Hottest July on record reported nationally; fairly strong cold front about to give summer a punch in gut, trigger storms Friday
The National Oceanic and Atmospheric Administration reported Wednesday that July has set the record for the hottest month on record in the U.S. (dating to 1895) based on average temperature over the contiguous 48 states (that is, all but Alaska and Hawaii), surpassing by two tenths of a degree July 1936. Interestingly, while several states reported a July that was among the 10 hottest on record, Virginia was the only state recording its hottest July on record, according to the National Climatic Data Center. A week ago, I wrote about how July was Roanoke’s hottest July on record, and second hottest month, dating to 1912, primarily driven by warm nighttime temperatures. How much, or how little, man-induced global warming has had an impact on this specific situation is a (pardon the all-too-obvious pun) hot topic in climatological/meteorological circles, with answers ranging from it being the primary cause, to an exacerbating factor, to a minimal component, to no effect at all above natural variation. Various studies in the academic world will be sorting that out over time. The short- term purely meteorological cause of the heat and drought, undoubtedly, has been a persistent area of high pressure primarily over the central U.S., expanding eastward at times. (It is important to note that while Virginia was the only state to have its hottest July on record, that doesn’t mean Virginia’s temperatures were hotter than states to our west that have hotter normal summer temperatures than we do). The question now is how long this “heat dome” will persist, or possibly how often it will return, into the remainder of summer and perhaps fall.
It looks certain that the heat dome will take a major body blow over the next 72 hours as a strong cold front is driven southward and eastward from Canada, almost reaching the Gulf Coast by Sunday. The punch of relatively cool, dry air into hot, humid air will fire thunderstorms, some of which may be severe, with damaging winds as the main threat. Those storms will be more likely in the Ohio Valley on Thursday, and then perhaps in the Appalachians and Mid-Atlantic come Friday. While Saturday will cool off to the 70s/low 80s for highs and 50s/low 60s for lows (perhaps coolest on Sunday morning, actually) across Southwest Virginia, our cool shot may be relatively short-lived, with temperatures beginning to rebound back to more summerlike levels by early next week. But make no mistake that in the bigger picture, this cold front is deflating the heat dome quite a bit, and there may be even more cool air pooling in Canada, perhaps even extending into the Great Lakes region. It’s not really the first hint of fall, but it is an indication that the big, bad heat wave of 2012 is not invincible.
Before the front arrives — Thursday will not really be a “heat wave” kinda day in Southwest Virginia, but it will be rather hot with highs in the mid 80s to low 90s over much of our region, and the typical scattered afternoon thunderstorms boiling up in the humidity.