April in the Peach State is a masterclass in atmospheric irony, a time when you can find yourself reaching for a heavy coat and an ice scraper on Monday, only to fire up the grill in shorts and sandals by Wednesday. Remember last month? We went from record highs in the 80s to snow flurries and 20s and back again, all within a week!
As we move from the damp chill of late winter to the muggy embrace of early summer, the month serves as a volatile tug-of-war between lingering Arctic air masses and the building heat of the Gulf of Mexico. It is, by all accounts, the most temperamental month on the Georgia calendar.
On paper, April looks like the picture of comfort. In the Atlanta metro area, the average high temperature sits at a pleasant 73 degrees, while the average low hovers around 51 degrees. Farther north in the mountains, those numbers dip slightly, with average highs closer to 71 and lows near 41.
April brings on two challenges. First and foremost, it is the official peak of North Georgia’s severe weather season. As warm, moist air from the Gulf surges northward, it often clashes with cold fronts sweeping across the Plains. This is the prime recipe for the “Tornado Alley” extension into the Deep South. We can get pop-up thunderstorms but the most devastating is the MCS (mesoscale convective system), the QLCS (quasi-linear convective system), and the Derecho. I know, this sounds technical but they are all basically squall lines of thunderstorms. Remember the polar vortex that slammed us last month? That was a QLCS that preceded the Arctic air.
April frequently brings these squall lines. In a typical pop-up thunderstorm, we have an updraft of warm, moist air that causes the storm to grow. Eventually it begins to rain. The rain-cooled air will overwhelm that updraft in about an hour, shutting the storm down.
However in the spring, the upper level winds are very strong. The change in wind speed or wind direction with height is called wind shear. When we have thunderstorms and wind shear, the upper level winds blow the thunderstorm tops forward, tilting the storm. Now, the updraft is no longer being squashed by rain-cooled air so it can go on for hours and hours. These storms turn into monsters with large hail, damaging winds, and tornadoes. These tornadoes are very dangerous, not so much for their strength, but for being nearly impossible to see on radar. They form on the leading edge of the squall line. They spin up quickly and with little warning time.
I have tracked a great many of these systems over the past four decades. See the accompanying radar image. You can see how the line bows out, indicating those strong upper level winds. Many times the storm’s downdrafts will bring those winds to the ground. The aftermath often looks like a tornado hit.
Squall lines are a big part of our April severe storm season. Now you know the science behind them.
Of course we welcome April showers and rain. It helps cut down on our other nemesis … the pollen! It should be a banner month!
Image: courtesy of NOAA
