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Effects of Topography on North Georgia (Atlanta, GA)
Severe Weather and Tornadoes

MATT MCCRARY

The City of Atlanta, GA and surrounding metro area is no stranger to severe weather and tornadoes, especially. Based on Tornado Climatology Statistics from the Peachtree City Office of the National Weather Service, there appears to be a small tornado alley in the northern part of the state. This alley is a triangular area stretching from a line(s) near Rome to Gainesville to Newnan and back to Rome, including the cities of Atlanta, Marietta, Canton, and Cartersville. From 1950-2006, this general area experienced over 130 confirmed tornadoes. The topography of the region plays an important role in the frequency and type of severe weather that occurs in this part of Georgia. Elevations, for the most part are over 1,000í and the City of Atlanta sits on the southern end of the Appalachian Mountain range. Also, Atlanta is effected by itís proximity to the Gulf of Mexico and the Atlantic Ocean, both of which provide an abundance of moisture needed to fuel severe weather. Most severe weather occurs here in very late winter into early spring. However, some significant severe events have taken place in November as is the case with most regions of the southern US. In particular, the forecasting of these events is very tricky due to the way the ridges on the southern Appalachian are accentuated. They run from southwest to northeast, which is the typical path of a super cell thunderstorm.

The typical atmospheric setup for a severe event in North Georgia usually involves a Low pressure developing over the southern plains. Usually, the severe event will have taken place across the southern plains and mid-south the previous day before arriving in North Georgia the following day. Strong upper-level troughs associated with strong low pressures will dig deep into the south, bringing up very warm, moist, and unstable air mass from the Gulf of Mexico. Cold air masses from the upper plains and Canada will move south behind the low, clashing with the warm, moist air in place. Typically, if the central part of Alabama receives severe weather, then the northern parts of Georgia, including Atlanta should expected to feel the effects also.

The severe season in North Georgia lasts for about two to three months beginning in late February and ending in April. There is also a small window in November when severe weather typically occurs in this region. Obviously, severe weather can occur at any time of year, but the times listed above are the most conducive for severe weather development.

One of the things that make forecasting severe weather in this area is the diversity of the geography within the forecasting area. The forecasting area in general is very large, covering about half the total area of the state. This forecast area includes the Piedmont Plateau, the Blue Ridge Region, and the Appalachian Ridge and Valley Region. The Piedmont is marked by a hilly landscape in the north where it meets the Appalachian regions at around 1,500 feet above sea level. The land loses elevation to the southeast, where the hills become more gently rolling and the land is only about 400 feet above sea level. The Blue Ridge Region is marked by mountain peaks, which rise 2,000 feet to almost 5,000 feet, including 20 peaks greater than 4,000 feet with Brasstown Bald Mountain the highest point in Georgia at 4,784 feet. The Appalachian Ridge and Valley Region are marked by valleys with parallel ridges running southwest to northeast. The area where the Piedmont and the Appalachian Ridge and Valley areas come together make up the heart of the forecasting area, including the city of Atlanta. This change in elevation makes it very difficult to pinpoint where severe weather will occur. I have witnessed a damage path that was remarkable in that it proves there is no truth to the old tale about tornadoes being unable to cross mountains. A tornado on March 27, 1994 in Pickens County, GA crossed from one valley to another by climbing about 1,000-1,500 feet of ridge to reach the other side, snapping pine trees all the way across. The recent tornado that struck downtown Atlanta in February 2008 was almost undetected until a warning was issued for Fulton County about 8 minutes before the downtown area received significant damage. If the SEC championship basketball tournament game being played at the Georgia Dome had not gone into overtime, there would have been thousands of fans leaving the dome while being confronted with winds associated with an EF-2 tornado. This makes for a horrific reality for any forecaster when confronted with such a serious potential for significant numbers of injured. Also, missing on a severe weather event results in considerable property damage as the entire North Georgia region has exploded in population the last few decades. Making sure the public is aware and that all areas of the forecast region have the potential to be effected by severe weather enables the public to protect from lose of life.

Most of these storms run out of Central Alabama to the northeast into Northern Georgia. Typically, these storms occur in the late afternoon or early evening. However, some of the worst outbreaks of severe weather have occurred overnight, such as the event on April 8, 1998 when several tornadoes struck metro Atlanta including areas in Smyrna, Sandy Springs, Dunwoody, and Norcross.

Other severe threats such as winter storms play havoc on North Georgia, in particular Ice Storms. Cold air damning on the front side of the Appalachians will sometimes trap cold air in North Georgia. This cold air in place combines with seasonal rainfall during the winter to produce significant Ice Storms in the metro Atlanta area, sometimes bringing the entire region to a standstill. Moisture feeds northward from the Gulf of Mexico and overrides the cold air in place allowing precipitation to fall and rain, although surface temperatures can be below freezing. The differences in elevation can also play a role in Ice Storm events as lower elevations south of Atlanta can experience above freezing temperatures while the northern areas can be several degrees colder.

Forecasting for severe weather in North Georgia is difficult because there has been a pattern of areas where severe weather occurs. The counties north and west of Atlanta, including Cobb, Cherokee, Bartow, and Hall all experience more tornadoes than the surrounding counties. That is a little misleading though as the area of landmass in those counties is larger than others in the same region. There is a pattern, though. That is not to say others in North Georgia are not affected by tornadoes as every county in the state has experienced at least one in the last 50 years. Significant tornadoes ravaged numerous mountain counties in North Georgia on March 27, 1994. Cobb County has the most recorded tornadoes since 1950 with 26, the strongest of which was an F4 on November 22, 1992, also the first time I chased such a storm. There were forecasts for storms that day however; many people were not prepared for the destruction that would take place. A warning was not issued until the tornado was already on the ground.

Much of the forecasting of severe weather has improved since the introduction of the National Weather Service Doppler Radar on Hytop Mountain, AL. Before, there was no available radar from Peachtree City south of Atlanta all the way to Morristown, TN north and east of Knoxville. After a series of destructive tornadoes struck Northern Alabama and Northern Georgia the National Weather Service put this Doppler radar into use. The use of this radar has increased warning times in Northern Georgia significantly. The general public were concerned about the lack of warning with the February tornado that struck downtown Atlanta. However, according to National Weather Service Meteorologist Mike Griesinger there was a warning issued about 12 minutes before the tornado struck downtown. This is about the average warning time. One thing that will continue to aid forecasters in predicting these severe weather events would be to analyze the upper air charts. Also, I would suggest placing a second National Weather Service Doppler Radar on the north side of Atlanta. This would enable Forecasters to better track storms capable of producing tornadoes and also provide a better resolution on the radar signature.