, five days of lead time was required to mobilize these forces. Thus, Gen. Dwight D. Eisenhower needed to know by May 31 whether the first week of June, the prospective target for the invasion, would provide favorable weather.

The forecasters foresaw a break in that year’s unusually stormy late spring and suggested June 5 would work. As the day approached, the team realized that a one-day postponement would offer better conditions, prompting Eisenhower to make the fateful decision to invade on June 6, under barely acceptable conditions.

Had the Allies delayed, the combination of lunar cycle, tides and weather almost certainly would have postponed the invasion for more than a month, likely costing the effort the tremendous advantage of secrecy.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: History, Meteorology

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How can you report tornadoes?

Posted on June 6, 2022 by WeatherGuys Editor
National Weather Service’s Skywarn logo
(Image credit: weather.gov)

The National Weather Service is always looking for trained volunteers to provide severe weather reports, including reports of tornadoes.

Its spotter training sessions are free and last between 90 minutes and two hours. Trained volunteers receive certificates and are added to the National Oceanic Atmospheric Administration, or NOAA, database of weather spotters. They also receive a Spotter ID. A virtual training option is also available.

The spotter program is informal and voluntary. Observations can be made as spotters carry out their daily routines.

The training covers the basics of thunderstorm development and fundamental storm structure. Spotters learn to identify potential severe weather features and how to report the information. Basic severe weather safety is also covered.

SKYWARN is a NWS volunteer program with more than 350,000 trained severe weather spotters. Volunteer spotters relay their severe weather reports to the weather service as they observe them. Their observations are used by forecasters to track storms and alert the public to dangerous weather situations.

NOAA is also seeking volunteers to help collect additional data about people’s severe weather experiences. Their new Tornado Tales citizen science tool is an online survey in which participants submit anonymous reports of their tornado experiences. The information will be used by NOAA’s National Severe Storms Laboratory (NSSL) to better understand how people receive, interpret and respond to tornado information from NOAA.

NOAA collects a lot of science data about storms from satellites, radars and weather stations. It has less data about what people do when tornadoes strike, or are about to strike. Tornado Tales — go.madison.com/TalesTornado — gathers information about what people affected by severe weather are really doing. Hopefully, this will generate understandings that NOAA social scientists can use to refine messaging about tornado warnings and watches.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Severe Weather, Weather Dangers

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What does the hurricane season look like?

Posted on June 1, 2022 by WeatherGuys Editor

The official Atlantic basin hurricane season runs from June 1 through Nov. 30, with an average of 10 to 15 storms each year.

2022 Atlantic Basin Hurricane Season Infographic. (Credit: NOAA)

The peak of activity in the Atlantic basic runs from mid-August to mid-October. During that subset of the entire season, more than 70% of all storms in the last 100 years have occurred.

The National Oceanic and Atmospheric Administration, or NOAA, predicts that there is a 70% chance that this year’s Atlantic hurricane season will include 14 to 21 named storms, or storms with winds of 39 mph or higher. Of those, NOAA predicts that six to 10 hurricanes will have winds of 74 mph or greater, and three to six major hurricanes will have winds of 111 mph. This could be the seventh year in a row that the hurricane season is more active than usual.

Hurricanes are given people’s names because it is easy to identify one storm from another and the names are easy to remember. When a tropical storm reaches wind speeds of 39 mph, the storm is then called Tropical Storm “Blank,” the blank being whatever name is next on the alphabetical list. A tropical storm is classified as a hurricane when it reaches wind speeds of 74 mph. If the storm turns into a hurricane, it keeps its original name and is referred to as Hurricane “Blank.”

The first storm of the 2022 year will be named Alex, and the next four will be named Bonnie, Colin, Danielle, and Earl, according to the National Hurricane Center.

The Saffir–Simpson scale rates hurricanes on a scale of 1 to 5 based on the damage their winds would cause upon landfall. Major hurricanes are those classified as Category 3 and higher on this scale. Category 3 hurricanes have a one-minute sustained wind between 111 mph and 130 mph. The one-minute sustained winds in a Category 5 hurricane are greater than 155 mph.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Severe Weather, Tropical

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Is 2022 an active tornado season for the U.S?

Posted on May 23, 2022 by WeatherGuys Editor

Tornadoes can occur anytime of the year, so it is too early to answer this question.

Tornado damage in the town of Dunkirk near Stoughton Wisconsin from March 2022.
(Photo credit: State Journal archives)

Tornado season is based on when the ingredients for severe weather come together in a particular region. 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 happen at any time of day or night. However, they thrive on solar heating and in some cases the ability of warm, moist air at the surface to penetrate a low-level capping inversion. Therefore, 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.

As of Friday, there have been 573 confirmed tornadoes in the U.S., causing 15 fatalities. The number of tornadoes puts this year at the 75th percentile climatologically.

The ongoing La Niña is predicted to continue through spring and possibly summer. La Niña conditions favor a more active severe thunderstorm and tornado season in the US.

Tornadoes are classified based on the damage the tornado does, which enables us to estimate the wind speed of its rotating winds. All tornadoes are assigned a single number from the Enhanced Fujita scale, abbreviated EF, according to the most intense damage caused by the storm. The weakest tornado is EF0, with winds less than 85 mph. The strongest is EF5, where the winds are greater than 200 mph. As of Friday, there have been no EF5 tornados, 15 EF3 and 2 EF4 tornados.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Meteorology, Severe Weather

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