UPDATE 1:30 PM, 6/8/2019: The flash flood watch, not unsurprisingly, has been extended until Sunday evening at 8 p.m., and now includes more localities, as Henry, Franklin and Montgomery counties have been added, as well as Roanoke County/Roanoke city/Salem. Redeveloping bands of rain are expected to continue pivoting through this afternoon through Sunday, adding 1-2 inches, locally more, on top of 1-5 inches that has already fallen in much of this region. Relatively dry conditions previous to the rain have staved off widespread flooding problems, but some localized flooding has already occurred -- the South Fork of the Blackwater River and some tributary streams went out of their banks in western Franklin County, resulting in some closed roads, and prompting a flash flood warning. A slow-moving low to the west, continually pumping in Gulf of Mexico moisture atop a cooler surface air mass wedged against the mountains by high pressure, plus a complex overlay of atmospheric boundaries and terrain effects, will continue to allow bands of rain to form and lift northward, with some thunder possible, through Sunday evening. Showers and storms are likely to linger intermittently through Monday and Tuesday until a cold front can clear most of it out of the way at midweek. END UPDATE
The potential for heavier bands of rain to focus along the Blue Ridge south of Roanoke overnight into early Saturday has prompted the National Weather Service office in Blacksburg to issue a flash flood watch for four counties in Virgnia -- Floyd, Carroll, Grayson and Patrick -- extending into northwestern North Carolina.
This watch is effective until noon, and reflects the possibility of some locations getting 1-2 inches of rain within an hour in the heavier bands, capable of swelling small streams beyond bank full and possible flooding some roads. It is possible additional localities will be added and/or the watch extended or reissued for a later time this week, pending the development of additional rain bands and any subtle changes in the atmospheric setup that could enhance them.
A low-pressure system taking its sweet time moving eastward across the Mississippi River toward the Tennessee River valley is circulating a deep vein of Gulf of Mexico moisture, and periodic upper-air impulses, atop our region. This moisture is overrunning cooler air being pushed southward at the surface by high pressure near the Great Lakes, interacting with a couple of atmospheric boundaries, and being glided up the mountain slopes by easterly fetches of wind between the systems.
Periods of rain will continue intermittently across our region Saturday and Sunday, with widespread amounts in the 1-3-inch range and localized amounts topping 5 inches where heavier rain bands or upslope enhancement occur. Additional rain is likely on Monday, probably Tuesday and possibly even early Wednesday before a cold front sweeps the moisture out of the way.
For the most part, this will not be flooding rain on a widespread basis, largely owing to entering this period relatively dry. But there is enough of a risk along the Blue Ridge south of Roanoke for a flash flood watch to be issued.