Friday, January 21, 2011:
I will be the first in line of those willing to admit that this winter is turning out to be much colder than expected before the season started. December went into the record books as one of the coldest on record for the Central Region (ninth coldest ever in a 116-year record set) and the Southeast Region (third coldest ever). So far this month of January is no better, with temperatures since the first of the month running easily below normal in the Plains, western/southern Corn Belt, Delta, and Southeast with some of that area seeing readings averaging four or more degrees below normal. We have a very strong La Ni¤a right now, with ocean waters in the NINO 3.4 monitoring region going to their coldest levels (on average) for this event last week and the Southern Oscillation Index value for December at about its highest level ever recorded. Typically such a La Ni¤a would mean mild winter temperatures for at least the southeastern part of the Nation, but obviously that has not been the case this year. For the second winter in a row, we are seeing the effects of El Ni¤o/La Ni¤a completely outweighed by things such as the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), and now the Pacific-North American Oscillation (PNA). We had a cold winter in 2009/10 despite the presence of El Ni¤o, due in large part to a persistently negative Arctic Oscillation index. So far this winter, the same thing has happened as a persistently negative AO (and NAO) have completely overruled the typical La Ni¤a impacts that are seen in the winter. Recently we have seen the AO reach neutral levels…so then why has it not warmed up (and in fact this morning is about as cold as we have seen it all winter, with lows of -20 and colder for northern Iowa and into much of Minnesota)? That’s likely due to the fact that the PNA, which has been negative for much of the winter (actually a warm signal) is now going positive and is forecast to stay positive. For especially the eastern half of the Nation, temperatures will continue to average below normal for at least the next two weeks and in fact we may not see a lot of abnormal warmth throughout much of the month of February either.
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