Where's winter? Changes in the winter forecast
State College, Pa. AccuWeather reports while we are still about a month away from the official start of winter, unusual warmth in the Central and Eastern states has people wondering what the winter will be like, now that Thanksgiving is upon us.
Some people in the East are saying it will be like the 1989 when a mild November was followed by wicked December cold. Others are saying it may be a year without a winter in the East. Folks in the Northwest may already have their answer.
With Old Man Winter and La Nina giving some early signs, there are some changes coming to the AccuWeather.com Winter Forecast. However, some aspects of the original forecast will likely remain unchanged.
The projection of a weaker La Nina this year compared to last year is on target so far. However, the La Nina is even weaker than originally thought. This October La Nina has been rated at a magnitude 8.3 while during October 2010, La Nina was rated at 18.2 magnitude.
La Nina is a cooler-than-average tongue of sea surface temperatures in the southern Pacific Ocean. When this occurs, it causes a northward bulge in the storm track over the northern Pacific Ocean. Correspondingly, the storm track dips over the United States.
Where this dip occurs is critical and depends, in part, on the intensity of La Nina and other weather patterns. A strong La Nina may override most of the other patterns and have more definitive results. A weaker La Nina is more likely to be influenced by other weather patterns, yielding a wide variety of potential outcomes for different parts of the country.
One of the key players thus far this fall has been the lack of a Greenland Block. This is a northward bulge in the storm track over Greenland that produces a southward dip in the storm track over the Eastern U.S.
According to Long Range Weather Expert Paul Pastelok, "Without this block, the dip in the storm track would be in the western U.S. rather than the eastern U.S. The Northwest would be cold and stormy, the East warm and the Ohio Valley stormy."
This has generally been the case so far during November with some exceptions.
Pastelok feels the Greenland Block is still a potential wild card for the winter.
"A lack of Arctic sea ice and the warm surface it produces could cause the Greenland Block to show up for a several-week period with little notice."
Such a visit could drive arctic air the storm track southward for multiple weeks in the Midwest and East, tipping the balance toward more snow.
Without the Greenland Bock storms are more likely to "cut" northward toward the Great Lakes with only limited opportunities snow in the I-95 Northeast. This would mean bouts of severe weather in the south-central U.S.
"The idea of cutting storms would have a good side too, bringing rain to needy areas in eastern and central Texas and the south-central U.S. in general," Pastelok said.
Unfortunately, much of the Southwest would still be drier than average.
As for the West, the nasty cold already showing up in Alaska and the storms that are kicking into high gear in the Northwest are signs of a typical La Nina pattern.
One thing Pastelok suspects will happen is that Arctic cold will continued to press southward in the west, driving the storm track southward as the winter progresses.
This will tend to drive powerful, moisture-rich storms into Oregon and northern California for one or more periods this winter. A pattern such as this could deliver a month's worth of rain (or snow) during a single week.
Winter 2011 officially arrives on December 22 at 12:30 a.m. EST.