The Weather Guys

Total precipiatable water measured by water vapor sensors on satellites depicting an atmospheric river from February 2017. Credit: CIMSS

An atmospheric river is a term used to describe a relatively narrow region in the atmosphere that transports water vapor outside of the tropics northward.

They are typically a few thousand miles long and a hundred miles wide. There can be three to five of these “rivers” at any time covering the hemisphere.

Atmospheric rivers are apparent in satellite images and are often in the vicinity of fronts over the oceans. They transport large amounts of water. A common atmospheric river is one that transports water vapor from Hawaii to North America, sometimes nicknamed the “pineapple express.”

While these atmospheric rivers are made of water in the gas form, or water vapor, they can carry as much water as a dozen Mississippi Rivers. The features can transport water into storms and result in extreme precipitation events which cause severe flooding in western coastal regions of the world, including the West Coast of North America.

Much of the precipitation and resulting flooding occurring on the West Coast is from water vapor transported across the Pacific Ocean by these rivers of water vapor. While some of these events lead to flooding and mud slides, most precipitation events simply provide beneficial precipitation important to the region’s water supply.

Much of the water vapor transport to Wisconsin comes from the Gulf of Mexico and not the tropical Pacific Ocean. Water vapor transport by atmospheric river coming from the Pacific Ocean has to flow over the Rocky Mountains, where much of the precipitation falls out.

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.

A frozen lake is a public good for all.
Photo Credit: John Hart, WSJ

It’s no secret we have had some amazingly warm weather the past few days. After reaching 60 Fahrenheit Friday February 17, the temperature soared to 62 F Saturday in Madison, breaking the previous record for the date (57 F) set in 1981.

In Milwaukee, the record-shattering was even more extraordinary as the Saturday high of 67 F broke the old record for the date – set in 1877—by 10 degrees! In fact, Milwaukee’s high of 67 F was just one degree short of its all-time February record of 68 set on Feb. 11, 1999.

Madison’s 62 F on February 18 was only two degrees shy of its all-time monthly record of 64 F set on February 25, 2000.

These are clearly exceptional temperatures!

Combining Friday and Saturday’s warmth with what appears likely in the short-term forecast, both cities may be in the running to record another unusual meteorological event by Wednesday of this week.

It is quite rare for six consecutive days in February to have high temperatures of at least 50 F. This last occurred in Milwaukee in 2000 while the wait has been much longer in Madison – not since 1930!

The expansive ridge of high pressure that is responsible for our warm spell appears reluctant to move so we may well see the longest February warm streak in Madison since Herbert Hoover was living in the White House.

In addition, it is not inconceivable we could set a daily record on every one of those days. It is a welcome respite from winter and should serve as an encouraging reminder of what is to come.