A Look Ahead … And … A Quick Look Back

The weather pattern has become quite active with Pacific storms hitting the west coast and moving straight across the country.

The late morning full disk satellite image shows three major storms over the northern half of the western hemisphere. One over the north Atlantic, one moving in to the Atlantic seaboard and a third coming onshore along the Pacific coast. These storms are essentially moving from west to east. This pattern tends to moderate temperatures to a degree but allows for stormier conditions.

The Friday morning forecast map above shows the east coast storm beginning to move off the Mid Atlantic and southeast coast with its’ expected mix of rain, sleet and snow. The Pacific coast storm has progressed into the Rocky Mountains.

The pattern will drag the storm out west across the Great Lakes and Mid Atlantic region late in the weekend or early next week. With extremely cold air bottled up in Canada the storm should be mostly rain for our region.

This situation speaks to Tom’s previous entry. You should check it out. It addresses the swings in patterns that have affected the region so far this winter.

The early portion of this winter season was a cold one with the jet stream diving deep into the southeast U.S. One storm moving around the periphery of this cold air mass gave the area the big December snow.

The path of the jet stream this time of year is more or less controlled by the “Arctic Oscillation”. It is a pressure field that sits over the Arctic region like a rock in a stream with the large scale northern Hemisphere air flow moving around. Its’ position shifts around during the winter months and that shift is responsible for things like cold waves and “January Thaws”. This year’s early winter was dominated by a “negative phase” of the oscillation and the polar jet stream dove deep into the United States, as Tom’s graphic in the previous entry demonstrates. The U.S. wasn’t the only region of the world affected by this “negative phase.

The analysis above shows that Northern Europe, Russia and Northern Asia were also hit by unusually cold air. News at the time was full of stories of extreme cold and snow in these regions. In contrast, the U.S. West Coast, N.E. Canada and Greenland, North Africa and Central Asia were warmer than usual.

At the moment the “Arctic Oscillation” is in a more positive phase and weather over the eastern U.S. is not quite as extreme as far as cold temperatures are concerned. The “Oscillation” will likely swing more negative over the next few weeks and another shot of very cold air is possible.

This year the shorter term oscillations of pressure patterns over the Arctic regions are laid over the longer term El Nino pattern over the equatorial region of the Pacific. This pattern tends to generate an active storm pattern across the southern U.S. The combination of the two patterns this year, on average, has made this winter season in the Mid Atlantic region much more active, and so far, colder.

John Collins


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