WRAL WeatherCenter Blog

Tropical assembly line continues

Posted August 29, 2011 7:37 a.m. EDT

Despite its status as a category one hurricane, Irene caused a great amount of trouble over the eastern half of our state, owing in part to its particular track, organization and the size of its wind field, all of which led to sustained winds that were generally below hurricane strength, but with frequent gusts between 50 and 70 mph over the eastern half of our viewing area and some 75-100+ mph gusts in the vicinity of the sounds and Outer Banks. The National Weather Service has released a preliminary map of peak wind gusts that I've included here. The 115 mph value shown over eastern Carteret County was reported by the Cedar Island Ferry office, and was by far the strongest measured value from the storm.

The highest sustained winds observed during the storm were, on average, about 60-70% the strength of the highest gusts, with peak gusts being the highest instantaneous wind speeds (in practice, a 3-5 second average), and sustained winds being defined as either one or two-minute averaged values, depending on the sensor network involved. Tropical cyclones are defined based on one-minute average winds of 39 mph or higher for tropical storms and 74 mph or higher for hurricanes.

With Irene now a post-tropical cyclone, we turn our attention to continued activity to our east, noting that we're not quite halfway through the Atlantic hurricane season yet. Tropical Storm Jose has been active since yesterday but doesn't appear to be any kind of threat to the U.S.

This morning also brought the declaration of Tropical Depression 12 by the National Hurricane Center. That system will very likely be named Katia sometime today or tonight, as it is moving into an area that appears favorable for strengthening, and it may well be a hurricane by Wednesday or Thursday of this week. It is now located in the far eastern Atlantic not far from the Cape Verde Islands, so we have a long time to watch it as it moves east. It should not be an issue for the U.S. in terms of the Labor Day weekend, but will bear some vigilance after that. Of course, beyond 5 or 6 days the results of computer model projections for these storms have to be viewed with very low confidence, but so far a look at the very long-range outlook for this system from both the U.S. Global Forecast System (GFS) model and the European Center for Medium-range Weather Forecasting (ECMWF) model indicate it may be on a path moving northwest toward Bermuda by the middle of next week (second image above). That would be a path that would likely bypass the eastern U.S., but again, it is way too early to count on the results from a computer model 240 hours out in time.

One other item we'll watch this week is how the rains from Irene affect the region, beyond the flash-flooding that occurred in some spots, and the limited follow-on river flooding. For example, how will the U.S. Drought monitor assess its impact on drought designations, which remained severe to extreme over coastal and southeastern NC before Irene's arrival, and also whether the 10 or so inches of rain the hurricane dumped on the "Lateral West" fire in the Great Dismal Swamp in southeast VA and northeastern NC put that one to rest for good. Just before Irene arrived, that fire was up to 6300 acres burned and was considered 35% contained. The incident report on that fire hasn't been updated since crews were evacuated prior to Irene, so I'm including a link to the incident site here as well.