* Period of snow/sleet early Sunday changing to long period of freezing rain remainder of Sunday into Sunday night and possibly into Monday morning. Minor snow/sleet accumulations of under 2 inches are expected at this time, with more possible along and north of the I-64 corridor. Ice accretion of 1/4 to 1/2 inch is projected. This would be enough for some damage to tree limbs and sporadic to scattered power outages. Greater amounts may be possible in some areas of the Shenandoah Valley and westward, north of Roanoke. Road surfaces will be slow to build ice at first due to recent warmth, but early snow/sleet and temperatures falling into the 20s may allow more as the day continues, with some roads at elevation, in shaded areas and especially bridges and overpasses becoming very treacherous. There are still variables in the amount of snow/sleet that falls to begin, precipitation amounts and duration, and the temperature pattern that could alter the weather from forecasts, so stay tuned.
From Merriam-Webster dictionary online:
* a gradual process in which layers of a material are formed as small amounts are added over time
* something that has grown or accumulated slowly : a product or result of gradual growth
You may notice the National Weather Service uses the word “accretion” rather than “accumulation” to describe ice amounts, whereas the latter term is favored for snow. The way ice forms on tree limbs and exposed objects is more of a process of layers being added over time rather than stacking up the way snow or sleet does on the ground or elsewhere it can cling.
The forecasting challenges with ice accretion are immense. Some are obvious: Mixed-in snow and sleet reduce freezing rain volume, and subtle temperature changes in borderline events can mean it’s 32.1 and not freezing 6 feet off the ground where most official thermometers are positioned but 31.9 and freezing a few feet higher. There are physics involved that can conflict with each other in warming and cooling effects of falling precipitation, humidity levels, and even the very act of water turning from a liquid to a solid (actually releases a slight amount of heat that can be significant in a large ice storm with near-freezing temperatures). Wind and varying temperatures of surfaces play roles too.
But the most crucial aspect of an ice accretion forecast comes down to how much falls and how much of that freezes. One fallacy that is repeated over and over again is looking at a total precipitation map on a forecast model and presuming that every drop of rain depicted will flash-freeze. It doesn’t happen like that. Seeing a zone of 1 inch of rain with temperatures below freezing does not mean 1 inch of ice will form. Rain, being liquid, runs off — more of it, the harder it falls. It freezes faster the farther the temperature is below the freezing mark, slower with temperatures nearer freezing. In most cases, ice accretion ends up being no more than 1/3 to 1/2 the total amount of rainfall with temperatures below freezing. You might get more if temperatures are well below freezing and rain falls lightly for a long time, less if temperatures are very near freezing and rain is falling heavily.
Bottom line: 1 INCH OF FREEZING RAIN DOES NOT MEAN 1 INCH OF ICE ACCRETION. But it might easily mean 1/3 to 1/2 inch of ice and that’s enough to cause plenty of problems.
That brings us to our coming winter storm. At left is the Weather Prediction Center projections for total liquid precipitation through Monday morning — most of Virginia from the Blue Ridge westward is in the 0.75 to 1 inch zone. (Bigger map linked here, with color chart for amounts). Of course it might rain more or less than this amount, but let’s just presume for a minute that 1 inch of liquid falls Sunday into Monday morning. Part of that will probably come as snow and sleet to begin. Part of that may occur as temperatures have risen slightly above freezing late Sunday night or early Monday morning — there are some uncertainties about when temperatures rise above freezing, but generally will occur south to north late Sunday night into early Monday. At the least, temperatures will rise to right at the freezing mark on Sunday night, and that will mean a slower rate of freezing on objects than with temperatures in 20s as might happen Sunday afternoon. Take an inch of rain, subtract out that portion which is snow and sleet (0.1 to 0.2 inch of liquid is a decent guess), and that which doesn’t freeze or freezes more slowly late Sunday and early Monday (0.2 to 0.4 is a ballpark guess) and that gets us down to roughly 1/2 to 3/4 inch of rain. If only 1/2 of that “accretes,” we’re down within the 1/4 to 1/2 inch ice forecasts being issued by the National Weather Service — not a widespread disastrous ice storm, but certainly problematic for many.
Again, it is possible more rain will fall, it will be below freezing longer than expected, or that there will be less sleet/snow on the front end. So it is possible ice will exceed forecasts — just as it is possible it will underachieve forecasts, if less falls, it is not as cold as expected or rises above freezing quicker, or there is more snow/sleet than forecast.
The best course of action is to PREPARE AS IF YOUR POWER WILL DEFINITELY GO OUT AND ROADS WILL BE IMPASSABLE (from ice or fallen trees) and then if it doesn’t happen that way, you’ll stay prepared for the next potential emergency.