Another round of wintry weather tonight will get the south off to an ugly crunch for the second Monday in a row. The first batch of snow and ice is already falling across a swath of real estate from Louisiana to North Carolina, while a more threatening round of sleet and freezing rain will affect Texas and Oklahoma on Monday morning.
The two slugs of precipitation will straddle the freezing line at the surface, which stretches from central Texas through Arkansas and extreme northern parts of Alabama and Mississippi. Precipitation south of the surface freezing line will fall as rain, along and near the line as freezing rain and sleet, and areas far enough into the cold air will see some snow on the ground by daybreak.
The type of wintry precipitation that falls is almost wholly dependent on atmospheric temperatures thousands of feet above the surface. Even it's freezing in your backyard, the snow still has to make it from the cloud to the ground as a snowflake without undergoing any melting. This storm—as with most storms in this region of the country—will have a delicate battle with warm air a few thousand feet aloft, bringing the second (and even third) ice storm in as many days to some parts of Dixie Alley.
Here's what this afternoon's run of the GFS model thinks temperatures will look like around midnight CST across the southern United States. I've (somewhat sloppily) highlighted the freezing line—north is below freezing, south is above. In this solution, areas south of the freezing line would see all rain at midnight.
Now, here's a look at the 850 millibar level of the atmosphere—roughly around 3,000 feet—showing temperatures in Celsius. Again, I've crappily highlighted the freezing line.
We see the tango between warm and cold air in the atmosphere occurring over pretty much the same areas, but the freezing line makes a noticeable northward march when you look at the atmosphere just a few thousand feed above the ground.
We can look at a model simulated SKEW-T chart—the graph tracing temperature and dew point data collected by weather balloons—to see what the vertical temperature profile of the atmosphere will look like at a given time. Here's a look at Dallas-Forth Worth International Airport around 2:00 AM on Monday.
The models (the Rapid Refresh Model, more specifically) predict sleet around the beginning of rush hour near DFW Airport on Monday morning. The line between sleet and freezing rain is very fine, and it can be hard to predict one or the other without some pretty dramatic atmospheric factors in play. It's pretty easy for sleet to develop and change over to freezing rain if mid-level temperatures are just one or two degrees warmer than forecast.
For the purposes of ice/sleet accumulations, I will not differentiate between the two storms. The first half is already occurring, with the second part (affecting Texas and Oklahoma) developing later on Sunday evening through Monday morning.
The National Weather Service's forecasts lump sleet in with snow since it accumulates in the same manner. Almost all of the snow accumulations on this map are really sleet, except for along the northern periphery of the storm and out west towards the Rocky Mountains.
Sleet is a very different animal from snow. These tiny ice pellets are frequently mistaken for small hail because of their resemblance to the latter. Sleet looks like snow when it's on the ground, but it has a tendency to freeze into a glacial sheet of rock-hard ice once it starts to accumulate more than one-half of an inch on the ground.
Those of you in Dallas, especially, may remember the "cobblestone ice" fiasco in December 2013 that coated area roads in inches of ice that caused major damage to vehicles and made it very hard to travel around the area for many days after the storm. That ice was caused by sleet accumulating on the road and freezing solid, making it impossible to remove unless crews chipped away at it or used graters.
Any accumulating wintry precipitation is dangerous when you have to drive or walk on it, but ice accretion from freezing rain is the most dangerous of the bunch. Freezing rain acts exactly as its name suggests—it freezes on contact with any exposed surface with subfreezing temperatures. Roads, sidewalks, vehicles, trees, railings...anything that's exposed to freezing rain will likely wind up with a glaze of ice after the precipitation stops.
Freezing rain becomes especially dangerous when ice accretion exceeds one-quarter of an inch or more; this is the point at which tree and power line damage becomes a possibility. Some areas of the south could come close to seeing accretions on the order of one-quarter of an inch—especially around the Arklatex—but any ice accretion is dangerous.
All it takes is a thin glaze of ice on roads and sidewalks to make even travel by foot too dangerous to attempt. If you're able to do so, stay home on Monday until temperatures climb above freezing long enough that ice has a chance to melt. If you have to venture out on Monday morning, drive very carefully and give yourself plenty of room between your car and the idiots around you. If you have to walk on an icy surface, walk with a flat foot to distribute your weight over a wider surface area (lowering your chances of slipping).
Winter's Many Last Gasps
The air behind these two systems will be pretty darn cold—low temperatures on Tuesday morning will clock in 20 to 30 (or more) degrees below normal for this time of the year, especially around the Ohio Valley and in parts of the Northeast. Lows at or below zero are likely from Indianapolis and Detroit straight east to the megalopolis on the I-95 corridor.
It's going to get even colder towards the end of the week, but we're approaching March and the sun is on a steady climb towards the Equator. It won't be too long before the warm air starts returning and battling the cold air for dominance—unfortunately, that often results in the severe weather we're all too familiar with during the spring months, but we'll cross that destroyed bridge when we get to it.
Winter is on its way out...eventually...and it's determined to go out in style.