Wednesday night’s threatened winter storm didn’t end up causing too much difficulty for the metro Atlanta area. The onset of snow between 5 and 5:30 PM managed to put about a quarter inch of the white stuff on my yard and caused the temperature to drop below freezing, but by 7:30 PM, temperatures rose above 32 degrees, and stayed constant or warmed up as the overnight hours passed.

Nonetheless, at least some freezing rain was noticed in parts of Gwinnett, even if it was in limited areas. I noticed that ice was still sticking to trees after noon today along Wisteria Drive in Snellville, and the pine trees along Ronald Reagan Parkway were similarly burdened. So much of what happens in a winter storm is temperature dependent, so just a variation of a degree or so can make a big difference.

The continually varying weather we’ve seen this winter has brought relatively normal precipitation to drought-starved North Georgia. December ended up with more than the normal amount of rainfall, and the official rain gauge in Atlanta is close to what we would expect so far in a normal January.

The latest drought outlook, released today and shown at right, calls for improving conditions for much of the northern part of the state including Atlanta, with at least some relief for the rest of the state. We should see another chance of rain and/or snow Friday night and Saturday morning, with more precipitation for the latter part of next week.  Both the 6-10 and 8-14 long range outlooks call a better than average chance of rainfall.

We’re not out of the drought yet.  But, with drier than normal conditions predicted for the winter due to the presence of La Nina, it can’t be a bad thing that precipitation is running close to normal.

It looks like things could get fairly nasty Wednesday Night and into Thursday for much of the Atlanta area north of Interstate 20, and especially for the northeast counties, including Gwinnett.  An incoming low pressure system in the Gulf of Mexico will bring precipitation to Georgia beginning Wednesday morning.  Meanwhile, the high currently over the eastern part of Kentucky will move northeast, running into the Appalachians, and causing a ‘wedge’, or cold air damming to affect northeast Georgia.

What happens is that the colder air gets trapped on the western edge of the mountains, and the approaching lower pressure, warmer air slides above the colder, heavier air. You can tell when we’re in a CAD situation when the wind comes from the east, and it doesn’t warm up much during the day.

The presence of warm air above cooler closer to the ground tends to favor the formation of sleet or freezing rain, since the warm air layer will melt any snow as it passes through, while the cold air close to the ground will either re-freeze the precipitation, or cause freezing when the water hits the surface.

In the last day, forecasters have increased the total amount of expected precipitation from the storm to almost an inch, while dropping the low temperature Wednesday night, especially in the northeast counties most affected by the wedge.

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While it may not get as bad as the winter storm in North Georgia two years ago, it looks like Thursday morning is going to be pretty nasty.

The Weather Service has issued a winter storm warning for the northern part of the state, including Gwinnett County. They have also issued a winter weather advisory north of a line from LaGrange to Macon to Augusta. The warned area could expect up to half an inch of sleet and a quarter of an inch of ice before Noon Thursday.

Cold temperatures today — the high was only 42 degrees — and dropping temperatures due to the still-clear skies will push the morning low below 30 degrees. Meanwhile the precipitation will arrive starting in the northwest corner of the state, and will most likely arrive in Atlanta after 1 AM, with the worst arriving shortly before sunrise.

The good news is that the precipitation will bring warmer air, with temperatures rising above 32 degrees by noon, and probably staying there through the overnight hours early Friday morning, so the worst should be over Thursday afternoon. Overall, we could get more than an inch of liquid precipitation before things start to dry up late Thursday night.

The longer range outlook keeps things cold in the Atlanta metro area through the middle of February. The Weather Service issued an updated forecast for the month, and while the predictions for the Southeast US didn’t change much, they are calling for colder weather than originally thought in the Northeastern states.

In a near-repeat of weather conditions from two weeks ago, the possibility of freezing rain has raised its ugly head in North Georgia.

As you may remember from this post, we had a situation two weeks ago with a low pressure system approaching from the south, and cold air in place. This brought the threat of freezing rain or sleet. Although there was a bit of freezing rain overnight, the ground was warm enough, and the precipitation amount was small enough that there weren’t any problems.

The situation this week is different. Unlike two weeks ago, when we set a new high temperature record, it’s been much colder, which increases the chances of any freezing sticking around. Also, instead of 1/10th of an inch of precipitation, we are likely to see about an inch between 7 PM Wednesday and 7 PM Thursday.

Right now, forecasters believe that what does fall could be freezing rain north of a line between LaGrange and Milledgeville. Even after sunrise on Thursday, the line between rain and freezing rain will be just south of Gwinnett County.

Everything is subject to timing of the storm, and the exact temperatures on Wednesday night. We’ll have a much better picture of what is to come on Wednesday morning.

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…