The first major winter storm of the season struck the northern third of the nation over the weekend, bringing an  ugly combination of snow, freezing rain, and rain to everywhere from Washington State to New York as a low pressure system combined with cold arctic air moved across the country.  Portions of Michigan reported up to ten inches of snow and 1/8th inch of ice from freezing rain this morning, and depending on where you were in Pennsylvania, you got from one to ten inches of snow.

Now, the storm is bearing down on New England, as you can see from the forecast map below:

The precipitation is made  much more treacherous because the freezing line runs right through the area the storm is hitting in Pennsylvania and southern New England.  The northeast is going to get a one two punch.  After the current storm moves out Monday night, a second clipper will track across the area on Wednesday.

Here in Georgia, we’ll be lucky to get half an inch of precipitation as the cold front associated with the low crosses the state overnight Sunday.  Following that, we’ll have some cooler temperatures this week before a warmup the week of the 10th, when temperatures will again rise to the mid 60s.. Don’t expect much rain during this period, though.  After tonight, we may not see rain until the second half of December.

Georgia State Climatologist David Stooksbury held a press conference today in Athens to discuss the latest Georgia drought conditions, and to explain what he thinks might occur weather-wise over the winter and spring of 2008. Much of what might happen through the next six months depends on the strength of the developing La Nina, including the chances of being able to refill, at least partially, Lakes Lanier and Allatoona.

There is now general agreement that La Nina conditions are developing in the Pacific. La Nina, and her partner El Nino, are descriptions of the two phases of the warmth of the Pacific Ocean surface temperatures compared to normal. In El Nino conditions, temperatures are warmer, while the reverse is true for La Nina. Most climate observers feel that La Nina conditions strengthened in September.

In a press release accompanying the news conference, Stooksbury said,

“The effects of the La Niña pattern differ with its strength. These differences are critical across north and central Georgia, potentially having major impacts on the current drought and the region’s ability to recover this winter and spring.”

If the La Nina conditions remain weak, then there is actually a greater chance of precipitation across extremely north Georgia, and normal precipitation across the piedmont area. In a moderate to strong La Nina, the chance of more than usual precipitation will be limited to the extreme northwest corner of the state. In either case, it’s likely to be warmer and drier over the southern half of the state. You can see the impacts in the image to the right (click on it for a larger version.)

We had best be hoping for a weak La Nina pattern. In the words of Stooksbury,

“If a moderate La Niña pattern develops, there is a high likelihood that north and west Georgia won’t be able to recover from the drought this winter.

The extreme- to exceptional-drought regions of the state may muddle through the winter and early spring. But without significant recharge of the soil moisture, groundwater, streams and reservoirs, conditions next summer could become catastrophic.”

Accuweather.com has released its Winter 2007-2008 forecast, and it calls for much warmer temperatures than normal for most of the country. Long range forecaster Joe Bastardi says that the beginning and end of the season will probably be closest to normal, but the period from mid-December through February could be one of the top ten warmest winter periods, with the bulk of the warmth centered over the Tennessee Valley and the Carolinas.

Bastardi also has a warning for the drought starved southeast:

“We will know in the next three to four weeks whether a storm from the tropics might arrive to help the southeast drought situation. If that doesn’t materialize, those states are looking at a very dry winter, with precipitation amounts less than half of normal in some areas.”

A combination of La Nina conditions and warming Atlantic Ocean temperatures are the chief culprits for the warm weather. East of a line from the Great Lakes to the Southwest could see temperatures above normal 75% of the time.

Update 10/25/07 - here’s a link to a more detailed winter forecast from Accuweather.

The National Weather Service has issued its predictions for the upcoming winter, and for most of the country, the outlook is for warmer than usual temperatures. The persistent drought in the Southeast is also a concern in the press release put out by NOAA. Here’s a look at the predicted temperatures from December, 2007 through February 2008:

While the greatest chances for above normal temperatures are in the southern Plains states, virtually the entire country with the exception of the Northwest is expected to be warmer than normal. Cooler than normal temperatures aren’t predicted for anywhere in the country. Part of this is due to the long-term trend towards warmer temperatures compared to NOAA’s base years of 1970-2000, and the other part is due to the La Nina conditions expected this winter, particularly in the southeast. Even though temperatures are predicted to be warmer than normal, the Weather Services says that on a nationwide basis, they should be cooler than the 2006-07 winter.

As far as the chance of precipitation goes, much of the southeast and southwest is predicted to be drier than normal, while the northwest is going to be wetter than normal, both due to the La Nina conditions. Also predicted to be wetter than normal is much of the Midwest.

The weather service stressed that this outlook isn’t a snowfall forecast for the season, claiming that it’s difficult to predict exact snowfall patterns more than two weeks out, which is true. I’m a bit surprised that NOAA used the long range outlooks they issued back on September 20th as the basis for their winter outlook, when an updated one is due out in a week, and should in theory be more accurate. Perhaps they wanted to be ‘in the lead’, prior to other forecasters making their predictions.

The Weather Service provided further confirmation to what it had been hinting at for the past several months yesterday, when it issued a news release reporting a developing La Nina condition. The expectation is that La Nina conditions will continue to develop over the next three months, and provide a wintertime La Nina weather pattern. This expectation is backed up not only by observation, but by computer modeling.

If La Nina conditions develop, what can we expect for this winter? According to statistics gathered during previous La Nina years, there is a 49% chance of less than normal precipitation in North Georgia from January through March, and a 60% chance that temperatures will be above normal. There’s only an 8 percent chance of below-normal temperatures, but there is still a 32% chance of above normal precipitation.

La Nina conditions occur when sea surface temperatures in the Pacific Ocean are below normal for more than three months in a row. It is the opposite of El Nino conditions, where equatorial ocean temperatures are above normal. The last time we had a La Nina episode was back in 2000.