The Weather Guys

The jet stream is a ribbon of high wind speeds near the top of the troposphere (about 6 miles above the surface of the Earth). The major jet streams flow west to east. The existence of fast winds moving from west to east was long suspected because of the movement of storms, cloud systems and volcanic debris high in the atmosphere.

Suspicions remained unconfirmed until World War II. During major U.S. air raids against Japan, the B-29 airplanes flew from east to west at altitudes of 10 kilometers, where they encountered a strong stream of westerly winds that slowed or even stopped the planes in mid-air. There were also balloon observations made in the early to mid-1920s by Wasaburo Ooishi, the first director of Japan’s upper-air observatory, that suggested these fast moving winds.

Today’s weather reports often discuss the position of the jet stream. The jet stream position is strongly tied to the southern edge of the dome of cold air centered on the North Pole. During the depths of winter, that cold dome expands considerably, extending nearly to the Gulf of Mexico. As the winter ends and spring approaches, the hemisphere begins to warm up and the cold dome shrinks dramatically. Its southern edge moves to central Canada by early summer.

The jet stream is associated with vigorous upward and downward vertical motions. The upward vertical motions are instrumental in producing thunderstorms. Thus, when the jet stream migrates northward as the weather warms in spring/summer, so does the greatest concentration of severe weather outbreaks. That season is heading our way, denoted by designating this week as Tornado and Severe Weather Awareness Week, with a drill day planned from 1 to 2 pm. on Thursday.

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 100 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. Atmospheric rivers that transport water vapor from Hawaii to North America are 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 that cause severe flooding in western coastal regions of the world, including the West Coast of North America.

In early December 2012, San Francisco received about 10 inches of rain from water vapor transported across the Pacific Ocean by one of these rivers of vapor. While some of these events lead to flooding and mud slides, most precipitation events simply provide beneficial precipitation important to a region’s water supply.

Because of these features’ importance to the water supply of a region, their movements are important to follow and forecast. They are monitored using satellite measurements, aircraft reconnaissance and a new set of instruments along the coast of California. These observations, combined with better numerical modeling, will likely improve forecasts that eventually will help improve water management.