Erin - An Unusual Tropical Storm

If you've been following our Hoot article series, you'll remember our article last month discussing the lack of tropical cyclone activity in the Atlantic basin. What a difference a month makes! During August and the first four days of September we've seen three new tropical systems, two of which made landfall in Central America as category 5 hurricanes (Dean and Felix), making 2007 the first year in recorded history in which two hurricanes have made landfall as category 5 storms. Hurricane Dean attained a minimum central pressure of 906 mb, making it the ninth most intense hurricane ever recorded in the Atlantic basin. However in a twist of fate, Tropical Storm Erin, the weakest of these three storms, is perhaps the most unusual and interesting of them all.

TS Erin first developed into a tropical depression late on the night of Aug. 14th in the central Gulf of Mexico from an area of disturbed weather that had been traversing the Caribbean and the Gulf for nearly a week. Erin was quickly upgraded to a tropical storm the next day when a NOAA Hurricane Hunter plane found tropical storm force winds within the storm. Erin made landfall along the Texas coast early on Aug. 16th, and continued inland on a northwesterly path.

Now over land, Erin weakened to a tropical depression, but maintained convection near its center of circulation along with scattered rainbands, moving first northwest, then north, and finally recurving to the east as it moved from the Texas panhandle into western Oklahoma late on Aug. 18--what followed was nothing short of spectacular. As Erin moved over central Oklahoma overnight it suddenly reintensified, with spiral rainbands and even the development of an eye feature, as pictured above. Maximum sustained winds of near 50 mph (with gusts to over 80 mph) were recorded by the Oklahoma mesonet, well above the threshold for tropical storm intensity, with reports of nearly calm conditions within the eye. As morning broke Erin weakened, but left behind a swath of destruction in the form of severe floods from six to ten inches of overnight rainfall.

The reintensification of a tropical or subtropical system over land is not unprecedented, particularly in Australia where inland systems with tropical characteristics called "landphoons" are occasionally observed. Such inland reintensification is rather rare, though, as tropical systems generally need to draw their energy from warm ocean waters. One possible factor in Erin's sudden resurgence over Oklahoma is a supply of energy and moisture from the low-level jet, a nocturnal feature that often brings warm, moist gulf air northward over the southern plains during the summer, and was in operation during Erin's sudden reintensification. The exact reasons why Erin suddenly came back to life over Oklahoma, however, will likely be an ongoing topic of research for months and even years to come.




Story is © Nate Snook, 2007