The Weather Guys

How wild a January are we having?

Posted on January 17, 2023 by WeatherGuys Editor

Here we are halfway through the month of January, and it has been remarkably mild for those of us in southern Wisconsin.

Through Saturday, January has been 10.3 degrees above normal in Madison and 10.6 degrees above normal in Milwaukee. In fact, since Christmas Day, the temperature in Madison has averaged just shy of 9 degrees (8.93 degrees) above normal in what is usually one of the colder stretches of the year.

Though weather has been savage in many other locations around our country, with floods in California and deadly tornadoes in the South, we have experienced very little wintry weather. Madison has received just 1.1 inch of snow so far — more than 4 inches below normal, with the measly 0.4 inches on Jan. 4 leading the way in this unusually dry month.

Temperatures around the Northern Hemisphere have been unusually warm as well. The areal extent of air colder than minus 5 degrees Celsius (23 degrees Fahrenheit) about 1 mile above the surface is one way to measure this. This year’s value — from Dec. 26 to Jan. 14 — is the record smallest value in the 75 years of data availability, and by quite a wide margin.

Temperature anamolies (departure from normal) on January 17, 2023

Thus, we have all just lived through the mildest three-week stretch at the beginning of Northern Hemisphere winter that has ever been recorded. And, though there is no guarantee this will continue through the rest of the winter, the current seven-day forecast does suggest that unusually warm temperatures are likely to persist across much of North America in the coming week.

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, Seasons

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Are there rivers in the atmosphere?

Posted on January 9, 2023 by WeatherGuys Editor

The term “atmospheric river” has been in the news recently due to the flooding along the West Coast.

An atmospheric river is a narrow band of concentrated moisture in the atmosphere. It is a narrow moisture plume that is a few thousand miles long and only about 250 to 375 miles wide. The term was coined in the early 1990s.

These long, meandering plumes of water vapor originate over the tropical oceans and flow toward the mid-latitudes. While small, atmospheric rivers account for more than 90% of the Earth’s north-south transport of global water vapor. According to the National Oceanic and Atmospheric Administration (NOAA), these rivers transport an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River.

Atmospheric rivers can result from the encroachment of a middle latitude weather system into the northern subtropics — latitudes of about 30 degrees north. The robust circulation of the middle latitude weather system drives air northward on its eastern side and equatorward on its western side. On the Pacific coast of North America, the tropical air can sometimes originate near the Hawaiian Islands, hence the name “Pineapple Express.”

Trans-Pacifc Atmospheric River pattern in late December 2022.

Atmospheric rivers are estimated to provide 30-50% of the precipitation on the West Coast. While they provide the needed water to the region, they can also bring heavy precipitation, as we have seen with the recent central and southern California flooding. When these rivers with high water vapor content 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 rain and snow to the area.

Atmospheric rivers cause $1.1 billion in yearly flood damage on average. About 85% of flood damage in Western states is associated with atmospheric rivers.

Category: Meteorology, Phenomena, Severe Weather

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How remarkable was the recent winter storm?

Posted on January 2, 2023 by WeatherGuys Editor

The recent winter storm that affected large portions of the United States just days before the Christmas holiday was remarkable in a number of dimensions.

It was an example of a “bomb cyclone” which simply means that the rate at which its central pressure dropped — about 2.5% in a single day — was extremely unusual. Even though a 2.5% change in central pressure does not sound like very much, it was responsible for revving up the extreme winds that brought wind chills into the minus 30s and ground blizzard conditions to a large portion of the Great Lakes states on Dec. 23.

In addition, this particular storm was the agent for delivery of an extremely cold Arctic airmass in its wake — dropping the temperature in Madison to minus 12 on the morning of Dec. 23. Since it subsequently only rose to minus 3 for that day’s high, the daily average temperature of minus 7.5 that day ranks as the seventh-coldest December day in Madison since 1939.

GOES East view of the powerful winter storm wrapping arctic air into it’s core (milky orange) while ushering bitter cold conditions south and east across the U.S. (Credit:NOAA)

Another aspect of the storm that was noteworthy was the remarkable degree of accuracy surrounding its prediction. Though there was some change in the days leading to the event, the forecasts for ground blizzard conditions, quite light snow amounts (Madison received 3.7 inches total), and desperate cold were fairly well locked in by Monday afternoon — three-plus days prior — at the latest.

This is a triumph of modern science that often goes unheralded. The combination of unceasing progress in theoretical understanding of the atmosphere, computer science, observing technologies (especially satellite observations), and data science over the past 50 years have led to stunning advances in predictive skill.

Though it is still true that forecasts are not 100% accurate, it is beyond doubt — even in the heart of the most ardent skeptic — that accurately painting the complexion of a future meteorological event days in advance has become the rule rather than the exception.

Category: Meteorology, Phenomena, Seasons, Severe Weather

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What is a mesonet?

Posted on December 26, 2022 by WeatherGuys Editor

In meteorology and climatology, a mesoscale network, or mesonet, is a network of automated environmental monitoring stations designed to observe meteorological phenomena on the mesoscale. In meteorology, “mesoscale” refers to weather events that range in size from about 1 mile to about 150 miles.

Mesoscale events last from several minutes to several hours. Thunderstorms, snow squalls and wind gusts are examples of mesoscale events. Due to the space and time scales associated with mesoscale phenomena, weather stations comprising a mesonet are spaced closer together and report more frequently than the larger synoptic scale observing networks run by the National Weather Service.

The Institute for Rural Partnerships is a great example of the Wisconsin Idea.

Just recently, UW-Madison received funding from the Wisconsin Alumni Research Foundation (WARF) and the United States Department of Agriculture (USDA) to purchase and install a mesonet across Wisconsin. A Wisconsin Environmental Mesonet and the data provided will support a wide range of research interests, including applications that involve many collaborators across the state.

A majority of Midwest states already have some type of mesonet guiding everyday use and decision-making. Wisconsin will now join that infrastructure.

Mesonets provide real-time weather and soil data for anyone to use. This helps support farmers, crop consultants and extension agents, support fire weather prediction, reduce economic losses in agricultural production, assist in weather warning issuance and forecasting, assist in emergency planning and preparedness, improve public safety, and support K-12 education.

Mesonets also provide data to a wide variety of researchers, scientists and teachers. Data from existing mesonets support a variety of weather and agriculture models. Observations from a mesonet can help support and improve the development of local temperature forecasts, as well as predictions for water demand by crops.

Category: Meteorology, Severe Weather

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Can a snowstorm have lightning?

Posted on December 19, 2022 by WeatherGuys Editor

Yes, and if you were awake late Wednesday night you might have observed lightning and heard thunder with the snowstorm.

This video compilation from AOSS rooftop cameras captured thundersnow on the UW-Madsion campus on Wednesday December 14^th around 11:30 PM. (Video courtesy of Pete Pokrandt, UW-Madison Dept of Atmospheric and Oceanic Sciences)

It is not a common occurrence, but when lightning and thunder occur during a snowstorm, the event is reported as “thundersnow.”

Thundersnow is a thunderstorm with snow falling as the primary precipitation instead of rain. It is a rare phenomenon in comparison with summertime thunderstorms, but the underlying mechanisms are the same. The storm has sustained strong vertical mixing which allows for favorable conditions for lightning and thunder to occur.

Lightning is a huge electrical discharge. Static charges form in a storm composed of ice crystals and liquid water drops. Turbulent winds inside the storm cause particles to rub against one another, causing electrons to be stripped off, making the particles either negatively or positively charged.

The charges get grouped in the cloud, often with negatively charged particles near the bottom of the cloud and positively charged particles up high. This is an electric field and, because air is a good insulator, the electrical fields become incredibly strong. Eventually, lightning occurs to neutralize the electric field.

The type of lightning in a thundersnow storm is the cloud-to-cloud variety, as opposed to lightning bolts that travel to the ground. At night, the lightning in a thundersnow may appear bright, as the light reflects off the snowflakes in the storm.

Thunder always accompanies lightning, but in thundersnow storms the sound is muffled by the snowfall. While the thunder from a summertime thunderstorm might be heard many miles away, the thunder during a thundersnow event will be heard only within 2 to 3 miles of the lightning. The thunder is more of a rumble than the booming sounds often heard in summertime storms.