UPDATE 6:50 PM, 9/1: Tropical moisture builds over the weekend, leading to showery pattern into next week
UPDATE 6:50 PM, 9/1: Slow-moving showers and storms in the Roanoke and New River valleys have led to a rash of severe thunderstorm and flash flood warnings. Very heavy rain may flood roads quickly in the heaviest downpours. Click here for latest from National Weather Service-Blacksburg. END UPDATE
Friday was a rude awakening — and ending — to a seasonable August. Roanoke’s high of 95 was the hottest day since July 26, and the 14th 95-plus day of 2012. With 14 95-plus days in 2010 and 18 last year, the 2010-12 period has had the most 95-plus days at Roanoke since there were 19, 28 and 19 in 1952, 1953 and 1954, respectively. Blacksburg wasn’t far off 90, hitting 88, also its hottest day since July 26. We will likely see temperatures 3-5 degrees cooler on Saturday, as more humidity builds in with an enhanced chance of afternoon showers and storms.
Isaac still spins. The center of the former hurricane, now a depression, has drifted northward into Missouri, the wrap of Gulf moisture clearly visible on radar (image from Friday evening linked here; you can click here for current Radar / Future Cast, which can be stretched out to see the whole nation) streaming through Arkansas and then around the top of the low-pressure center in Illinois and Missouri. It has produced heavy rainfall amounts of 3-9 inches in some bands across the parched central U.S., and spawned a few damaging tornadoes near St. Louis. The northward progress of Isaac’s center is being slowed now as it begins to interact with a slow moving cold front just to the north in Iowa. Tropical moisture bouncing against this front is likely to mean some very heavy rains falling in some of the same locations repeatedly in Illinois and northern Missouri overnight into Saturday. The front will also start to nudge Isaac’s remnants — the moisture field very slowly become unwound from a gradually dying circulation center — toward Virginia by late Sunday and Monday. But, before actual pieces of what used to be a hurricane can make it our way, Isaac’s remnant low is already swirling Gulf of Mexico moisture northeastward. This moisture will become more obvious in Southwest Virginia on Saturday, likely a hot, sticky day, with highs in the 80s to low 90s. This streaming moisture, saturating many layers of the atmosphere, is often invisible on radar views, except in the afternoon hours when showers and storms pop up in daytime heating east of the main rain area. We’ll see some of those pop-up showers and storms on Saturday afternoon, and even more so on Sunday afternoon, any of which have the potential to dump locally heavy rainfall. It is possible that a showery pattern, rather than a long continuous rain, may be what we experience right on through the holiday weekend and well into the next week. Once this tropical moisture is introduced, it won’t fully clear out until a much stronger cold front can push down from Canada, perhaps next weekend.