Imagine looking down on the continental United States from a satellite. If you could have done that over the last six weeks, you would have seen that the Mississippi River Valley represents a great divide as to where storms have tracked.
“Everything to the east of the valley is far wetter than normal,” says Barry Keim, regional climatologist at the Southern Regional Climate Center in Baton Rouge. “On the west side, things deteriorate quickly — by the time you get into Texas, Oklahoma and parts of Missouri, drought conditions are evident.”
Contrary to popular weather theory, Keim says the Mississippi River is too small to have any major impact on the steering of storms.
“What's steering the storms is the location of the jet stream, paths of the tropical cyclones and things of that nature. The Mississippi River itself is an arbitrary boundary — it just happens that's where the divide is currently. For whatever reason, the central U.S. has a ‘trough’ that has held storms to the east.”
Those who forecast the weather like to blame everything on the jet stream, says Keim.
“The jet stream has been in a particular configuration where all the storms are tracking along the spine of the Appalachian Mountains. This has been a very persistent pattern for the last six to eight weeks. That's very typical for the atmosphere.”
With weather, there seems to be a built-in persistence, says Keim. Once a pattern is set up it tends to stick. While there are movements away from the pattern, storms will retrograde back until, without any notice months later, they'll snap into a new position.
“We can't predict when those changes — those snaps — will occur. That's why everyone gets down on us for our long-range forecasts.”
Regarding long-range forecasts for the Delta, temperatures should be normal, says Keim.
“Through November, forecasters are giving temperatures (in the Delta states) what's called ‘equal chances.’ That means there are even odds that the temperature will hit the norm — there are no indicators that it'll swing one way or the other.”
For long-range precipitation, Keim says, there are suggestions that the extreme southeastern U.S. will be wetter than normal. The states that could be most affected by this are Florida, Georgia and the Carolinas. But this potentially wet region extends into parts of Alabama and Mississippi.
The reasons for the wet forecast are varied. Given the move into La Niña conditions, Keim and colleagues are expecting an enhancement in the tropical season.
However, there's little suggestion that the western side of the Gulf of Mexico has anything to worry about storm-wise. Other than Mississippi, that holds true for Delta states as well. The Delta should see normal rainfall, he says.
And while the forecast is for a more active tropical storm season, there's no forecast on whether those storms will ever hit land.
“On paper, the tropical storm season begins June 1 and ends the last day of November. But, in truth, the ‘bite’ of the season begins in mid-August and runs through September with the peak on Sept. 10. So, we're starting the heart of the season right now.”
While Keim says the Delta forecast for harvest season is a good one, he offers a caveat. “All it takes is one or two good storms and the complexion of this will change dramatically. For example, last year's tropical storm Isidore produced over 25 inches of rain in extreme southeastern Louisiana. Almost all of Mississippi had 10 inches or more. That storm was impressive and it could happen again. Everyone should keep an eye on the tropics.”