Summer showing signs of slowly dwindling away, but it’s still a bit too early to say what winter “will” do
I saw a Christmas tree in a store lobby. Campaign attack ads have been on the television for months for a November election. In a similar vein, the winter prognostication season has gotten under way with a bold prediction by Accuweather: “Following a snow drought during winter 2011-2012, the mid-Atlantic and southern New England will get a snow dump this winter. ” The prediction largely centers around the expected return of El Nino, the irregularly recurring warming of a strip of equatorial Pacific waters. As you may recall, El Nino was a key influence in the 2009-10 snowy winter we had, pumping a series of wet Pacific storm systems across the southern U.S. into cold air that became trapped much of the winter over the eastern U.S. But as an article today on The Washington Post’s “Capital Weather Gang” and my own Weather Journal column from September 2009 both note, El Nino winters tend to be “feast or famine” for snowfall in the Mid-Atlantic and central/southern Appalachians largely based on whether or not the El Nino coincides with high-pressure blocking over the Arctic Circle and Greenland. So how to take the Accuweather prediction: It’s way too early to say winter WILL do anything, but it’s an interesting idea if (1) El Nino develops as expected (2) it coincides with Arctic air forced southward by blocking high pressure systems in the far northern latitudes.
That, believe it or not, brings us to the current weather pattern — strong blocking highs have developed over Greenland and the North Pole, and that has in turn led to a weather pattern allowing a series of shots of Canadian air into the eastern U.S. It doesn’t work quite as efficiently at our latitude in August as it would in January because of the mean jet stream position being farther north — it won’t dip south all the way to the Gulf of Mexico, usually not making it much farther south than the Great Lakes — and the tendency for the built-up heat of summer (in this case, an extreme summer) to moderate the cool air shots as they dip south. But the atmospheric regime change has been striking enough that we will now consider a possible high in the low 90s in Roanoke by Friday as a “heat spike.” This short-lived warmup — starting today with some mid to upper 80s – is preceding the arrival of a rather strong Canadian cold front late on Friday or early on Saturday, that will likely drop highs about 10 degrees between Friday and Saturday. The air mass behind it will be moderating as it heads southward, but it will be broad enough (and trapped enough by high pressure) to have some staying power, with perhaps 4 days or so of 70s-low 80s highs and 50s-near 60 lows from Saturday to Tuesday in Southwest Virginia. Some 40s lows are possible in the usually coldest spots — high exposed ridges and protected valleys — if we have a night or two of clear skies and calm winds to maximize radiational cooling. The arrival of the cold front is likely to trigger a round of showers and thunderstorms, with some localized severe storms and/or heavy downpours possible, but a widespread general rain does not appear to be likely. There is some chance a low will develop along the front and hang some moisture back for a few showers on Saturday, but at this time it appears the front will push far enough through to allow the cooler, drier air to set into western Virginia by Sunday, at least. This may not quite qualify as a “fall-like” air mass, but some of the mornings will provide at least a taste of fall. While it appears likely that a gradual warmup will occur next week, there remains absolutely no indication that extreme heat will build back into the East anytime soon. I think we’re long done with triple-digit heat this summer in Southwest Virginia, and with each day this northwest-flow-dominated air mass hangs on, the chances of another mid-upper 90s day in the Roanoke Valley and points east diminishes. This beast of a summer that gave us the hottest high temperature in 29 years and a fierce windstorm that will rattle our memories for a lifetime may yet fade into history with a only a whimper.