Two pretty cool mornings lead into typically warm August week ahead; next weekend may bring autumn preview
When a heat wave shifts to the West Coast, that usually means that cooler air is slipping into the East. As high pressure shifts westward, its clockwise rotation spins in cooler air from Canada. The cold front that passed through early Saturday is the first of perhaps 2 or 3 installments of Canadian air we’ll see over the next 10 days or so. Sunday and Monday mornings will feature many lows in the 50s, with low 60s likely in the Roanoke Valley and eastward. Those numbers are not astounding for mid-August — Blacksburg’s normal low is 59, Roanoke’s is 66 — but one or both will likely be the coolest mornings we’ve had in Southwest Virginia since the June 28 sudden flip (59 to 97 that day in Roanoke) from a remarkable summer cool snap to an intense record-setting heat wave.
While this week will feature many days with temperatures close to mid-August norms (normals for Blacksburg are a high of 82, low of 59 through August 20, while Roanoke shifts from 87 to 86 for a normal high today and from 66 to 65 for a normal low on Wednesday), a weather pattern similar to what we see with a mid-winter Arctic blast may be possible next weekend. This series of temperature anomaly maps off the 12Z Euro model (courtesy of Allen Huffman’s RaleighWx model page)– linked here for Wednesday, Friday and Sunday (insets from top to bottom in time order) — show the progression of a pocket of cooler than normal air (the blue colors) from near the polar region southward into the central U.S. and then spreading eastward. By Sunday, (lowest inset image on left) the darkest blue core of air is over West Virginia, showing temperatures about 9 degrees Celsius below normal (that’s about 16 degrees Fahrenheit) at the 850 millibar level — about a mile up. If that verified, it would certainly make for a downright chilly night or two at higher elevations like Mount Rogers/Grayson Highlands in Southwest Virginia and Snowshoe Mountain in West Virginia. That level of anomaly won’t necessarily translate to lower elevations at the same magnitude, but it would not be unreasonable to think temperatures 10 degrees below mid-August norms would be possible for a day or two by next weekend IF this model verifies. Air masses like this do tend to modify quickly, especially with us just barely past the peak solar period of the year, so the models may be overdoing this some. But the synoptic pattern to deliver a cool Canadian air mass like this appears to be in place, with many forecast models in broad agreement.
Though the correlation for our region is weakest in mid-summer, the North Atlantic Oscillation is spiking deeply negative, meaning high pressure is building over Greenland — this is obvious on the models. This can block the jet stream and force it to dip more southward over the Eastern U.S. (note that it was also somewhat negative during our heat wave period in early July — that played some role in keeping the Northeast from being devoured by the same heat wave that fried us, and that juxtaposition of air masses contributed to the June 29 derecho). Like I said, it’s a weak correlation as far south as we are, and by itself wouldn’t mean much in August — but combined with some other factors, it plays a role. The Arctic Oscillation is also sinking into negative territory, meaning cooler air typically held close to the North Pole will be allowed to circulate farther outward away from it. But perhaps most importantly, the Pacific-North Atlantic pattern is spiking quickly into a positive phase. This is the key mechanism, as the models show strong high pressure building through the western U.S. and ultimately into central and western Canada, which in turn dislodges this pool of cooler air southward and eastward. On the linked maps, the yellow/green areas of above-normal temperatures over the northern Rockies and western Canada, eventually spreading into the Northern Plains (days after that region gets some downright cold mornings) roughly correlates with the presence of this high-pressure system.
If this modeled pattern verifies to its fullest, we may see an early preview of autumn next weekend. Roanoke’s record low for Aug. 19, next Sunday, is 52, while Blacksburg’s record is 48 — those would be within the realm of possibility IF the pattern maximized its potential. But of course, projected patterns 7+ days out usually don’t maximize — just too many things that can vary it, a little or a lot. On the warmer end of possibilities, a couple of relatively cool mornings are ahead, at least compared to the muggy lows that have dominated this summer since June 29, and then we get several days of close-to-normal August temperatures, reinforced by a mid-week cold front, followed by what is likely to be at least a moderate cooldown next weekend. Not a bad week at all for those who didn’t care for early July.