Now about halfway through the month of December, the balmy start to the month has put back into play the possibility that 2012 could be Roanoke’s warmest year on record, something I wrote off as recently as Nov. 21. The first 15 days of the month averaged 48.2 degrees. The month as a whole would need to average 46.8 for 2012 to surpass the yearly average temperature of 59.6 from 1931. It would need to average 45. 6 to at least tie that mark. An expected cold shot of air next weekend will likely bring the month’s average down quite a bit, but it still has some room to drop, especially if we rebound to somewhat milder weather after Christmas. After the last 2 months of the second-warmest winter on record, the warmest March on record starting off the warmest spring on record, and then the hottest July on record splitting a summer that was fairly mild on the front and back end, it appeared that cooler weather in October and November had derailed the year’s obvious march toward record status. The first 60-degree average year appears to be off the table now, requiring a 50.4 average for the month. (Incidentally, 47.8 is the record warmest December for Roanoke, set in 1956. Last year’s 43.8 was the 8th warmest December … and the 31.3 average of the previous December in 2010 was the fifth coldest.) But a record tying or setting year is still possible — unless next week’s cold spell hangs in for the balance of the year. Joining 1921, 1931, 1990 and 2007 as the fifth 59-plus degree year is very likely. Nationally, 2012 is virtually assured to be the hottest year on record, dating to 1895.
Back to the here and now, with rain potential. The NAM (North American Model) and GFS (Global Forecast System)models — the two major U.S. forecast models — are fighting it out about whether Southwest Virginia will be a rain bullseye or rain shadow, respectively, in regard to the storm system the next couple of days. The Hydrometeorological Prediction Center has more or less split the difference, showing our region generally around the 0.5 to 0.75 inch range over the next 3 days — more than the GFS shows, less than the NAM shows. The reason such a configuration could occur is what often occurs here, with the best moisture staying to our south while the best storm dynamics to lift and condense moisture go around us to the west and north. If you’ve lived here a few years and paid attention to the weather map, you’re all too familiar with this two-step. The good news for those wanting moisture to ease the dryness is that we are likely to get at least some, as Gulf of Mexico moisture overruns cooler surface air on southwesterly mid-level winds overnight into Sunday, triggering some showers. By Monday, a low-pressure system and cold front will move closer, adding more lift to squeeze out the moisture. Roanoke’s biggest rain since Oct. 1-2 was only 0.41 on Nov. 12-13, so it wouldn’t take a whole lot of rain to make this the rainiest system in more than 2 months. Once the cold front passes late Monday or early Tuesday, a few mountain snow showers may develop on northwesterly upslope breezes, but the air mass behind it is not that all that cold, and southwesterly flow ahead of the next storm system tracking northwest of us may get us close to 60 again in some spots by Thursday.
Late week, a surge of Arctic air remains likely Friday into the weekend, with blustery northwest winds, highs not topping 40 on at least a day or two, and the potential for a more vigorous round of upslope snow showers. Forecast models today do want to move low-pressure over the Great Lakes eastward a little faster, rather than stalling it, so a multi-day snow shower event appears less likely. Next week has lots of questions with it, but on the whole, it appears likely to be the coldest average-temperature week so far this month — not a terribly impressive feat, considering how mild it’s been, though there is at least some model support for a very cold Christmas week — with perhaps some potential for a storm system in the days after Christmas. Whether the temperatures rebound after next weekend’s cold surge, or remain at least seasonably cold, will go a long way to determining if a potential system after Christmas would bring wintry precipitation or mainly rain.