A disruptive ice storm is likely going to unfold across parts of North Carolina and Virginia tonight as a fast-moving disturbance skirts the coast and drops freezing rain. The system threatens to produce significant accretions of ice along I-95 from Florence, S.C. to Richmond, Virginia, making travel impossible at times.
The National Weather Service has issued winter weather/freezing rain advisories and warnings across wide swath of real estate from the Appalachian Mountains to the Atlantic Ocean, with advisories stretching from just west of Myrtle Beach all the way up to Williamsburg, Virginia. Forecasters are increasingly confident that a disruptive freezing rain event will play out tonight across North and South Carolinas and Virginia this evening, with more than one-quarter of an inch of ice accretion possible in areas that experience the heaviest and steadiest rainfall.
Northerly winds associated with a ridge of high pressure over the Midwest are pumping cold air into the Mid-Atlantic and Carolinas this afternoon, with many readings steadily dropping through the 30s as of the writing of this post. Many locations across the region will bottom out with temperatures solidly in the 20s tonight. However, thanks to a process known as cold air damming (where cold air pools up against the mountains), this cold air will only exist at the surface. Temperatures a few thousand feet up will warm well above freezing, and this is why we're staring down the barrel of an extended period of freezing rain.
To better understand why the region is expecting freezing rain and not snow, we need to look at a model forecast sounding—a simulated weather balloon launch—for Raleigh, N.C. at 4:00 AM on Wednesday.
- The red line on the chart shows the predicted temperature through the atmosphere, in Celsius.
- The green line on the chart shows the predicted dew point through the atmosphere, in Celsius.
- The gray, horizontal dotted lines show altitude, in thousands of feet.
- Blue/aqua dotted lines stretching from bottom-left to top-right are temperatures, in Celsius.
- If you want to know the temperature at 2,000 feet, find where the red line intersects the altitude mark, then trace it down one of the temperature lines. In this case, the temperature at 2,000 feet is around -5°C, or about 23°F.
The temperature and dew point lines almost overlap one another through most of the atmosphere, signaling high moisture (and in this case, rain). This sounding also tells us exactly why forecasters are calling for an ice storm in Raleigh.
Precipitation will begin as snow in the mid-levels of the atmosphere and it'll stay snow until it reaches the freezing level at around 8,000 feet. At this point, the snowflake will begin falling into increasingly warmer air between 8,000 feet and 3,000 feet. The air will get so warm—almost 40°F at its peak—that the snowflake will completely melt into a raindrop, with no trace of ice left in the liquid.
The raindrop will then fall into the sub-freezing air starting at 3,000 feet, but it can't freeze into an ice pellet (sleet) because it's pure liquid. Ice crystals require a nucleus around which to form; if there's no nucleus, the water will cool below freezing, becoming something known as a "supercooled liquid." This supercooled raindrop will continue to fall through very cold air until it reaches the surface (which is also below freezing), at which point it will freeze on contact with the road, sidewalk, cars, trees, or any other surface that's exposed and sub-freezing.
The best way to deal with freezing rain is to stay home if you can. Cars don't do ice very well, and neither do pedestrians. The best way to walk after an ice storm is to try to walk on the grass if you can safely do so. If you have to stick to the pavement, walk with a flat foot—it's hard to get used to at first, but if you walk heel-toe, heel-toe, you will slip and fall. Walking with a flat foot spreads your weight over a greater area and provides more control over your movement. Also, if you have rock salt, sprinkle salt ahead of you as you walk to give you some traction.
If you have to go driving, go as slow as possible and don't jerk the wheel or make any sudden stops or starts. If your car begins to slide, take your foot off the gas and the brake and turn your wheel into the spin. That's all you can do at that point. If you wreck, get out of your car (when it's safe) and move as far off the road as you can. Do not stand next to your car—if you spun-out, chances are someone else will, too, and you don't want them to slide into you. Oh, and don't pour hot water on your windshield unless you want to buy a new one.
[Images: AP, NWS, SimuAWIPS, Bufkit]