Article and Forecast by Meteorologist Peter Forister
Graphics by Denver Murray
— Peter Forister ❄️☃️❄️ (@forecaster25) February 17, 2021
A very strong arctic airmass has setup to the northeast of the Mid-Atlantic, and is forcing a layer of cold air to run down the spine of the Appalachians. This “CAD” (Cold air damming) ensures that temperatures at the surface will remain below or near freezing for most of the day Thursday.
A low pressure system will track through Tennessee and to the south of the Mid-Atlantic, bringing with it plenty of moisture. This moisture will “Overrun” the cold air at the surface. The remarkable thing about this particular storm system is that the mid levels of the atmosphere will be unseasonably warm (possibly exceeding 50 Degrees several thousand feet up) and will be transported north over Virginia by VERY strong mid level winds (over 80mph). This process of moisture and temperature movement is called “WAA” (Warm Air Advection) by meteorologists, and is what will make the forecast so tricky and potentially dangerous on Thursday.
Currently, most models indicate that this “WAA” will overrun the cold air and produce an ugly mix of sleet, freezing rain, and a bit of snow (for areas north of Interstate 64). The exact strength of this warm area aloft will determine which areas will get sleet and freezing rain. We expect that areas north of Highway 460 will have primarily sleet through the day, and areas south of 460 will remain as freezing rain (ZR).
This is a particularly tricky forecast because small differences in the strength of the WAA will mean big differences at the surface. Take two models for example: At 10am Thursday, the HRRR model has WAA winds at 65kts and the NAM model has WAA winds at 75kts. The HRRR keeps the sleet mostly north of Route 58 in Virginia, while the NAM model has sleet all the way south to Charlotte, NC. As a rule of thumb, WAA is generally slightly stronger than modeled, so in this case the HRRR is the more reasonable solution of the two. In areas where the sleet does not fall for more than a couple hours, the freezing rain impacts will be much worse.
Current NWS products and the reliable HREF FRAM model indicate that an area of 0.5″ – 0.75″ of freezing rain (ZR) accumulation is likely along Route 58, Route 360, and Route 460 in areas west of I-95. This amount of ZR would be “Catastrophic” (per the Wakefield NWS) and leave areas without power for multiple days and make most roads impassible. The NWS blend of models (a forecasting system that has proven to be quite good) keeps ZR accumulations a bit lower than this, ranging from 0.25″ to 0.4″ over the same areas. This would be less destructive, but still lead to widespread power outages especially in the wake of last week’s storm.
Areas to the north of Highway 460, along Interstate 64 and further north, will likely start with wet and heavy snow in the morning, but will quickly transition to sleet. Accumulations of sleet could reach 3-4″ in the heaviest areas. It is unlikely that anywhere in Virginia will remain as snow through the whole day. The areas furthest north in Virginia (such as Winchester, areas along route 50 and the northern I-81 corridor) could see wet sloppy snowfall of 4-8″.
Sleet: Bad for roads. In areas of heavy sleet, even treated roads will quickly be covered with a layer of ice. This can easily cause widespread traffic accidents, but will not impact electricity lines.