In winter we spend a lot of time talking about whether there’s blocking or not. Last week’s wintry precipitation setup and the one that’s happening this week show us it can be important. Take a quick look at the 500-millibar chart here from the initiation frame of Wednesday’s 12Z European model — essentially what the model was interpreting the atmosphere to be like from present data at 7 a.m. EDT on Wednesday morning. The area circled is around Greenland, the North Atlantic, and some of the Arctic Ocean. The blue indicates low atmospheric pressure, while the yellow to red colors are high pressure. If you notice, there’s a lot more blue in the circle than any other color. Red colors would indicate high pressure, creating a “block” that would force that big blog of blue and purple and even peach — which has very cold air under it — farther south. That would lock in Arctic air and drive the storm track — roughly where the green colors are between the blue and yellow — farther south. As it is now, the cold air comes down but escapes north and east, because there is no blocking to keep it around. And the storm track drifts a little bit more north than would be optimum for winter storms in our region. Because of cold air damming/wedging against the mountains, some of the cold air can hang in long enough to encounter moisture from systems moving by, but ultimately the milder air the storm system draws up from the Gulf of Mexico overwhelms the cold air. That is what we’re again facing this weekend. A storm system will track somewhere across the southern half of the U.S., but is likely to drift too far north to keep from lifting milder, moist air northward to erode the cold air. And the cold air will be moving away, anyway, without the blocking high near Greenland. So the forecast for Southwest Virginia for this weekend involves a period of wintry precipitation late Friday night into early Saturday, changing to rain. It’s unclear how much and exactly what type of precipitation will fall, and honestly, forecast models seem to be leaning away from a longer period of snow/sleet and more toward a shorter period of sleet/spotty ice.
There is no real sign blocking will develop in the next 10 days near Greenland. BUT … all the red and orange you see on the same map over Alaska and the Pacific represents strong high pressure that is continually forcing cold air southward across Canada into the central United States, eventually spreading eastward. If we continue to see cold air masses like this come down, sooner or later a storm will probably squirt along the edge just in time for a more substantial snow situation than what we’ve seen so far (which has been almost nil). There is going to be no shortage of cold air between now and Christmas, though it may come in spurts with moderation (and perhaps even downright mild air a day or two) in between. The “H” amid the yellow off the Southeast coast is also something interesting to watch. At one extreme, this could become a “Bermuda high” that would deflect cold air and storminess away from our part of the country, entirely. But a weaker high positioned fairly close to where it is now, combined with some of the blocking we’ve talked about, could guide a storm system perfectly out of the Gulf up the East Coast for something big and wintry. We’ll watch how the chess pieces move in the days and weeks ahead. For now, they suggest periods of cold for us, but not exceptional chances of a significant winter storm.