Tropical Storm Isaac, as of this hour, is not that impressive. Its central circulation has become disorganized and it’s clinging to 45 mph winds, 6 mph above the baseline for tropical storm status. But it’s also not even in the Caribbean, yet, with lots of warm ocean water and a fairly conducive upper air environment ahead of it to grow and strengthen. Largely because of the Republican National Convention being scheduled for Tampa next week (Aug. 27-30),and the fact that that there hasn’t been a Category 3 or stronger hurricane landfall on a U.S. coastline since 2005 (covered in Weather Journal and a USA Today front-page article today) there is going to be a lot of attention — even a little more than usual — on Isaac’s track and strength over the next several days. At this stage, here is what is known: (1) Isaac likely WILL affect the U.S. coast in some form next week and (2) it is not at all certain that it will be a major hurricane (Category 3 or stronger, 111 mph or higher winds) at U.S. landfall, if there is one. Between its current location and the U.S. coast are several land masses that could weaken the storm, particularly the 8,000-10,000-foot “hurricane killer” mountains of Hispaniola. Isaac’s track depends largely on what happens with high pressure to the north of it — the same high pressure that will be bringing us several days of similar weather, with highs near normal (80s highs, 60s lows) and scattered afternoon showers and storms. Since we’re close to football season, think of a running back looking for a hole in the defense to slip through. Isaac will follow whatever weakness develops in high pressure to the north. Detecting when and where such a weakness occurs is a major reason forecast models vary all the way from the Savannah, Ga., to New Orleans with the projected U.S. landfall of Isaac. (Inset map shows the 12Z European model’s depiction of Isaac on Thursday morning, approaching the Louisiana/Mississippi Gulf Coast. The “L” and “H” in this case are not about atmospheric pressure, but about winds speeds at about a mile up, with L marking the calm spot of the eye and H marking a 114-knot or 131 mph burst in the eastern eye wall.) As we move forward in time, the forecast models should come into at least somewhat better agreement on Isaac’s track by moving closer on the position and strength of high pressure to the north. Remember that on the tropical atmospheric playground, it’s not big bad hurricanes that are the bullies that push everything around, it’s the bulky, stagnant high-pressure systems.
Southwest Virginia will be affected by Isaac, either directly or indirectly. Direct impacts could occur if the system moves northward from the Gulf or its moisture becomes swept north or northeastward by a frontal system moving southeastward. Flooding rains, gusty winds and tornadoes are possible with inland-moving tropical systems, depending on many factors. Indirect effects, especially with a more westward path, could simply mean more of a southeast wind flow and a tendency to pump up high pressure to the northeast of it. If the latter is the case, Isaac may assist in summerlike heat (mid 80s-low 90s highs) and humidity re-establishing itself next week.
Before it ever becomes an issue for the U.S. mainland, Isaac has the potential to be very troublesome for Puerto Rico, the Dominican Republic, Haiti and Cuba, to name only a few Caribbean nations. It’s still 5-7 days away from the U.S., and the average forecast track miss on hurricanes that far out is about 500 miles.
As if Isaac weren’t enough, we’ll probably soon have Tropcial Storm Joyce following on its heels.