UPDATE 5:20 PM: Might Friday be Roanoke’s last 90-degree day of 2012? Weekend cold front brings autumnlike temperatures
UPDATE 5:20 PM: It won’t be Roanoke’s last 90-degree day of 2012 — the high made it only to 88. The last 90-degree day of this year was either last Saturday, Sept. 1, or will be a day in the future during a warm spell. I’m leaning to last Saturday, with the possibility of reinforcing Canadian air shots in the next couple of weeks. Typically, we don’t have 90-degree heat in late September or October unless there is a continuing or intermittent heat wave extending from August into September — but of course, it’s still possible. END UPDATE
Over the past 10 years, Sept. 10 has been the average date of the last 90-degree high temperature in Roanoke. During the past decade, it came as late as Oct. 9 in 2007 (10 days earlier than the record latest in 1938) and as early as Aug. 18 in 2009 (the last 90-degree day has been in July on 4 occasions since 1912). So we are definitely in the part of the calendar when it’s not highly speculative to suggest that an expected 90-degree day could be the season’s last. Friday might be that day, as summerlike high pressure builds over for a hot and very likely dry day, with highs in the 80s most places west of Roanoke and upper 80s to low 90s in the Roanoke Valley and points south and east. It’s possible that last Saturday was the last 90 of 2012, or that there will be a later one, even this Saturday if the cold front is later arriving than forecast. But I think Friday has a pretty good chance of being that day, the 30th 90-degree day of 2012 — actually a tiny bit below the 100-year average of 33 90-degree days. Keep in mind we didn’t have our first one until June 19.
This weather map for Tuesday morning, posted by the Hydrometeorological Predition Center, shows that the cold front that will push through Southwest Virginia this weekend is a really big deal in the progression of summer into fall. By Tuesday morning, the front actually clears the Florida peninsula and pushes deep into the Gulf of Mexico, the first time we’ve seen that since May 11. The map also shows high pressure centered over Southwest Virginia — if that verifies reasonably well, and we get a clear night with calm winds, there could well be several 40s low temperatures in our region; certainly, widespread low to mid 50s are likely. After Saturday pushes into the 80s and storms develop ahead of the front, highs are expected to drop back into the 70s Sunday behind the front and remain there at least through midweek. This is a substantial shot of Canadian air that will clear out whatever’s left of Isaac’s tropical residue. It doesn’t appear likely to be a major rainmaker – cold fronts approaching from the west usually aren’t, unless there is a strong low riding up them from the southwest, and it may well be that the heaviest storms jump from west of the mountains to east of us in the Piedmont and coastal plain. But there will be some threat of strong to severe storms on Saturday, considering the strength of the front and the lingering warm, moist air in our region.
As of late Thursday night, it does not appear that Isaac’s offspring in the Gulf of Mexico is going to become a tropical cyclone — the storm is disorganized, with convection mainly southwest of the circulation center, and it has encountered both upper-level shear and dry air to keep it in check. Hurricanes Leslie and Michael continue to churn in the open Atlantic. The latest forecasts have Leslie veering a little east of Bermuda, and possibly scraping east of Newfoundland as well. That will still be close enough to create some higher surf and dangerous riptides along the East Coast — beware if you head that way the next few days.