JACKSONVILLE, Fla – If you're like me and grew up in the heart of Dixie, snow was a novelty, not a nuisance. The rare nature of snow made each moment it did a kodak moment. But why is it so rare in the deep south?
Snow has been reported in all 50 states and as far south as Homestead, FL. On January 19, 1977, that event knocked President-Elect Carter off the front page of the Miami Herald and replaced with "Snow in Miami!"
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Those events are few and far between---perhaps once in a lifetime for those living in Miami or Tampa.
Ice however is the ugly sister of snow and far more common in the south. The reason is pretty straight forward: we live in the sub-tropics where warmth is abundant.
The atmosphere is 3D, not one dimensional. It's measured in terms of pressure layers; where high pressure is at the surface and lower pressure up high. We call this a 'column.'
All precipitation, even in the Summer, starts as snow. That's important to remember. But in order for the snow to reach the surface the entire column from surface to top (in this case 1000 millibars [surface] to 850 millibars [dendritic layer---snow-making level at 5000 feet]) must be below freezing. That is a hard feat to accomplish at 30 degrees latitude---aka Jacksonville.
In the deep south, our weather is often influenced by warm, sub-tropical moisture from the gulf or Atlantic that injects a layer of warm air above the surface. This warm air may only be 33 or 34 degrees but it's just warm enough to melt the snowflakes as they fall. The precipitation then falls as rain. If the cold air at the surface is deep enough, the rain will then refreeze in the air and will fall as sleet. If the warm air is deeper, the rain may not refreeze in the air but will do so upon contact with the ground. This is the most dangerous of winter weather: freezing rain.
In order to cool our atmosphere from top to bottom with sub freezing temperatures, usually a series of arctic cold fronts are required to keep the supply of refrigerated air in place and keeping the warm-nose at bay. The last time this happened was in December 1989 when a trio of strong arctic fronts kept the air exceptionally cold just long enough for a disturbance to develop and pass overhead. Viola: snow. Most of the time however our timing is just pitiful. It's either cold enough for snow but we have no moisture available or we have the moisture while the cold air lags. If we could just get the two to link up. Easier said than done, ain't it?
Here's a look at some 'behind-the-scenes' meteorology:
Here is what we call a Skew-T diagram. This is plotted via weather balloons that are released twice a day at Weather Service offices around the country. The vertical lines that diverge as they rise are temperature and dewpoint. As you can see, both lines are left of the blue freeze line. This would indicate an all-snow event because both the temperature and dewpoints from the surface up are below freezing--or left of the freeze line.
What is more common in cities like Atlanta, Birmingham, Jacksonville, New Orleans, Houston, Dallas, ect, is the Skew-T diagram on the left. In this example we have the same vertical lines that diverge as they rise. Again they are temperature and dewpoint. What's different about this chart verses the one above is you'll notice that the black lines briefly move to the right of the diagonal freeze line which indicates the temperature and dewpoint briefly rises above freezing above the surface. This allows the snowflakes to melt. Look a little closer and you see the lines cross back to the left of the freeze line as you hit the surface. This chart indicates that freezing rain is the likely precipitation type due to the 'warm-nose' seen in the chart.
Move even further south, basically Jacksonville southward, our 'warm-nose' is almost always too big to get much of anything frozen to fall. While it is possible and it will snow again in Jacksonville, it's usually not wise to try and forecast it. It takes an exceedingly rare set of circumstances to transform this place into a winter wonderland.