John Grymes, Boudreaux, James E., Pyzner, John R. | 4/22/2005 2:03:51 AM
In addition to being drier-than-normal for most parishes, the 2004-2005 winter has been relatively mild in terms of seasonal temperatures, according to AgCenter Climatologist Jay Grymes.
Although December temperatures were a bit below the norm, January and February temperatures were above average, with winter seasonal averages running from one to two degrees above the mean.
"That may not seem like much of a departure, but monthly and seasonal temperatures tell only part of the winter story," Grymes says, adding, "In many cases, the seasonal average temperature may be far less important to the overall impacts of winter weather than the occurrences and severity of the occasional bitter-cold spells for Louisiana."
These winter blasts occur when the upper-air steering currents in the atmosphere - the jet stream patterns - provide an "expressway" for the rapid southward transport of extremely cold-and-dry air from the frozen North all the way to the Gulf Coast.
Although rare, these "arctic outbreaks" in the past have produced single-digit temperatures across much of the state. Needless to say, these freezes can result in widespread environmental, agricultural and economic havoc for the Bayou State, even if they last for only a day or two. Indeed, they have proved deadly in the past.
"Fortunately, Louisiana has been spared from these frigid air masses this winter," Grymes says, noting, "In fact, the occurrences of freezing temperatures in general are running below average for the season." For some areas north of the I-20 corridor, the number of "freeze days" (days when the minimum temperature was 32 F or below) are only about 70 percent of the winter season norm.
The reason for the relatively mild winter weather is, at least in a large part, a function of this winter's jet stream configurations. A dual jet stream pattern has persisted through much of this winter season, with the "northern branch" of the jet stream frequently confined well to our north, and the "southern branch" meandering near and over the Gulf of Mexico. This dual pattern tends to keep the bitterly cold air masses bottled up over the northern states.
"That pattern doesn't mean there will be no cold days when the dual jet is present, but it tends to limit the duration and severity of the cold snaps that do make it as far south as the central Gulf Coast region," Grymes says.
The climatologist adds, "And, let's face it, our mild winter weather has been good for Louisiana residents and businesses, considering the savings they've realized in reduced heating demands. This is especially true this winter, given the recent, rapid rises in the costs of energy."
But these same mild winter temperatures have been a concern for many in Louisiana’s tree fruit industry. Most non-tropical fruit species remain dormant during the winter until they receive a specific number of hours of temperatures below 45 degrees – referred to as "chill hours."
The chilling requirement of a given variety is generally controlled and constant for that variety. When the rest period is satisfied, the plant becomes active with the warmth of spring. Some varieties may have rest periods of only 200-300 hours. Others may have chill requirements of more than 1,000 hours.
Plants that do not receive sufficient cold to satisfy chill requirements do not perform well. Such plants often are delayed in both leafing out and blooming. The bloom period is scattered over a long period and there is a reduced fruit set. Blossoms and fruit may drop prematurely, and fruit quality is often reduced. This year’s peach season may be less productive than hoped.
North Louisiana normally has between 700 and 1,100 chill hours. South Louisiana normally has between 400 and 700 chill hours, and coastal Louisiana has between 300 and 500. Chill hours accumulations in most areas of Louisiana are a couple of hundred hours below normal.
In light of the mild fall and winter, Louisiana citrus growers are grateful for no late-season hard artic freezes, according to LSU AgCenter horticulturist Dr. James Boudreaux. The warm fall did not let the trees slow down growth and develop their resistance to cold. Just before the navel orange harvest in late November, the trees were blooming. That meant the trees were in an active stage of growth and very susceptible to cold injury. If a hard freeze had occurred this year, serious freeze damage to the citrus trees would have resulted.
The mild temperatures were a blessing for strawberries growers, too, Boudreaux adds. It provided them with early berries to harvest in December and January. These early berries helped initiate the market season and bring a premium price to the growers. Early high-price berries help offset the high cost of growing the fruit.
Mild winters also can increase the overwintering success rates of insect pests, especially mosquitoes. With increased concerns across the Pelican State regarding vector-borne diseases such as West Nile virus and encephalitis, health officials and insect specialists will be watching closely during upcoming weeks to see how these pest populations may have responded to the moderate weather.
"So we need to keep in mind that there are some positives to be considered when it comes to Louisiana winter freezes," Grymes says.
Since the first day of spring - officially March 20th - the threat of a freeze or heavy frost effectively ended for the coastal parishes and is very low for all areas south of the I-10/I-12 corridor. Areas north of the I-20 have recorded freezes in late April, but even that is the odd exception rather than the climatological rule.
"Our statistics indicate that the chance of a late freeze drops to less than 1 in 10 by the first week of April for most of the state's northern parishes, with the freeze threat all but ended across central Louisiana," Grymes notes.
Those are long-term statistics, but what about this year?
"Although we cannot completely rule out another brief cold snap - especially the chance for a moderate to heavy frost across the northernmost parishes - there is good reason to expect that the same jet stream pattern that has produced mild weather through most of early 2005 will continue into the early spring. Bottom line: I suspect that most, if not all, of Louisiana has seen the last freeze of the winter," Grymes predicts.
For freeze statistics in your parish, contact Jay Grymes. 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.
On the Internet: LSU AgCenter: www.lsuagcenter.com/
Jay Grymes (225) 578-6870, or JGrymes@agcenter.lsu.edu
John Pyzner (318) 644-5865, or Jpyzner@agcenter.lsu.edu
James Boudreaux (225) 578-2222, or JBoudreaux@agcenter.lsu.edu