UPDATE 6 PM: Hydrometoerological Prediction Center has posted a slight to moderate risk (small sliver of high risk, west of I-77) of 4-plus inches of snow for Friday across western Virginia. Will post a new entry later this evening. END UPDATE
There will be finally be no doubt that it is winter on Tuesday, a windy though sunny day with highs that may not reach 20 in many locations west of Roanoke and won’t reach freezing anywhere in Southwest Virginia. Temperatures may even slide downward during the day as the brunt of Arctic air arrives, the biggest shot of it we’ve had in at least a couple of years. There will probably be some single-digit lows west of Roanoke and no higher than the mid teens anywhere on Wednesday morning. There is a weak disturbance that might trigger some snow showers by Wednesday afternoon and evening, but doesn’t look to be too significant.
Then there is the late Thursday/Friday storm system. Sound familiar? Just like last week? In timing and iffy-ness (if that’s a word), it is just like last week. In almost everything else, it is entirely different. The first important difference is that this week we are dealing with northern-stream energy, diving in from Canada through the central U.S., not a southern-stream disturbance moving in from Mexico. Secondly, we’ll have Arctic air parked over us, though starting to edge eastward, when the storm system arrives. Tracking how much cold air remains at various layers of the atmosphere, how far south the northern-stream system can dig, and how much moisture it can pull up and fling into the cold air is a three-ring circus the forecast models are flailing about trying to outguess right now. Arctic air of the caliber moving in the next couple of days is very hard to dislodge at the surface, especially with the Appalachians tending to hold it in place. However, it is much easier to move out aloft if there are 100-200 mph winds blowing through the mid and upper layers of the atmosphere with the approaching system. Cold air through the atmosphere from cloud to ground leads to snow, but warm some layers aloft, and we start getting into sleet and freezing rain. The prognosis generally remains the same as before: Wintry precipitation will probably fall for some period of time late Thursday and/or Friday. Most guidance has trended colder, so chances have increased on the snow/sleet end of things and moved a little away from freezing rain/rain. The Hydrometeorological Prediction Center best-guess weather map for Friday, inset at left, shows a low in Kentucky essentially moving right at us with a bent stationary front ahead of it illustrating how Arctic air is still entrenched all the way almost to Atlanta. It’s certainly not a typical way to get significant snow here, but not much about this winter has followed the rulebook so far. I still have some doubts about the level of moisture and the potency of the storm system as it approaches us, so I’m not entirely sold on this being a significant winter weather event just yet.
For those who want the cold weather to settle in for weeks, and for those who want the cold to leave and be replaced by mild weather for weeks, you’re both out of luck. After a very cold weekend, maybe as cold or even a little colder than the next couple of days, temperatures are likely to rebound next week ahead of a storm system tracking far to our west. But it won’t last, as new Arctic air is likely to follow that storm. It’s a dynamic pattern that will hold lots of potential for strong storm systems in the days and weeks ahead.