(Karen Braun is a Reuters market analyst. View expressed are her own.)

By Karen Braun

Dec 16 (Reuters) – The extent of snow across the Northern Hemisphere is alarmingly low, especially considering the increased risk of much colder weather come January.

North America, Europe and Western Asia have all been consistently two to four degrees Celsius warmer than average over the past month. Record warmth across the eastern United States last weekend had residents enjoying weather more typical of late summer.

Seasonably mild conditions thus far have led to the lack of vital snow cover across grain and oilseed production regions of the Northern Hemisphere, and widespread snowfall is unlikely through the end of the year.

This leaves winter crops exposed and vulnerable to the elements, and atmospheric indexes are already indicating greater chances for frigid conditions during January and February.



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Most winter grains across the Northern Hemisphere are currently in good condition, but crop ratings could take a tumble if the snow cover does not arrive before the mercury drops.LESS SNOW, MORE PROBLEMSThe few snowstorms that have been observed across the Northern Hemisphere so far this season are quickly becoming distant memories. For farmers and winter enthusiasts alike, adequate snowfall this winter may seem like a pipe dream.Snow cover is the reason that autumn-sown crops are able to survive harsh winters in climates that would otherwise be unsuitable. The layer of snow provides insulation to the soils and the plant’s root system.Farmers also rely on snow melt as a vital source of soil moisture replenishment once spring arrives. Soil moisture in the Northern Hemisphere is generally sufficient, save in Ukraine, Russia, and parts of Eastern Europe, where moisture deficits are near record lows.Of course for this very reason, too much snow can be detrimental as flooding can occur in the spring upon melting. Also, excessive snowpack is heavy and may damage the fragile grains, especially if the snow turns to ice. So for farmers, a steady, average snowfall pace is ideal.At this time last year, snow cover extended into the majority of the winter crop regions of Russia and Ukraine. By Dec. 31, 2014, the Eurasian continent was blanketed in snow from Germany eastward.But right now, almost every winter crop field is without snow in Europe, including Russia and Ukraine, and given the forecast, this is unlikely to change much through the end of the year

The last time Eurasian snow cover was this sparse was 2011, which led to disastrous wheat harvests for Russia and Ukraine in 2012 following a punishing winter and a drought-like spring.

Snow cover in the United States is also below average, and the chances for snowpack to build anywhere in the Northern Hemisphere are grim through at least the end of 2015.

Warm temperatures are expected to persist for the next couple of weeks in North America, Europe, and Western Asia, meaning that any precipitation is likely to fall as rain, and any snow that does manage to fall is likely to melt right away. Additionally, any snow that is currently on the ground in these regions is likely to vanish.



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RED FLAGS EXIST ALREADYThe lack of snow cover across the Northern Hemisphere is not necessarily a problem so long as winter temperatures remain mild, but there may be a heightened risk that this will not be the case come January.Although many look to El Niño for winter weather clues, phenomena such as the polar vortex or the Arctic Oscillation (AO) are much better indicators in the Northern Hemisphere.If the polar vortex is strong between October and December, history indicates it is likely to be weaker in January and February, releasing cold, Arctic blasts down into the mid-latitudes.Following a strong October, the polar vortex reached record strength in November, explaining at least in part the recent warm weather experienced throughout the Northern Hemisphere. So far, this strong trend fits the mold of a polar vortex likely to take a nose dive in January.The AO is an index based on pressure anomalies over the Arctic region. In the Northern Hemisphere, positive values typically spell out warmer winter temperatures while negative values lead to colder conditions, similar to the AO’s “cousin,” the North Atlantic Oscillation (NAO) (http://tmsnrt.rs/1QKp4FP).Since late October, the AO has been trending positive, which along with the polar vortex has caused the delay in the arrival of winter. (http://tmsnrt.rs/1O30TNP).But the AO could plunge into negative territory during January and February. A linear regression model developed by weather analysts on Thomson Reuters’ Research and Forecasts team indicates that the December-February AO value may average out below zero (http://tmsnrt.rs/1QKnQdU).This model takes various October atmospheric variables into account, including Siberian snow cover. If the AO for December-February is to be as low as the model indicates, January and February values will have to be quite negative in order to offset December’s positive trend.If the AO turns negative at the same time that the polar vortex loses strength, temperatures in the Northern Hemisphere would almost certainly plummet, potentially to extreme levels if the AO ends up considerably negative.Neither the polar vortex nor the AO index is well-correlated with precipitation, so it is difficult to know at this point if snow would be able to accompany the cold – or preferably, precede it.But for now, the welfare of dormant crops across North America, Europe, and Western Asia relies heavily on receiving snow before the Arctic air arrives. If snow remains elusive, global wheat supply may not be able to reach last year’s record-high levels.