UPDATE 6:50 PM, 10/21: Up and down we go on autumn temperature roller coaster: Warm days ahead this week, then cold blast likely
UPDATE 6:50 PM, 10/21: A frost advisory is in effect Monday morning for every single Virginia, North Carolina and West Virginia county of the National Weather Service-Blacksburg’s forecast area — except for the typically coldest county, Greenbrier County, W.Va. The reason: Greenbrier County is the only county in Blacksburg’s forecast area that has been determined to be past its growing season. While patches of other counties may have had killing frosts, most areas have had patchy frost or no frost. Widespread lows in the 30s are expected on Monday morning, before a warm week commences — and then the freezer door opens wide from the Arctic next weekend or early the following week. END UPDATE
A frost advisory has been posted Sunday morning for Southwest Virginia counties from Roanoke, Floyd, Botetourt and Rockbridge westward (plus Amherst sticking out farther east). Frost advisories typically indicate the expectation of patchy areas of frost, especially in low-lying, rural areas. (By low-lying, I don’t necessarily mean the lowest elevations, but any pockets between higher land into which cold air can sink under clear skies with calm wind). Lows tonight will be in the 30s to low 40s across the region, and probably will be similar Monday morning, after highs mostly in the 60s to near 70 Sunday under bright, sunny skies.
October’s run of amazing days for outdoor activities will continue this week, though it may be a tad warm for autumn to some cool-weather tastes. The rest of this coming week is going to be dry with warm days and cool nights, with highs in the 70s (maybe near 80 Roanoke south and east by midweek) and lows in the 30s and 40s, becoming 40s to low 50s by mid to late week. We’ll be under the control of high pressure over the Southeast, while the storm track stays well to the west and north, and chilly, wet weather builds into the Pacific Northwest and northern Rockies. This is a classic PNA-negative pattern, for those into the alphabet soup of climatic oscillations. But there are many indications on the forecast models that this pattern will collapse suddenly and rapidly into an Arctic blast about 8-10 days from now. The 12Z European model shows cold air ranging up to 16 degrees Celsius below normal (yes, that would be nearly 29 degrees Fahrenheit) about a mile up pouring south into the central and eastern U.S. on Oct. 30. As depicted, this would be an extreme Arctic blast for so early in the season, and would probably be accompanied by the first widespread case of Appalachian upslope snow showers. It is several days out, so details of the timing and intensity of the cold blast are not refined, but it appears likely the period of warmth we experience this next week will be reversed in the following week.