It was the storm that disappointed snow lovers across Atlanta and Athens. However, the storm exceeded expectations for those that lived in North Alabama and Northwest Georgia. For several days, our weather models indicated the potential for a winter storm on Wednesday, February 25, 2015. However, details on timing, intensity, and location of the storm was up in the air. How good or bad were the forecasts? How and why did this storm form? Why was it tricky to figure out? I’ll answer all of those questions below.
Most of the snow verified north of I-85. Snow fell in the northern Atlanta Metro area and points north and eastward through Gwinnett and into the mountains starting Wednesday afternoon around 1-4 p.m. EST. However, the snow was only temporary. Warm air several thousand feet in the air began to push into the Atlanta and Athens area. The warm air was much stronger than what the models indicated, and the snow/sleet/rain mix became a cold rain event. As the sun set, the warm nose of air continued to push as far north as Dahlonega. The warm air did not reach Northwest Georgia or the mountains, which helped to contribute to higher snowfall totals. As the storm continued to push to the east, a band of snow on the northwest side of the low wrapped around North/Central Alabama and was pushing into Northwest Georgia. This band of snow is technically called the “deformation” band. When this forms, strong dynamics in the atmosphere can result in heavy snow and possibly produce thunder. As it forms, it also brings in dry air to the south of this band. This pocket of dry air pushed through Atlanta and Athens and along I-20. The end of the storm finished with snow north of I-85, and light showers to little precipitation falling from the sky along I-20 as dry air was pushing in.
We knew that a surface low would form along the Gulf Coast and push to the east. Meanwhile, cold air and additional energy to enhance the storm push eastward across Texas thanks to another low pressure (upper level low). The two were able to phase into a decent storm. Moisture was not an issue. However, temperatures would be the key factor in determining who would see snow versus a cold rain. A low further north would keep us warmer with a rain solution. A low more suppressed and further south would keep us colder and have a higher chance of seeing snow. The big question about this storm: How far north will the low go?
The National Weather Service issued a winter storm watch for all of North Georgia early Tuesday morning. They issued the watch late to help avoid confusion on the wintry weather that was pushing through North Georgia Monday evening. By Tuesday afternoon, a winter storm warning was issued for counties along I-20 and points north. The forecast from Peachtree City NWS was for 2-4 inches of snow with areas across the mountains possibly seeing higher totals.
Tuesday evening, we issued our snowfall map:
How did we do?
Our last forecast snowfall map was made around 1 p.m. EST Wednesday afternoon before the moisture was pushing into the area. We noticed that temperatures a couple of thousand feet in the air were too warm to fully support snow. We can look at these temperatures by looking at soundings that measure temperatures as you go up in the air. Peachtree City issued a couple of soundings around 10 a.m. EST and at 1 p.m. EST. We also looked at temperatures recorded from instruments on airplanes.
Here’s what a sounding looks like. This particular image is showing how the temperature warms up as you go up from the surface:
We also calculated the wet bulb temperature. We can determine how cold it can get at the surface based off the air temperature, dew point temperature, and barometric pressure. With a temperature of 40°F at the surface and a dew point of 28°F, we knew surface temperatures were going to stay above freezing all day. Ice/freezing rain was not a concern. With all of the warm air in place, we shifted the rain/snow line further north to include Atlanta and Athens. We also increased the snowfall totals across parts of I-85 and points north.
Here was our last update issued at 1 p.m. EST Wednesday prior to the storm:
Here was the forecast totals from NWS Peachtree City:
Finally, here are the preliminary snow totals that actually occurred:
In our post on Tuesday, February 24, 2015, we wrote that impacts would not be as big in Athens as it would be for those along and north of I-85. However, we underestimated the warm air and the shift of the storm which was slightly further north. We also stated the high uncertainty of snow chances for those along I-20. We had a concern that nothing would materialize. However, we found it necessary to be safe than sorry and show some snow totals just in case the storm over-performed. After all, majority of the models were hinting at 2″ of snow for Athens. Why ignore that?
I would personally give us a B for the forecast. The forecast verified for the North Georgia Mountains where many areas recorded over five inches of snow. Our general idea was great. The spots we highlighted 1-3 inches of snow should have been pushed further north.
Finally, temperatures Wednesday night into Thursday morning were not as cold as we thought it would get. The idea of patchy black ice never materialized. We did see several schools close or plan a delay Thursday morning after the storm. Question: Was the closing of a school system worth it? Should schools hold and wait on a decision until the morning hours instead of the night before? Every school system is different, and I think we need to find a way to prevent unnecessary “snow” days.
Here is a look at the entire event via radar. You can see the rain/snow line start south and push to the north as warmer air aloft was pushing into the moisture. Video courtesy of 13WMAZ Weather!
Storm had a spring element to it too!
While North Georgia/Alabama/Mississippi had to deal with a winter storm, the southern part of the storm was actually producing spring-like weather. Temperatures were in the 60s and 70s across Florida. A line of severe storms formed along the area of low pressure located near Tallahassee, Florida. Several severe thunderstorm warnings were issued along with a tornado watch. It was a very dynamic system.
Why mostly rain and not snow in Athens?
The area of low pressure was simply too far north. Also, we really didn’t have a strong surge of cold air from the north to really help cool us down and form snow.
Here is what the surface map looked like for February 25, 2015. It shows an area of low pressure near Tallahassee, Florida. High pressure was not north of our area to help spill cold air into North Georgia completely.
Remember the snow event of January 10, 2011? If you didn’t, we recorded between 6-10 inches of snow across the Athens area. The snow line was much further south.
Do you see the location of the surface low in the image below? It is much further south near Tampa, Florida. Since the low was further south, colder air from the north was able to give all of us a good snow across North Georgia. Plus, a strong area of high pressure to the north really helped us out by giving us very cold temperatures. Plus, it didn’t hurt that we were in the heart of winter either!
Finally, our very own Chris Davis wrote up an excellent article last year explaining how we get snow through several examples. You’ll find that setups like yesterday are typically favored to produce significant snows across the Southeast.
Bottom line: Winter Weather Forecasting is super challenging. We have to figure out the placement of the storm. Meteorologists are not only concerned about surface temperatures, but also look at temperatures up into the atmosphere to determine precip type. Spots 20 miles south could see nothing but rain while points 20 miles north could get hit with several inches of snow. It all depends on where things line up. It was a tough forecast to nail down. It wasn’t perfect, but the general idea of an I-85 and points north special was spot on. The models were showing it, and our very own NWS indicated it as well.