For those who bore the brunt of storms that raged through areas of the Delta a few weeks ago, it must seem strange to hear that others in the region are still suffering from lack of moisture. But that's exactly what's happening, says Barry Keim, regional climatologist with the Southern Regional Climate Center in Baton Rouge, La.
“In looking at the last 30 days across most of the Delta — especially in Louisiana — some areas are deficient in precipitation. Drought conditions prevail across significant portions of both Texas and Louisiana,” says Keim.
Too much or too little
But there are marked pockets of exception. The Gulf Coast of Mississippi is at 150 percent “or more” of normal. Keim also says it appears most of Mississippi is doing okay rainfall-wise, at least for the last 30 days or so.
Western Tennessee is hurting worse than most areas of the state coming in at 50 percent or less of normal precipitation. The exception to that is the extreme southwest part of the state, around Memphis. That area is quite a bit above normal precipitation levels.
“But if you cut Tennessee in half, east-west, and go to the western half and put a bulls-eye right in that middle, that's coming in at 50 percent or less of normal,” says Keim. “There's a sort of spine that runs from northwest Arkansas through Memphis and on through parts of northern Mississippi. That swath of land is above normal on the precipitation meter. Once you get away from that, though, things dry off very quickly. There are plenty of spots within that area, though, that are above 150 percent of normal.”
Coming tropical season
The tropical season is expected to be above average.
“There's a gentleman at Colorado State University named Bill Gray. He puts out a hurricane forecast annually. His most recent prediction (which came out at the end of May) shows that all indicators are showing a strong hurricane season.”
Yearly, the average number of named storms (tropical storms that don't reach hurricane status) is 9.6. This year, Gray is predicting 14 named storms. For actual hurricanes, the long-time average is 5.9 annually. This year, Gray says to expect eight hurricanes.
Why more hurricanes?
In predicting more hurricanes, the main indicator to Gray and others is the La Niña effect. El Niño, which was experienced last winter, fell apart in March and April. Keim and others say we're expected to move into a La Niña as we approach the second half of the year.
“Such forecasts are somewhat sketchy, but there are indicators that we'll soon move into a La Niña,” says Keim.
What is La Niña?
Keim says under normal conditions, off the coast of South America there's cold water that's been dragged there from the Antarctic. During an El Niño, there's a “back-sloshing” of warm water from the tropical Pacific that moves in to that area. During a La Niña, there's an intensification of the cold water. That results in a host of impacts that are usually opposite of the things seen during an El Niño.
“La Niña conditions create a very favorable environment for hurricane formation in the north Atlantic basin,” says Keim. “Even though the La Niña phenomena takes place in the Pacific, it actually affects what goes on in the Atlantic.”
Sea surface temperatures have also been higher than normal for much of the year in the north Atlantic. That also favors more hurricane activity.
“A couple of good hurricanes could bust the drought. Of course, we don't want the big one that causes a lot of damage. We'd prefer a weak system that offers a few days of rain to dispel a lot of the current problems with drought.
“The bottom line is we have no real indication yet when these hurricanes could hit. The heart of the tropical season runs from mid-August through September. Most hurricanes take place during that window.”
In terms of a seasonal forecast, the Climate Prediction Center puts out good data for long-range forecasting, says Keim. But he has a caveat.
“Right up front let me say that long-range forecasting is still not very good — these are just indications. But they're showing that most of the southern tier states — all along the Gulf Coast, through Texas and all the way to California — should expect a warmer than normal summer. Precipitation-wise, they're predicting a normal summer across nearly the entire country.”
The latest drought monitor is showing that across southern Louisiana both agricultural and hydrological drought is being experienced. Most of the region is classified as “moderate” drought although there's a pretty significant pocket growing of “severe” drought, says Keim. A moderate drought is classified as a D-1 (drought levels go from a “moderate” D-1 to “exceptional” drought at D-4). Parts of extreme southwestern are now in a D-2 stage.
Although there are parts of Mississippi that are abnormally dry, none of it is classified as being in drought. The southern half of Arkansas is also abnormally dry.
“Those areas aren't at drought yet, but clearly if things don't change they'll soon move into a drought category,” says Keim.
In the area of Louisiana classified as in both an agricultural and hydrological drought, the implication is the drought is occurring on two time scales.
“The agricultural drought is a shorter-term, soil moisture perspective. The hydrological drought is a much longer-term perspective on rainfall deficiencies that will affect reservoirs, ground water and things of that nature. My guess is that to make the agricultural drought go away, a good 3-inch to 4-inch soaking over a few days would be what the doctor ordered. We don't want those inches in just an hour but stretched out over a few days.”
The hope is that as we shift into summertime, “a normal, convective afternoon shower pattern will emerge. Lately, we've had the heat and humidity in place, but not the mechanism to generate the afternoon showers. My hunch is that will arrive — hopefully very soon — and many of these dry conditions will be alleviated.”