It’s not quite a week into December, but some snow fans in Southwest Virginia are getting impatient because of the lack of cold weather, Thursday’s 40s highs and 20s/30s lows (47/31 at Roanoke, 46/22 at Blacksburg) only being a tease before a return to warmer than normal temperatures (60s highs, again) through the weekend. Maybe that’s because the Dec. 5-7 period has often produced a significant snow in recent years in all or part of our region — 2002, 2003, 2009 and 2010 coming immediately to mind. And many folks are wanting some wetness of any kind, with drought continuing to grow and worsen. There are a few showers popping up early on this Thursday evening from a weak disturbance and some growing moisture on southerly wind flow aloft, but not enough to settle the dust.
Neither of these issues are going to change with the flip of a switch. We often talk about pattern changes in the atmosphere as if they happen all at once. Sometimes, they do change quickly, but more often, it’s more of a pattern evolution that can take many days and even weeks. Over the next week or two we are going to see pieces of the Arctic air mass over Canada interact with the milder air that has built into much of the U.S. around the flow of the recurring Pacific Northwest low-pressure system. Low-pressure systems will develop along the boundaries of the contrasting air masses, and circulation around those systems will help swing cold fronts through. The first of these looks to occur about Monday or Tuesday, with a heightened chance of showers (and maybe even some thunderstorms, as mild as it will be ahead of the front) followed by a shot of at least seasonably cold air for a few days (sort of like Wednesday). There will probably be some snow showers behind the front as cold northwest winds blow up the Appalachians western slopes, but no organized winter storm appears to be in the offing. There may be two or three more similar storms in the next 2 weeks, each one dragging down some colder air and ever so slowly eating into the mild weather pattern we’ve been experiencing, one that will keep springing back to life between the fronts until there aremore large-scale changes, such as that Pacific Northwest low not redeveloping and being replaced with high pressure. These storm systems will each bring a chance of rain, and perhaps will eventually dig into the Gulf of Mexico for more significant, widespread rain than we’ve seen in while a time or two within the next couple of weeks. The Climate Prediction Center is tilting us toward wetter than normal in the 8-to-14-day period — but keeping us in warmer than normal as well, despite the quick, sharp shots of cold behind fronts. But for those wanting more winterlike weather, or at least wanting halfway decent rain, this slow storm-by-storm process is what it will take to change the atmosphere to become at least a little more conducive to what you want. For those who enjoy springlike weather — this will be another good weekend for you.