I hope everybody had a chance to get out and enjoy the great weather today. The high temperature was 7 degrees above normal, and the abundant sunshine was a welcome change from what we’ve seen recently.  Unfortunately, today may have been the nicest day we’ll see until the weekend.

We are in an interesting weather pattern now, with what meteorologists would say is a ridge in the west, and a trough in the east.  In layman’s terms, think of it as a mountain of dense air in the western half of the country, and a valley of lighter air east of the Mississippi.  Now, imagine yourself as an air mass in the Pacific trying to move from west to east.  You encounter the air mountain, and are forced to move north to get around it.  As a result, you get colder.  Then, after passing the mountain, you find yourself drawn southward, into the valley of lower pressure air.

The result of this air movement is that the southeast gets a fast moving storm system as the air mass passes through.  Then, we get colder weather because of the temperature of the air that follows the storm.  Finally, things begin to warm up again due to the warmer angle of the sun.  Then, the pattern repeats itself, as it will do tomorrow morning, on Friday, and again next Tuesday.

All in all, it’s not that unusual a pattern for early spring. In some ways, it’s a good thing, since it indicates that the effects of the La Nina pattern are loosening, which could bring more rain to an area that sorely needs it.

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.

With Spring officially arriving tonight, and the weather warming up, it’s time to start thinking about planting outdoors. There are two weather-related things to keep in mind before you run out to the garden center to pick up those summer annuals.

The first is the last freeze date. You don’t want those newly planted flowers to freeze. For Georgia, the last frost date ranges from March 20th along a line from Columbus to Macon to Augusta to April 20th up in the mountains of Northeast Georgia near Clayton. According to the Weather Service, metro Atlanta to Athens has its last freeze sometime between March 30th and April 10th.

In reality, though, the last time Atlanta had a spring freeze in April was back in 2000, and before that, you have to go back to 1992. There have been a number of freezes in late March, however. In addition, Atlanta tends to be a bit warmer than the suburban counties, so the actual last freeze date would be a bit later here.

In addition to thinking about the last freeze, consider the soil temperature before you plant. Most annuals won’t grow much until the soil temperature remains above 60 degrees, and some plants, prefer temperatures above 70. I keep track of the soil temperature with a probe about 6 inches down in my backyard. A graph of the temperature is below:

The image above is live, and as I look at it, I see the soil temperature reached 56 degrees during last week’s warm spell, then dropped to 48 over the chilly weekend. It’s now on the way back up. So, as of today, it’s still too early to plant, even if you were positive we weren’t going to see any below 32 degree weather. When you see the temperature above 60 and staying there, that’s a pretty good indication that it’s safe to plant.

There’s a reasonable chance that metro Atlanta might get its first wintry weather Wednesday night and into Thursday, but the forecast is tricky, and a lot of what will happen will depend on exactly how conditions set up.

During the day Wednesday, North Georgia will be under control of a high pressure system located over Virginia. This will make for a chilly day again. Meanwhile, a low pressure system is moving east. It’s the same one that has brought freezing rain to Texas earlier this week. Whether we get a major winter event depends on how these two systems interact.

As the warm, moist air moves into Georgia, it will meet the colder air. Since the colder air is heavier than the warmer air, it will remain below the warm air. This effect is called Cold Air Damming, or popularly, ‘the wedge’. The possible effect is that instead of temperatures rising with the incoming rainfall, it will remain cold enough at the surface to cause sleet or freezing rain.

Below is the Weather Service graphical forecast for 7 AM on Thursday:

You can see that below Macon, there will be heavier precipitation, and it will be all rainfall. There’s an area north of that that may see heavier icing, since it will be in the more active part of the rainy sector, while also under the influence of the wedge. North of a line from Columbus to Athens, including Gwinnett County, freezing rain is also predicted.

So what happens on Thursday morning is a function of how strong a wedge develops, and how fast the precipitation moves. Since the ground is warm because of the springlike weather we saw before today, there isn’t likely to be any accumulation on the ground. However, if we do get freezing rain, it will affect trees and power lines, and things could turn nasty. If it’s too cold, we get sleet, which won’t be a problem. And, if the wedge is weak, we may get all rain as temperatures close to the surface remain above 32 degrees.

Stay tuned…

Well, in Georgia, not very much. But, if you’re in the Northwest United States, they are very good. Below is a map showing the probability of a white Christmas based on data from 1961 through 1990:

According to the folks at the NOAA National Data Centers, who put together the graphics, the chances of a white Christmas is greatest in Savannah and Brunswick, oddly enough, with a probability of under 5%. Elsewhere in the state, the chance of snow in virtually zero. The report (PDF) says that there are five places in the US that are virtually guaranteed of having one inch of snow on Christmas day, including Marquette and St. Ste. Marie, Michigan, Hibbing and International Falls, Minnesota, and Stampede Pass, Washington. You can also find out the probability of snow on Christmas for many other cities in the report.

You may have wondered about the accuracy of the different weather forecast services out there, since they often make different predictions about what is going to happen on a given day.

According to a recent Wall Street Journal article, someone up in Ohio wondered the same thing, and started a website called the Forecast Advisor that compares the accuracy of weather forecasts from major providers, including weather.com, the National Weather Service, and Accuweather. The comparisons are done by major city or ZIP Code. If you look at the results for Lawrenceville, for example, you’ll find that the Weather Channel has been the most accurate over the last year, followed by the National Weather Service. You can also look at previous versions of the forecast for a given day, to see how they have changed over time.

The Advisor reports the accuracy of both the National Weather Service and the NWS Digital Forecast. Although they are both produced by the Weather service, the general forecast is for a specific weather zone, for example metro Atlanta. This is the forecast you see on the Lawrenceville Forecast page, and you can see it covers everywhere from Gwinnett to south Fulton counties, which is actually a rather large area. The NWS Digital forecast, on the other hand, tries to produce a forecast for a specific latitude and longitude, accurate to within a several mile radius. That’s the forecast used for the Lawrenceville Pinpoint Forecast, as well as the quick forecast on the home page. Because it covers a smaller area, it tends to be more accurate.

The National Weather Service has developed some new web-based tools to help people understand and use their three month seasonal forecasts. In the past, the long range forecasts, made over three month rolling periods were distributed in the form of maps showing areas where the temperature was expected to be above or below normal.

Now, they are providing a series of graphs and charts at the local level, which in North Georgia includes Atlanta Hartsfield, Athens, Cartersville, Rome, Newnan, and Gainesville. For example, the chart below shows the probability that the actual temperature will exceed a given temperature.

So, you can see that there is an almost 100% chance that the average temperature will be greater than 68 degrees, while almost a 0% chance that the average will be greater than 74. Because the green forecast line is to the right of the red actual line, overall, there is a greater chance of warmer than normal temperatures from August through October. You can look at the latest 3 month outlook for Atlanta here.

In addition to the Probability of Exceedence graph shown above, they have a number of pie charts and tables to illustrate the data in other ways, including temperature ranges and the chances of temperatures being above or below normal.

As January prepares to end, people all over the country are wondering, “Where is Winter?” Temperatures continue to be much warmer than normal, and despite several model predictions and teleconnections, not much has changed in the weather since the beginning of the month. We continue to see a largely zonal air flow from the Pacific, combined with a heat ridge over the Atlantic that is keeping much of the East in a springlike pattern.

If you believe the models, all of this is going to come to an end beginning in the middle of next week. Here is the National Weather Service 8 to 14 day outlook for the period between February 7th and 13th:

The predicted pattern of colder than normal temperatures in the east, and warmer than normal temperatures in the west (plus warm in Alaska, which isn’t on this map), match the typical negative Arctic Oscillation and North Atlantic Oscillation teleconnection cycles. However, both these signals seem to have bottomed out, and are rising again. The forecast models also haven’t been particularly reliable this winter either.

So, we’ll have to wait and see. Expect warm weather this week, with temperatures in the low 60s, with a chance of a storm on Thursday. As we’ve seen this season, temps will be a little lower following each storm, but will rebound quickly. For next week, if the forecasts verify, we’ll see highs in the upper 40s, which is about 5 degrees colder than normal for the second week in February.

I had a chance to get down to the WeatherFest at the World Congress Center on Sunday, and met several prominent meteorologists, including Paul Kocin, the winter weather expert at the Weather Channel and Max Mayfield, Director of the National Hurricane Center, who assured me that last year’s hurricanes had nothing to do with global warming.

I also had a chance to speak to Elliot Abrams, Chief Forecaster for Accuweather. I listened as he explained how a change in the polar vortex at 10 MB of pressure should cause the polar jetstream to move south. Such a change has occurred within the last week or so, but as Elliot was the first to admit, he’ll be the first to chuck the theory if it doesn’t get cold soon.

The American Meteorological Society is holding their annual meeting here in Atlanta from January 28th through February 2nd. Several thousand weather forecasters and atmospheric scientists will discuss issues including climate change, droughts, floods and weather highlights.

While the convention sessions themselves are closed to the public, the organization is sponsoring a WeatherFest on Sunday, January 29th that is free and open to the public. Representatives from the Peachtree City office of the National Weather Service, the National Hurricane Center, and the Storm Prediction Center will be present, along with folks from Accuweather, the Weather Channel, and area TV and radio stations. There will be kid-friendly exhibits as well, so this should be an event for the entire family.

WeatherFest is Sunday, January 29th from noon until 4PM in the registration area for conference halls A and B at the World Congress Center (the part that faces Philips Arena). Find out more at the AMS WeatherFest page.

A month ago, I posted on the relationship of the Arctic Oscillation teleconnection to the colder weather we experienced in December. Essentially, when this indicator goes negative, you can expect colder winter weather in the Southeast, while when it’s positive, the weather is more likely to be warm and dry.

There is another teleconnection that can shed some light on likely winter weather patterns, the North Atlantic Oscillation. The NAO, as it’s called, measures the differences in unusual air pressure in the northern and southern Atlantic. When the index is positive, the eastern United States tends to have milder and wetter winter weather, while when the index is negative, the east coast tends to be colder, with more snow.

The graph below shows recent measurements of the NAO, along with the forecast for the next two weeks:

As you can see, the NAO has been in a positive phase since late December, corresponding to the milder weather we’ve seen so far in January. However, since the second week of the month, its effects have been getting weaker, and if you look at the second graph, which shows the 14 day predicted position, it should go into a negative phase soon.

The third chart shows the Arctic Oscillation prediction. After being in a positive phase in early January, it too is predicted to go negative.

While not as accurate as your typical three day forecast, these two teleconnections provide some indication that we may very well see more normal winter weather for February. The CPC’s 6 to 19 and 8 to 14 day outlooks through the first of the month still call for warmer and wetter conditions than normal through the first of the month, although the warm area shrinks in the longer-range forecast.

The weather service will issue its long range forecast for February tomorrow. It will be interesting to see what they predict.