Official USA Winter Forecast 2012-13

In recent months I have presented to you background explanation as to why I expect the US to see a colder winter this year compared to last winter and here is the final product: my official USA Winter Forecast 2012-13.

Courtesy of Mark Vogan

Given my expectation of a winter dominated by high latitude ‘blocking’, this winter should be colder than normal for the eastern two thirds while overall, the West should average out slightly above normal but that doesn’t mean it won’t get cold. I feel confident that this year will be very different to last year where it was downright balmy for the most part. The timing of cold spells will not be too dissimilar to that of the UK, given the teleconnection.

I expect November to be somewhat of a roller-coaster with a potentially cold first part, followed by a milder second part, similar to the UK. An early cold spell could hit hard and bring more record cold for a large chunk of the country. However I expect a zonal mild flow to kick in mid-month onwards.  In saying that, November may end with a flip from cold to warm over Alaska and when this happens, one should watch out for cold dropping into the Lower 48. This would send a trough into the Great Lakes and East, so not only cold, but lake effect snows could be expected.

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Arctic Air Enters West In Early December, Dominates East Second Half

The period from November 28th through December 8th may see a deepening trough over the Interior West, which sees a plunge of bitter cold into the Rockies. Ridging would replace a cold trough up over Alaska. All the cold air built up over the region would come pouring south from November 15th onwards but this should dive into the West first and then migrate east during the second half of November. While the front end of November could be significantly cold, the back end may be unusually mild but this could be in reaction to the deepening of the western trough and cold pool.

From December 1st onwards, watch for this Arctic air migrating east from the Rockies to Ohio Valley. The timing of this may coincide with the return to a -NAO and as pressure rises over Greenland, the trough would get forced east. Watch out from December 10th towards Christmas for a major cold period from the eastern Plains through Great Lakes, Ohio Valley to East Coast. This period, with a potentially -NAO and possibly negatively tilted troughs, would provide an excellent breeding ground for at least one or two snowstorms of 3-6 inches for the East Coast. Watch out for some very cold air driving all the way down to central Florida too.

Warm Spell to End December/Start January?

The period between December 1st through to December 15th should see cold in the West and from the 15-25th in the East with coldest compared to normal, centred over the Ohio Valley. While I am leaning towards a significant period of cold, what I am unsure of, is whether there is a full latitude or long wave trough which has the majority of the country locked in the cold or, whether the trough is shorter in wavelength and therefore it’s cold in the west and warm in the east. That idea remains to be seen, however.

From near Christmas through to January 5th, we may see a recovery with the flow flattening out and the Arctic air retreating. This would coincide with my idea of the NAO recovering back to neutral or even temporarily positive. Some may think that winter is over early but given the kind of NAO we’ve had all year and how warm those waters are in the North Atlantic, the block should redevelop.

Major Mid to Late January Arctic Outbreak?

From January 5th onwards, the second major cold wave may come down as strong blocking redevelops. This second polar plunge may bring longer, and stronger, cold directly into the Plains and on to the east. I believe there is a possibility that we may see a near repeat of the fierce Arctic outbreak that we saw back in 2004, which sent temperatures to -24F in Minneapolis, -7F in Boston and 0F in New York. This outbreak could last over two weeks following a strong stratospheric warming event.

Expect to see some big winter storms coming down from the Gulf of Alaska, striking the Central California coast and crossing the South-west.

The split jet would keep the cold shots firing into the central and eastern states, while a series of storms during January may bring significant rains to Southern California and big mountain snows.

Significant Cold May Impact The Southern Plains, South-East Late January

Even the Southern Plains may get their share of cold rains and even snow. I expect with the January cold outbreak that we may see record cold all the way to Dallas, perhaps Houston on east to the Carolinas and down to Florida. Mid to late January would be the time frame and this would come after a Southern Plains and South-east snowstorm. Could we see a 2-4 inch snowstorm in Atlanta?

The Appalachians may see a big year for snow.

Similarly to late December, another spell of easing may occur at the end of January, or start of February, with a near nationwide thaw possible. Isn’t a major thaw with a strong Pacific zonal flow not likely to end winter? No, I believe February, and even March, will present more cold shots, focusing from the Great Lakes to the South-east.

Courtesy of Mark Vogan

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