Spring Can Mean Stormy Weather Reports LSU AgCenter Weatherman

John Grymes  |  4/22/2005 2:06:39 AM

Although tornadoes can occur during any month in Louisiana, roughly half of them strike between March and June.

News You Can Use For April 2005


"When it comes to agriculture, home gardening and the local environment in general, spring is probably the most important season of the year from a weather perspective," says LSU AgCenter Climatologist Jay Grymes. "For most years, late winter and spring rainfall establishes the moisture situation for the entire mid-year growing season across the state."

Grymes says it can take weeks or months to recover from moisture shortages that develop during the early months of the year. "Sometimes we never recover," he recalls, as occurred for many Louisiana parishes during the drought years of 1998 to 2000. At the other extreme, an unusually wet spring - like that in 2004 - can delay fieldwork and crop development and even alter crop strategies for the entire season.

Rain is not the only aspect of the season that bears watching. Although the hurricane season (June 1 – November 30) may garner the greatest attention in terms of weather threats, history suggests that the Bayou State's most active weather tends to arrive during the first half of the year.

"Most people forget about the increased potential for severe and violent weather that occurs in the winter and spring," says Grymes.

For example, although tornadoes can occur in any month in Louisiana roughly half of all Louisiana tornadoes occur during the four-month period of March through June. In addition, although flooding from tropical systems can extend over a large area, most of the state's river-basin flooding is associated with frontal weather and not tropical storms and hurricanes.

Why are these winter and spring advancing fronts such a threat? Grymes offers two reasons. First and foremost, air masses colliding along the frontal boundary tend to display vast differences in air temperatures and moisture content at this time of year compared to other months. The advancing air masses from the north tend to be much cooler and far drier than the mild-and-moist air masses from the Gulf. Major differences in temperatures and moisture content between these air masses promote greater "instability," which usually means more and larger thunderstorms.

Second, in addition to the enhanced instability, upper-level winds in the winter and spring tend to be more vigorous, and the input from these winds aloft (the jet stream flow) can further energize developing thunderstorms.

How do we stand through the first months of 2005?

"So far, the winter and spring weather has been reasonably good. Temperatures have been a bit above the norm over the past few months," Grymes remarks, adding, "And it is true that we are a little behind in terms of normal rainfall, but not so far as to have any serious impacts as yet."

The 2005 rainfall through mid-March across the northern parishes was running between 2 inches and 3 inches below the long-term norm, with area totals 1 inch to 2 inches below average for most central parishes. The "driest" parts of the state tended to be across the Florida Parishes, where most stations were reporting rain deficits of 3 inches to 4 inches through the first two and a half months of the year. By contrast, southern parishes tended to report near-normal to slightly-above normal rains thus far.

"These are certainly not critical shortages in northern and central Louisiana, since a single rain could more than make up for those early year shortfalls, Grymes says, noting, "In fact, these modest departures are a bit of good news in some ways, since our traditional spring-season flood threats are reduced somewhat because the landscape is not fully saturated." Grymes stresses, however, that a swing into a few weeks of unusually wet weather could change that quickly.

So what is the outlook, and what is the current thinking regarding the El Niño that developed last year? El Niño occurs when surface waters in the central equatorial Pacific Ocean become warmer than normal. This oceanic warming can have a major impact on the circulation of the atmosphere and the paths of winter storms. For Louisiana and the Gulf Coast region, an active El Niño often results in a wetter-than-normal winter and spring, in part by strengthening the sub-tropical jet stream, which supports the development of winter storms in and near the Gulf.

Fortunately for the Bayou State, the recent El Niño proved to be weak, and most El Niño experts believe that it is dissipating. The lack of intensification by the recent El Niño may explain, at least in part, the state's drier-than-normal to near-normal rainfall over the last several weeks. Without that "warm ocean trigger" of El Niño to fuel storm development, long-range forecasts are calling for "equal chances" of near-normal, below-normal or above-normal rainfall over the next one to three months.

"The truth is, forecasters don't really know which way the rains will turn in the upcoming weeks," Grymes says. "All we can say at this point is 'so far, so good,’ and acknowledge that as long as rainfall over the upcoming weeks is somewhere close to the norm, we should head into the heart of the growing season in very good shape."

For 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: www.lsuagcenter.com/
Source: Jay Grymes (225) 578-6870, or JGrymes@agcenter.lsu.edu

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