Hurricane Isaac has hit pretty hard in the parishes (what Louisiana calls counties) south and southwest of New Orleans, with some areas there reporting worse flooding than occurred during Katrina. About 600,000 are without power in the region. More on the storm and its ongoing effects linked here from the Associated Press.
There is no “Tropical Landfall for Dummies” book that tropical systems can follow once they make it to shore. Every inland trek of a former tropical system is different. Some wash out almost entirely soon after landfall, becoming barely recognizable. Others continue well inland as an enormous rain producer, but not much else. Some spawn lots of tornadoes, most spawn a few, but others spawn very few or none away from the immediate coast. Some maintain a recognizable circulation for several days, sometimes regenerating if the circulation happens to get back over an ocean (Camille did this over the Atlantic in 1969, after its almost inconceivable dump of terrain-altering rain on central Virginia.) A few line up just right along a boundary between warm and cold air and successfully make the transition into a powerful extratropical low-pressure system, capable of producing hurricane-like winds far away from any ocean — the Galveston Hurricane of 1900, Hazel in 1954, Opal in 1996, Ike in 2008 are a few examples of that. Perhaps the most common thing a tropical system will do inland is slowly see its moisture sheared off from the circulation center, which then kind of dawdles around and slowly dies, cut off from any meaningful energy source. That appears to be the likely fate of Hurricane Isaac. Slowly and surely, it will wobble northward up the Mississippi River Valley, its moisture gradually becoming more unwound from its circulation center, and eventually pulled loose along a cold front sinking southeastward into the Upper Midwest and the Ohio Valley. The C-shaped pattern of the Hydrometerological Prediction Center forecast map shows the expected area of heaviest rains during the next 5 days, coming directly over some of the driest agricultural lands in the country, where many rivers are at near record lows and not expected to flood even if these amounts verify. At first, the rain will be closely connected to the circulation center of Isaac, but eventually, more of it will get pulled loose ahead of the cold front.
As the front advances south and east and possibly stalls across the Ohio/Tennessee Valley, central Appalachians and Mid-Atlantic, it is possible that some of Isaac’s moisture will find its way to Southwest Virginia. The HPC map even suggests an inch or so of rain may be possible in some of our region by late in the weekend and early next week (Yes, Hokie fans, there may be a chance of Isaac-inspired showers for the season opener with Georgia Tech in Blacksburg on Monday night — hopefully not the stormy mess of 2000 when the same two teams tried to open the season at Lane Stadium.) The HPC weather map for Sunday morning still shows the remnant low of Isaac centered over Illinois, weakening but riding the front. While this front will likely not provide a sharp enough boundary to help Isaac spin into a powerful inland low, the former Isaac may have enough rotation left to keep spinning southwest winds along the front and over the mountains, bringing some warm, moist Gulf of Mexico air that can be squeezed out by the front and any upper-level impulses that move along it. The short answer to whether Isaac will affect our weather is that it will not directly impact us the way Frances, Ivan and Jeanne did in 2004 or Tropical Storm Lee did last September, moving right at us from the coast with heavy rain and a tornado threat, but a little of what it once was may get mixed up in our atmosphere by late in the Labor Day weekend and enhance our chance of showers.
Until then — more seasonable, mostly dry days with highs in the 80s and lows in the 60s. Perhaps a little better chance of showers by Thursday and Friday as Gulf moisture starts building a bit.