What is intermittent snow?

Dane County snowplows clear the Beltline during a 2019 snowfall. (Photo credit: John Hart, State Journal archives)

A weather forecast of snow indicates a steady fall of snow. The forecast may be modified by terms such as “light,” “moderate” or “heavy” to indicate the intensity of the snowfall.

The intensity is measured in terms of visibility during the snowfall:

  • Light snow — visibility of 1 kilometer (1,100 yards) or greater.
  • Moderate snow — visibility between 0.5 kilometer (550 yards) and 1 kilometer (1,100 yards).
  • Heavy snow — visibility of less than 0.5 kilometer (550 yards).

A forecast for intermittent snowfall often refers to light snow falling for short durations. No accumulation, or a light dusting, is expected. The National Weather Service defines snow flurries as intermittent light snow. Snow flurries are short durations of snowfall and produce no measurable precipitation, only trace amounts. Snow flurries tend to come from stratiform clouds.

The term “snow shower” refers to a short period of light to moderate snowfall and is characterized by a sudden beginning and ending of the snowfall. Some accumulation is possible. The atmosphere is susceptible to the development of snow showers when the difference between the surface temperature and the temperature some distance above the ground is large.

Snow squalls are brief, intense snow showers accompanied by strong, gusty winds. Accumulation of snow on the ground may be significant during snow squalls.

Blizzard conditions include winds greater than 35 mph with heavy snow or blowing snow. Visibility is poor, less than a quarter of a mile, and these conditions are expected for a period of three hours or more.

Snow is difficult to measure because it can be moved by the wind. Newly fallen snow is reported in inches to the nearest tenth of an inch. The measurement is taken as soon as the snowfall has stopped. Meteorologists use a snowboard — a flat wooden surface -– and measure in a location where drifting does not usually occur.

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, Seasons

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What is the status of sea ice around Antarctica?

Sea ice around Antarctica reached the lowest extent ever recorded in December 2022 when NASA’s Aqua satellite captured this image of the Antarctic Peninsula and adjacent sea ice.

The Southern Ocean around Antarctica freezes to form sea ice every year. Scientists use observations from satellites to monitor the sea ice extent throughout the year.

Sea ice melt in Antarctica is a seasonal occurrence that usually begins in September (the end of Southern Hemisphere winter) and continues into February (end of Southern Hemisphere summer). As the pole warms during summer, the sea ice around the continent breaks up and melts.

Sea ice extent increases and decreases with the seasons. The decline in the amount of sea ice from winter to summer is very steep in the Antarctic. The seasonal variation is much larger than the interannual difference. Since the year 2013, Antarctic sea ice has exhibited its highest and lowest extents on record, with the highest-ever winter maximum in September 2014, and possibly this year for the lowest-ever summer minimum. The extent of sea ice around Antarctica at the end of December 2022 was the lowest in 45 years of satellite records.

Overall, the long-term annual trend in the amount of Antarctic sea ice is nearly constant. Sea ice at the Southern Hemisphere surrounds a large continent and has more room to expand in the winter than in the Arctic. Because of this geography, that ice also stretches into warmer latitudes, leading to more melting in summer.

Less Antarctic sea ice survives the summer than in the Arctic. On average, in the Southern Ocean only about 15 percent of winter ice cover survives the summer, while about 40 percent of the Arctic Ocean’s winter ice cover remains at the summer minimum. Since so little Antarctic ice persists through the summer, most of the Antarctica’s sea ice is only one winter old and is relatively thin, often about 3 feet or less.

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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How wild a January are we having?

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?

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.

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, Severe Weather

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How remarkable was the recent winter storm?

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.

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, Seasons, Severe Weather

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