UPDATE 8:20 PM: Moist wedge building in overnight; record hot July pace for Roanoke; widespread U.S. drought
UPDATE 8:20 PM: Easterly winds are banking relatively, cool moist air against the mountains. The upslope effects plus some temperature contrast are triggering small but heavy showers, perhaps accompanied by some thunder, in and around the Roanoke metro area, southward along the Blue Ridge through Franklin and Henry counties. (Radar linked here) Brief periods of heavy rain will be possible with these showers. As the easterly winds continue to bank moisture overnight, a layer of low clouds and scattered drizzle is likely to develop — such a layer kept much of northern Virginia in the upper 60s and low 70s throughout today. How long this layer can hold in place on Sunday will determine whether we have a cooler day with a high in the 70s or a warmer, sticky day in the 80s. END UPDATE
This weekend: Warm and sticky, highs mostly in the 80s, with scattered showers and storms possible as daytime heating interacts with a stubborn front and thick moisture. Some will get heavy rain (like Galax did Friday evening) and/or gusty winds, others will get lighter rain and a little thunder, and yet others will get sprinkles or nothing. Can’t really be much more specific than that. Same old midsummer dog days refrain.
We are 20 days into July, and Roanoke is on track for some possible monthly record heat. As with the record hot summer of 2010 and runnerup in 2011 (a merely normal June average temperature may make it hard for this summer as a whole to equal the previous two), it’s more about warm overnight lows than hot afternoon highs. Through Friday, Roanoke’s average high temperature for July is 91.9 degrees — that would rank fourth were it an end-of-month average. But more impressively, the average low has been only 71.2 degrees. Should a similar number be posted after 11 more days, it would crush the current July record of 69.9 degrees set just last year and even overturn August 2007′s record 70.6 for warmest average low for any month. Roanoke low temperatures have been no lower than 66 on any day this month and have been 70 or above on 12 of 20 days. The normal low is 67 — only July 12 at 66 has been below that, every other low has been 1 to 9 degrees above normal. Averaging the mean high and low together produces an average temperature for the first 20 days of July of 81.6 degrees, which, were it a month-end average, would set a new record for hottest July and be just less than half a degree off the 82.1-degree record warmth of August 2007 for hottest month on record. I bring this up now because we are two-thirds through the month with absolutely no sign of a significant weather pattern change. High temperatures go down with cloudy days like Friday (only 82 at Roanoke, 76 at Blacksburg) and passing cold fronts, but muggy low temperatures are hanging on even in the somewhat cooler periods because truly cool, dry air from Canada has been blocked from coming this far south.
Drought begets more drought. Drought begets more heat. More heat begets more drought. Vicious cycle much of the U.S. is in. We’re on the eastern periphery of the real trouble (Thursday’s Drought Monitor map shows the darker colors mostly to our west), but until there are major pattern changes that can bring large-scale storm systems to soak at least some of the very dry areas, there will be little change to prevailing mostly hot, mostly dry weather. Short of a large tropical system moving deep inland, an unlikely event for now because of an infusion of Saharan dust that has tracked westward from Africa over much of the Atlantic basin, those kind of drought-busting events probably aren’t going to occur for at least another 45 days, if not longer, when seasonal changes in upper-air patterns begin to take hold. So we’re pretty much stuck with a dome of hot, dry air over the central U.S., expanding or wobbling eastward over us at times, and at other times aiding in pushing at least some fronts and disturbances southeastward into the Appalachians and Mid-Atlantic regions. The heat and lingering humidity (plus terrain factors in the mountains) will be sufficient at times to fire scattered afternoon storms, but at other times hot, dry air moving aloft will block upward growth of storm clouds. A dip in the jet stream into New England may be able to erode the heat dome near us in coming days and weeks, which might lead to a somewhat cooler pattern — but widespread, general rainfall remains doubtful.