Here's the observed sounding from KJAN (Jackson, MS) from 12z (6 AM) this morning. The red line is the environmental temperature with the green line (it's basically on top of the red line) is the environmental dewpoint temperature. When the lines are so close, such as shown here, that means the atmosphere is nearly saturated up until the point where they spread apart. In this case, the atmosphere was saturated up to about 500mb (18,000 ft). Notice the temperature lines (the blue straight lines) that run diagonally from bottom left to top right. From the surface up to about 650mb, the temperature is actually above freezing (above 0 C) which is why we have been dealing with rain showers all day today. But notice above the 650mb level, the temperature is below freezing so today's rain is actually starting as snowflakes but as they descend through the atmosphere into the warmer layer, they melt and turn into raindrops.
However, things will begin to change dramatically over the next 12 hours. A strong upper level low pressure center, currently over eastern Texas, will begin moving our direction tonight and actually close off/strengthen. A good moisture channel is established ahead of this feature as seen in the water vapor imagery below, which measures water vapor content in the atmosphere at roughly 600mb. The white/lighter colors represent moisture.
This moisture transport will be enhanced with the approach of the aforementioned upper-level low. As the low approaches from the west, very strong uplift will result in rain showers developing once more after Midnight, possibly starting as early as 10 PM tonight over west Mississippi. This precipitation will gradually expand in coverage to the east and northeast as more uplift overspreads the rest of the state. Initially, the precipitation will be rain thanks to the warm layer aloft hanging tough. However, evaporative cooling will begin to take effect in the mid-levels of the atmosphere and this will begin to lower temperatures sufficiently for the rain to begin to change over to snow.
Here is a forecasted skew-t model sounding for 15z (9 AM) tomorrow morning, for the KGTR airport. Remember, this is a modeled skew-t and these data could be flawed. However, notice the atmosphere is saturated from the surface all the way to 600mb. If you follow the -10 C to - 15 C isotherm up to where it intersects the temperature/dewpoint lines, you can see it is saturated at that location which is commonly referred to as the dendritic growth zone, or the snow growth zone. In this region, dendrite-shaped snow flakes can develop and begin falling. Since the atmosphere is saturated all the way to the surface AND is below freezing, it is conceivable to assume that the precipitation will be snow by this point and probably a few hours before this sounding time as well.
I think the primary timeframe for snow, at least locally in the GTR, is sometime between 4 AM and 10 AM. After this timeframe, the best combination of cold air/moisture/dynamics will be shifting eastward into Alabama. Accumulations of 1-4" are possible across east-central Mississippi, but trying to pinpoint the exact location of the heaviest snowfall is very difficult, especially with these types of setups.
Forecasting snow in the south is always challenging, and the common meteorology maxim is "upper-level low, weatherman's woe." So, some surprises are certainly to be had with events like these, so it will be fun to watch and wait to see how things unfold tonight!
Enjoy any snow you see, and God Bless!
IW
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