The idea of “persistence” — a regional weather pattern that holds and repeats until somehow being wrenched from it — gets a lot of attention among climatologists.
“Once we get locked into a pattern — and, as an aside, these patterns can play themselves out on various time scales — even for one season, the chances for the next season to be in the same pattern are higher than the chances of change,” says Barry Keim, an LSU climatologist stationed in Baton Rouge, La. “If we have a very dry summer, there's an enhanced probability there will be another dry summer to follow. There is clearly some persistence built into the atmosphere like this.”
One of those patterns affecting Louisiana is the afternoon convective summertime shower. Keim recalls his time as an LSU graduate student from the late 1980s through the early 1990s. At that time, “you could almost set your watch by the daily afternoon showers.
“Then, I went away for eight years. When I came back, those afternoon convective showers had faded. They just don't occur with the same regularity.”
Keim checked the drought index for Louisiana, which, on average, is the wettest state in the union. Since 1999, 64 percent of the statewide drought values for the state have been negative.
“So two-thirds of the months in the last 10 years have seen some moisture stress compared to the long-term averages.”
Using the persistence factor, Keim expects the coming summer to be arid.
For the June/July/August temperature outlook, “there are normal conditions expected in the state, although there is an axis of chances for below-normal temperatures running across the Southeast into Tennessee, southern Illinois and Missouri.”
For precipitation, Louisiana is expected to be in the above-normal range, despite Keim's hunch.
What about hurricanes? “Folks who watch hurricanes say that the tropical season will be busy. There are two factors that point to an active season. One is a fancy index that looks at the sea surface temperatures of the north Atlantic. That index shows that certain parts of that region — which is, in essence, the breeding grounds for hurricanes — have temperatures that are warmer than normal. That means we can expect more hurricanes.”
That is modulated by the El Niño/La Niña phases. “An El Niño tends to mitigate hurricane seasons. If we're in a deep El Niño, it creates a hostile upper air environment over the Atlantic Ocean and that breaks up a lot of storms and we don't get quite so many.”
However, a La Niña “creates a very favorable upper air environment over the Atlantic Ocean and we get more storms.”
Going into this season, “we have warmer sea surface temperatures along with a weak La Niña. Put those together and the forecast shows an active hurricane season — not incredibly active, but higher than long-term averages.”
Of course, no one knows where the storms will go.
“Last year, we had storm numbers above the long-term averages. But the really bad storms tracked away from the United States. In fact, the two worst storms went across the Caribbean and into Central America and the Yucatan Peninsula.
“It was just the luck of the draw that they were steered in that direction instead of (the U.S.) coast line. Actually, I don't like to use the term ‘luck.’ Yeah, we lucked out in the sense that it didn't hit here. But we've had our fair share of those type storms in recent years. Last year, it wasn't our turn.”
In 2007, there were 15 named storms. Ten named storms is the long-term average.
“The perception by most Americans was it wasn't an active year. But that's only because the really bad storms didn't hit us. And thank God for that — the last thing we need in Louisiana is another bad hurricane to hit.”