A significant winter storm is expected during the next 24 to 36 hours across the Deep South. Depending upon location, there will be a wide array of wintry precipitation types starting early on Tues., Jan. 28, 2014 and continuing into the morning of Wed., Jan. 29, 2014. Look for snow, sleet and/or freezing rain anywhere from east Texas across the Gulf Coast States into the Carolinas. While amounts of precipitation won’t rival that experienced in some ice storms further north, even the mention of these precipitation types brings concerns to people in southern climes. Several tenths of an inch of freezing rain, up to a half an inch of sleet and/or up to four to six inches of snow could fall in various places in the area (Fig. 1, Fig. 2 and Fig. 3). In parts of eastern North Carolina, snowfall may locally top eight inches. For specific details and the latest, updated, local forecasts, look to the National Weather Service (NWS), local TV stations and other reliable weather sources.
While this is an unusual event for the Deep South, it is not unprecedented. In March 1993, the “Storm of the Century,” blasted the Deep South (even coastal, Mobile, AL) with large amounts of wind-driven snow. According to a National Weather Service snow study for Alabama, at least one inch of snow has fallen in extreme southern Alabama seven times in the 1969-1999 period (31 years). That equates to one one-inch snow event every four to five years. Further north, the frequency increases dramatically, reaching two one-inch snowfall events annually, on average.
Most meteorologists have keyed on the two main ingredients for this wintry storminess: cold air and overriding moisture. Well, there’s another ingredient that is going to play a significant role. That ingredient is dry air in the lowest four to five thousand feet of the atmosphere (Fig. 4). The reason is simple – evaporative cooling.
You may not recognize the term, “evaporative cooling,” but you probably know what it is. Evaporation is a cooling process. It requires heat to be used during the process of transforming water (liquid) into water vapor (gas). Watch a pot of boiling water on a stove to see what happens.
Anyone who has ever stepped out of swimming pool or a shower into air that is cooler than skin temperature and/or less humid than 100 percent relative humidity has likely experienced a bit of a chill. That chill is mainly due to the evaporation of water from your skin. Dry off and the chill vanishes. Yes, the process works, even in low heat situations.
So, as cold air infiltrates the Deep South on brisk northerly winds, dry air (actually very dry air) is accompanying it. For example, at New Orleans, LA, the dew point has fallen from the mid 50’s Monday afternoon to 28 early this Tuesday morning. Montgomery, AL has seen its dew point collapse from the low 50’s to the low single digits in about the same time period. Jackson, MS had a dew point of 4 degrees, while Atlanta, GA reported 1 degree at 1:00 a.m. C.S.T. this Tuesday morning (Fig. 5).
As overriding moisture (at altitudes of five to ten thousand feet) starts to produce precipitation, that precipitation will fall into the drier air and evaporate before reaching the ground. This will moisten the dry layer and also cause temperatures in that layer to cool further. By the time is process is complete, and precipitation reaches the ground, in many places, there will be warm air in the clouds and much colder air below. This will allow rain to fall into the colder air and either freeze on the way down (sleet or ice pellets) or reach the ground and freeze on impact (freezing rain or glaze).
To the north of this zone of icy precipitation, the atmospheric column will be colder and cloud precipitation will be snow. Snow, falling through cold air, will stay as snow.
So, look for a band of snow across inland locations of the Deep South. To the south of this band is where the sleet and freezing rain are most likely to occur.
Without the dry air allowing for cooling below cloud level, precipitation would likely be just rain or just snow.
The various forms of frozen precipitation (snow, sleet and freezing rain) will make driving and walking hazardous. Ice accumulations on trees and power lines could cause power outages, even after the precipitation ends. Gusty northerly winds will lead to even colder wind chill readings, in some cases single digits or lower.
The band of snow and ice may also affect temperatures in Florida. As northerly winds blow across the region covered by the wintry precipitation, the cold air will not moderate, as it does moving across ground that has been heated by the sun. Thus, if winds moving south across the Florida peninsula don’t turn to the northeast, an over water trajectory (but instead remain north to north-northwest, an over land and ice/snow), temperatures across central Florida may actually be colder than advertised.
© 2014 H. Michael Mogil