NWS ice accumulation 111418

Areas along the Blue Ridge south of Roanoke have the highest risk of heavy ice accumulations starting late Wednesday night and continuing through much of Thursday.

A wintry mix of freezing rain, sleet and snow looks very likely for western Virginia on Thursday morning, so much so that locations from Roanoke south were upgraded from a winter storm watch to a warning this morning, likely to follow later farther north.

The primary concern is the potential for icing to weigh on trees and power lines, leading to tree damage and power outages. We have yet to have a prolonged deep freeze, and are only a few weeks removed from summerlike temperatures, so the soil and many street materials are still retaining a lot of warmth, and won't easily collect ice. That said, bridges and overpasses will ice up quickly, and there may be patches of ice on other roads. Do be careful on the roads and don't presume that there is no ice anywhere just because most of the main roads may be just wet.

Effects are likely to be unevenly distributed across our region, owing to subtle temperature, terrain and precipitation type and rate differences. It Is very possible that some locations have tree-breaking, power-crashing ice while others not far away have minimal glazing. If it seems like a "bust" on your doorstep, do not presume it is for everyone in the region.

The way this is expected to transpire is that, during the late evening and overnight, precipitation will overspread our region from south to north. In most places, this will begin as rain or a rain/sleet mix, with temperatures somewhat above freezing at first. But evaporational cooling, the process of the air cooling as precipitation falls into a cooler, drier air mass, will drop the surface temperatures into the upper 20s and lower 30s. As that happens, sleet will increase, maybe even some wet snowflakes for a while, and any remaining liquid rain will begin freezing on objects at the surface. There may be enough sleet and snow to turn the ground white, or even accumulate an inch or maybe 2 at some spots.

For a few hours Thursday morning, sleet and freezing rain will be the predominant forms of precipitation, going over more to freezing rain as the air warms overhead and the layer of cold air near the surface thins. Many factors affect ice accretion, including the rate of freezing rain (heavier rain runs off faster and doesn't freeze as readily as lighter rain), exact temperatures (the colder it is, the faster it freezes) and how much sleet mixing there is (sleet bounces off instead of freezing on things). Icing amounts will vary across our region based on these subtle factors, but will likely range from about 1/10 to 3/10 of an inch for most, with locally up to 1/2 inch. We typically start seeing sporadic power outages with about 1/10 inch of ice and about 1/4 inch of ice begins more widespread power outages and limb-breaking, but this bar may be a bit lower with so many trees still having some leaves.

Eventually, temperatures will warm above freezing and freezing rain will change to plain cold rain. This will happen unevenly due to terrain and urbanization factors, so ice may continue to form in a rural valley hours after it has turned to rain in the city. Some ridgetop areas may actually warm above freezing before lower elevations, with warming moving aloft rather than upward from the surface.

A low-end scenario would be if temperatures don't quite cool as much as expected, mostly low to mid 30s, with only patchy freezing rain within a larger area of just-above-freezing rain. Urban areas of the Roanoke Valley sometimes make a 33-degree escape in situations like this. Again, do remember that just because there isn't ice on your trees doesn't mean they aren't already sagging with weight for someone at a different location not far away.

The high-end scenario would be a major ice storm with more freezing rain falling steadily but not too heavily for a longer time and temperatures in the 20s. There would be widespread power outages and tree damage with 1/2 to 3/4 inch of ice if the upper-end potential of this storm system is met.

There is a slight chance of another higher-end scenario in which cold air holds deeper for longer and more of the precipitation is a snow/sleet mix rather than freezing rain. If this occurs, we could get 2 to 4 inches of snow/sleet mix and less glaze ice before turning to rain. Many factors argue against this occurring, but I've seen it happen just enough over the years not to discount it entirely. This is probably what will happen farther north near the I-64 corridor and beyond.

By late afternoon, the main band of precipitation will lift north of us, but some showers will continue. On the backside of the system, northwest winds will return enough cold air to change most of what's left to snow on Thursday night and early Friday, though it appears significant amounts will be confined to the typical upslope bands in West Virginia and in higher elevations of extreme western and southwestern Virginia near its border.

The weekend looks pretty nice -- sunny with highs in the 50s.

Contact Kevin Myatt at kevin.myatt@roanoke.com. Follow him on Twitter @kevinmyattwx.


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