Jet Stream To Blame For Flip-flop Weather Says LSU AgCenter Climatologist

John Grymes  |  4/19/2005 10:28:34 PM

Unusual spring and summer weather can be blamed on the jet stream. It refuses to act “normal.”

News You Can Use For September 2004

Thus far, 2004 has been anything but "normal" for most of the Bayou State, regarding climate. The mid-year months proved to be no exception, according to LSU AgCenter climatologist Jay Grymes.

"May and June were the wettest May-June periods ever for many parts of the state, with some areas across the southern parishes recording from 25 inches to more than 35 inches over an eight-week period!" Grymes notes, adding, "Needless to say, Louisiana agriculture really took it on the chin during these critical weeks for many state crops."

The torrential rains eased back during early July, with dry weather setting in for July and early August. In fact, it was too dry for many areas, as farmers who tried to recover from the excess rains of the May and June were receiving too little moisture in the heat of mid summer.

Why the apparent rainfall flip-flop? "Blame the jet stream," Grymes says. This summer has been characterized by an unusually active jet stream, that "river" of rapidly moving air typically found five or more miles above the earth's surface.

The jet stream, and its large-scale pattern of upper-level ridges (northward reaches) and troughs (dips to the south), serves as the highway along which storms travel in their west-to-east migration.

Grymes explains that during the summer, the jet stream's path generally retreats to the north, tracking near the U.S.-Canada border. Although the jet stream may occasionally dive southward during a "normal" summer, that southern shift is usually short-lived, with the summer-season jet stream returning to its preferred northern position until the early fall.

"But the 2004 summer jet stream persistently has strayed from that traditional mid-year pattern," the LSU AgCenter climatologist points out. Instead of its usual northward retreat, the upper-level pattern over the lower 48 states has been marked by frequent upper-air ridges and troughs.

During much of the time between May and early July, upper-air troughs (southward dips in the jet stream's path) repeatedly formed over the western and central United States, a pattern that delivered stormy weather to the Bayou State and accounted for the record and near-record rains for so many parishes.

A transition during late June through early July produced a period of more typical mid-summer upper-level flow, with weak ridging across most southern states and a retreat of the jet stream northward. But by mid July, the jet stream had once again pushed southward, only this time the orientation was a flip-flop from the May-June pattern: a persistent ridge over the western states accompanied by a rain-making trough over the east. That new jet stream pattern extended into August.

This July-August configuration caused a dry pattern for Louisiana, especially for the northern half of the state, as the ridge to the west tended to stabilize the atmosphere. In other words, ridges tend to reduce storminess, and frequently result in warmer-than-normal weather in the summer months.

"Of course, few farmers would consider less rain and higher temperatures in July and August as weather assets!" Grymes says, adding, "The result: a tough year for many Louisiana crops, as reflected by LSU AgCenter production forecasts."

And in the tropics, the hurricane season got off to a slow start – with Hurricane Alex not developing until late July – but don't be fooled.

"Most hurricane experts are standing by pre-season expectations for an active hurricane season, and tropical activity picked up considerably during August," Grymes cautions. Also, keep in mind that September is the peak month for tropical cyclone activity for the Atlantic Basin, and history tells us that more than 40 percent of Louisiana landfalls occur in this month.

"Bottom line: it may be September, but the threat of landfall along the Louisiana coast continues, and the Gulf bears a close watch!" Grymes says.

For useful information on related topics, visit the AgCenter Web site at www.lsuagcenter.com/.  For local information and educational programs, contact an extension agent in your parish LSU AgCenter office.

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On the Internet: LSU AgCenter: http://www.lsuagcenter.com/
Source: Jay Grymes (225) 578-6870, or JGrymes@agcenter.lsu.edu

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