OMAHA (DTN) -- Another round of dry conditions for the already drought-stricken Southern Plains and a second helping of above-average snowfall in the Northern Plains highlight the DTN Ag Weather group's winter forecast.

These general trends are due to La Nina re-developing in the Pacific Ocean and the prospect of blocking high pressure in the far northern latitudes -- a prominent wintertime occurrence over most of the past five years.

La Nina exists when the equatorial waters in the Pacific are consistently at least one-half degree Celsius (about 1 degree Fahrenheit) below normal and, in addition, an ocean barometric feature known as the Southern Oscillation Index (SOI) has a consistently positive reading of at least +8.0 over a 30-day period. Those two criteria were met in September. It's the second year in a row that La Nina is putting in an appearance for the U.S. winter season.

Last year's La Nina was the strongest in almost 50 years. The current La Nina does not have such intensity -- for example, the SOI 30-day value on Friday, Oct. 14 was +11.2, compared with the monthly reading for October 2010 of +18.2. The National Oceanic and Atmospheric Administration (NOAA) and the Australian Bureau of Meteorology classify the current La Nina as "weak." Nonetheless, even a weak La Nina promises to still have an impact on the U.S. winter weather pattern.

For the main U.S. crop areas, that La Nina impact is most noticeable in the likelihood that the Southern Plains -- already coming out of a year when drought damage was historic -- will face a second winter season of very dry conditions.

"The strongest signal for La Nina impact is for dryness in the Southern Plains," said Telvent DTN Senior Ag Meteorologist Mike Palmerino. "And with the severe drought that they've already had there, I just don't see any other way to go. And even with their recent rainfall, you're still looking at soil moisture levels that are below where they were a year ago. It doesn't look good for that region."

The runner-up in the winter forecast signal regatta is the Northern Plains through the northern Midwest, where another cold and snowy winter at this point appears to be on the way. Such a trend, of course, offers a new threat for wet soils to delay field work in the 2012 crop year, along with the potential for a new flood threat in the Missouri River basin after record rainfall and snowpack devastated the basin in 2011.

The Northern Plains outlook has some La Nina relationship, but its likelihood also depends on whether blocking high pressure over the far northern latitudes develops to the extent that it did during the 2010-11 winter season. This feature (blocking high pressure) is much more variable than La Nina.

"Blocking can set up almost without warning," said Telvent DTN Senior Ag Meteorologist Doug Webster.

If blocking does develop, it is capable of altering storm tracks and cold-air flow significantly. This feature, when it develops, is capable of blocking the progress of storm systems working eastward from the Gulf of Alaska across Canada. Instead, those storm systems head southward into the Northern Plains and the northern Midwest. This is what happened last winter; it brought very heavy snow to the northern Rockies and across the Dakotas and Minnesota. Additionally, when the far northern blocking high develops, its influence can lead to a strong push of cold northern Canadian air south and east along the Atlantic coast.

This feature is already prominent in a private weather forecast issued earlier this week for Europe, where below-normal temperatures are predicted for the British Isles and Western Europe, largely due to the presence of the blocking high.

Elsewhere in the central U.S., the outlook is mixed. In the central Plains through the western Midwest, mostly-below-normal temperatures and seasonal precipitation are expected. But, in the southeastern Midwest (Ohio River Valley), heavy precipitation could be on the way during midwinter.

"Precipitation charts show that the Ohio Valley and Delta have a high frequency of wet conditions during a La Nina winter in December-January-February," said Telvent DTN Senior Ag Meteorologist Joel Burgio. Also, the Ohio Valley through northern Delta may be prone to a healthy dose of severe weather when spring rolls around if La Nina is still in effect. "La Nina has a strong signal for severe weather in the Midwest," Webster said.

For the Southeast, the outlook is for some early-season chilly conditions, followed by milder temperatures, while the general precipitation trend is drier once again. But, the development of blocking high pressure can turn this area's temperature trend almost on a dime.

"In a true La Nina, Florida would be warm and dry," Palmerino said. "That's why the blocking and its influence on the cold air flow out of Canada is so key to what happens."

While the Southern Plains dry trend appears to be well-established, it's worth watching what happens early in the season regarding the Northern Plains and northern Midwest forecast for heavy snow.

"If we start to see snow cover established early in the northwestern U.S. and the Northern Plains it enhances the chance of a snowier-than-usual winter," Webster said. "Look for snow in these areas by mid- to late-November -- that's what happened in late fall 2010."

Bryce Anderson can be reached at bryce.anderson@telventdtn.com

(CZ/SK)

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