Persistent rainfall and flooding across the central US is being caused by an unusual condition in the jet stream that has meteorologists scratching their heads. Normally in midsummer, the jet stream rises to the north and becomes weaker, flowing over central Canada and occasionally dipping down into the US, bringing outbursts of summer thunderstorms with it. This year, however, it has been acting in an erratic manner. In June, it looped high into the arctic, bringing record temperatures to Alaska and Siberia and accelerating summer ice melt dramatically. Then it dropped further south than usual, the summer melt ended and became normal, but the central US began to experience a cooler and wetter summer. Until August 10, one loop of the stream was stationary over the US midwest. This caused extensive flooding until the stream finally looped down, bringing drier weather to most of the region.
Why is this happening? Some experts believe that it is due to the very high levels of polar melt that took place earlier in the summer. Others are less sure. It seems likely that the reduction of solar output that has been taking place over the past few months has caused the melt to reverse and destabilized the jet stream. Normally, solar output would be rising at the height of a solar maximum, but so far this has hot happened. While the variations are small, they do have a demonstrable effect on Earth's climate. If solar output continues to decline during and after this exceptionally quiet solar maximum, and this is indeed an influence at this time, then the jet stream will return to normal at some point over the next six weeks, and the northern hemisphere will have a normal autumn and winter.
Strange weather has now been with us for some time. The spring and summer of 2012 brought extreme drought conditions. This spring and summer have been at the opposite extreme. The winter of 2011--2012 was unusually mild, the winter of 2012-2013 unusually violent, with more than its share of northeasters.
In October, Superstorm Sandy was one of the most unusual weather systems ever recorded. Not only did it expand to record size, it moved insland across New Jersey instead of rolling up the US and Canadian east coasts.
This spring, some of the most violent tornadoes ever recorded took place, but last spring saw a record low in tornado numbers.
The situation is so complex, with so many unknowns, that it is likely to continue to bring surprises. For our best evaluation of mid-term climate and a quick read of current underlying conditions, keep up with Unknowncountry.com's Climate Watch.
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