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During the past 24 hours, the models have come into better agreement about how Sandy will interact with several unusual weather features. Those converging events include a large dip in the jet stream into the eastern U.S., a powerful subtropical jet stream moving across the southern U.S., a massive area of high pressure that will be parked over northeastern Canada and southwestern Greenland, and a storm in the Central Atlantic. These features may help steer Sandy right into the Mid-Atlantic or New England.

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The high pressure area near Greenland, in particular, may act as a block (it's technically known as as a “blocking high”), which may help prevent Sandy from moving out into the open ocean. While it is not unusual to have a high pressure area in that location, its intensity is striking for this time of year. As Jason Samenow of the Capital Weather Gang wrote, the North Atlantic Oscillation, which helps measure this blocking flow, "is forecast to be three standard deviations from the average — meaning this is an exceptional situation." (my emphasis)

Recent studies have shown that blocking patterns have appeared with greater frequency and intensity in recent years, which some scientists think may be related to the loss of Arctic sea ice as a result of global warming. The 2012 sea ice melt season, which just ended one month ago, was extreme, with sea ice extent, volume, and other measures all hitting record lows. The loss of sea ice opens up large expanses of open water, which absorbs more of the incoming solar radiation and adds heat and moisture to the atmosphere, thereby helping to alter weather patterns. Exactly how weather patterns are changing as a result, however, is a subject of active research.

I'm often asked by friends and neighbors what my forecast for the coming winter is, but I usually shrug and ask them to catch some woolly bear caterpillars for me so I can count their stripes and make a random forecast. Making an accurate winter forecast is very difficult, as there is too much that we don't know. I've learned to expect the unexpected and unprecedented from our weather over the past two years, so perhaps the most unexpected thing would be a very average winter for temperatures. The one portion of the winter forecast that does have a high probability of being correct, though, is the forecast of dry conditions over Texas and surrounding states. Extreme droughts tend to be self-reinforcing, by creating high pressure zones around them that tend to deflect rain-bearing low pressures systems. The unpredictable AO doesn't affect weather patterns that much over Texas, so we can expect that the fairly predictable drying La Niña influence will dominate Texas' weather this winter.