Kevin Myatt is The Roanoke Times' weather columnist.

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Last year, it was El Nino. This year, if the experts are right, it will be La Nina, and that might not be good for anyone wanting to end the dryness or see a lot of snow this winter.

The National Oceanic and Atmospheric Administration issued a statement Thursday that La Nina is expected to develop in the next three months.

La Nina refers to the occasional rapid cooling of central Pacific waters from the coast of Peru westward. It is the opposite of El Nino, which refers to the warming of those same waters.

The central Pacific has been on the verge of La Nina since the last El Nino collapsed in midwinter, with slightly below-normal sea surface temperatures observed over part of the central Pacific. But computer forecast guidance is now showing that the cooling will become more organized and widespread in the weeks and months to come.

El Nino and La Nina are entirely natural phenomena that have been observed for centuries, and more scientifically documented for the past several decades. Each is a recurring event, but neither follows any detectable pattern or rhythm, so it is difficult to detect an El Nino or a La Nina until one is under way.

Each has been linked to various weather patterns around the world. But neither acts by itself, but rather in connection with various other oscillating ocean and air patterns around the world.

So any statement about what La Nina or El Nino causes is only a generality, not a certain cause-and-effect relationship valid in all cases. The effects of either can be overwhelmed or enhanced by a multitude of other atmospheric or oceanic factors.

Last year's El Nino provided a strong example. El Nino has been observed typically to produce a strong subtropical jet stream steering wet storms across the southern tier of the United States.

Last fall and winter, however, many of these storms veered northeast into the central U.S. instead of moving along the Gulf of Mexico coast. As a result, the Plains states got soaked, ending severe drought and setting the stage for flooding this summer, while the Southeast dried to a crisp.

Similarly, La Nina has been historically linked to mild and dry winter conditions through the southern U.S. We're a little north of the area designated, but looking at some of the data, you can see a loose correlation between La Nina and drier, milder winters even in Southwest Virginia.

For instance, in 12 La Nina events from 1950 to 2001, Roanoke's snowfall has been at least 6 inches below normal in seven of them, and has only been 6 inches or more above normal in one. So the odds favor less snow in a La Nina winter.

The principle behind La Nina being linked to drier weather is that less storminess fires in the Pacific because of the cooler water, and this in turn means fewer wet storms crossing the southern U.S.

If it behaves as expected, La Nina could be a development that keeps the Southeast and Mid-Atlantic from getting rounds of desperately needed widespread precipitation in the months ahead.

But as we saw with last year's El Nino, the weather pattern doesn't always follow the script. And what's more, this La Nina hasn't quite developed yet, but is projected to do so. It is somewhat surprising that it has not already developed.

We'll also have to see what other atmospheric players enter the scene as summer begrudgingly gives way to the fall and winter ahead.