by James Hudgins
The affects that tropical cyclones or their remnants bring to the region can vary from flooding rainfall to tornado producing mini super cell thunderstorms. These tropical systems again most often occur in the months of August and September when ocean water temperatures are at their warmest while upper level jet stream winds for the most part remain north of the region. This heavy rainfall can result in catastrophic damage due to flooding along with property and structural damage seen in the stronger tornadoes.
Many factors including residual frontal boundaries, strength and track of the storm, and upslope flow (air moving up the ridges) due to the mountains contribute to how the system might affect the region. One of the biggest factors involves upslope flow along the eastern slopes of the Blue Ridge (Fig. 1) where enhanced rainfall totals often occur along and ahead of the track of the residual tropical cyclone. This was highly evident during tropical depression Frances that moved along the spine of the Appalachians in early September of 2004. This system dropped up to a foot of rain across the Blue Ridge Mountains of northwest North Carolina and up to 8 inches in Virginia.
Fig 1. Favored upslope locations (white circles) and wind directions (white arrows) along the Blue Ridge. Terrain color coded with scale in upper left (thousands of feet).
Some of the other prolific rain producers include Beryl in August 1994 and Camille in August 1969 which interacted with residual frontal boundaries to produce torrential rainfall along parts of the Blue Ridge. Many systems that make landfall closer to the region, such as in the Carolinas, similar to Isabel during September 2003, tend to produce the heaviest rain along the track of the system given its strength and closer proximity to the Atlantic Ocean.
Tornadoes in conjunction with these tropical systems (Fig. 2) are most evident along the foothills and piedmont where less friction and better heating often occur outside the higher terrain. Local research suggests that the majorities of these tornadoes occur as a result of frontal interaction, often in the afternoon or early evening, and are mainly of the weaker F0 or F1 variety off the Fujita Scale. In addition a majority of tropical systems that produce tornadoes across the region were of hurricane strength at landfall primarily along the Gulf Coast or Florida. One of the bigger outbreaks occurred with Ivan on September 17, 2004 when over 50 tornadoes touched down across the Virginias and Carolinas. Some of these were of F2 (111-150 mph) caliber and resulted in significant structural and tree damage in Bedford, Henry, Franklin, and Campbell Counties in Virginia, as well as Rockingham and Guilford Counties in North Carolina.
Fig 2. Past tracks of tropical cyclones producing tornadoes across the area since 1950.
The 2006 season (Fig. 3) got off to a slow start with only one named storm, Alberto in June, and 8 additional systems through the time of writing in early October. Of these storms, 5 have reached hurricane strength, with Gordon and Helene reaching major hurricane status (Category 3) while passing over the open Atlantic waters southeast of Bermuda. The first named system of the season, Tropical Storm Alberto, produced some heavy rain across parts of Southside Virginia and Northwest North Carolina on June 14th. However the most significant affects were associated with Ernesto which was the first hurricane of the season. This system made landfall on the coast of southern North Carolina and proceeded to move north across Tidewater Virginia on September 1st. The heaviest rain and strongest winds with the remnant depression passed east of the region producing flooding across extreme eastern Virginia. However a residual frontal boundary along with moisture in advance of Ernesto, combined to result in 2 to 6 inches of rain along and east of the Blue Ridge Mountains. The remaining systems including Beryl, Chris, Debby, Florence, Gordon, Helene, and Isaac tracked offshore the U.S. mainland.
Fig 3. 2006 tropical cyclone tracks through October 10th.
The lack of storms this season in comparison to the last couple years appears related to strong high pressure over the central Atlantic Ocean and extensive dry air associated with this feature. In addition, slightly cooler than normal ocean water temperatures and strong easterly trade winds have acted to disrupt tropical waves moving across the Atlantic from the African coast. This has resulted in an overall near average hurricane season regarding the number of systems but with much less impact to the United States.
The latest updated forecast (via William Gray, Colorado State University) calls for only 2 named systems in October and November bringing the total number of predicted storms for the season to 11 (average 10). Of these, 1 is expected to reach hurricane status, resulting in a total of 6 (average 6) for the 2006 hurricane season. No additional intense hurricanes (Category 3 or stronger) are expected, leaving the seasonal total near the average of 2 per year. This below normal trend for October and November looks to be driven by developing El Nino conditions in the Pacific Ocean which tends to inhibit tropical development across the Atlantic basin. The season officially comes to an end on November 30th.