There has been an incredible difference in the character of the weather pattern over the central US during the early spring from 2012 to 2013. We have gone from record warmth in 2012 to some very cold weather in 2013--including some freeze damage to jointing wheat in Texas and Oklahoma. And in Iowa, the difference in the state average temperature in March of 2012 compared to March of 2013 is an astounding 23 degrees Fahrenheit.
Possibly the main reason for this incredible difference is the presence--or lack--of high latitude blocking. We have spoken of blocking before and its impact on the weather patterns in the US.
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When we see a lot of blocking (high pressure) over Alaska, northern Canada and Greenland, it promotes cold weather in Canada with some of the cold air moving southward into the US. It also promotes an active southern branch storm track between the cold air to the north and the warm air to the south. This is the pattern we are dealing with now that got underway in mid-February. When blocking is lacking the cold air recedes into northern Canada. The southern branch of the jet stream is much weaker due to the lack of temperature contrast from north to south. This can allow for subtropical high pressure to build northward out of the tropics into the US. This is what happened in 2012.
The problem with blocking is that it is not well forecasted by the models. You do not receive a lot of notice from the models that blocking will develop. They will often try to weaken the blocking pattern once it becomes established which we have already seen a few times since mid February without success.
The impacts of high latitude blocking have been found to be so significant on weather patterns in the US as well as the entire northern hemisphere that a strong effort will be made by the National Center for Atmospheric Research to better understand and forecast this feature.
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