The Heart of Hurricane Season Keeps Beating Strong
Hurricane Isaac has left a soggy mess along the southern Gulf Coast. First and foremost everyone who has put time, effort, and money into the New Orleans levee system since Katrina can take a bow. Looking at what happened in Plaquemines Parish (and others not protected by levees), its safe to say the $14 billion in improvements saved the city, state, and federal government billions in clean up costs. Most importantly it saved lives. We also want to send our thoughts and prayers to everyone suffering this morning. Isaac gave the region a considerable blow, due to the long duration of tropical storm force winds, heavy flooding rain, and high surge. Isaac is currently 95 miles west of Little Rock, AR and is still dropping heavy rain and spawning tornadoes. The tornadoes are unwelcome (for most!) but the rainfall is extremely beneficial.
Even though Isaac has moved along, the tropics remain active. The National Hurricane Center has been busy this past weekend issuing advisories on Hurricane Kirk and Tropical Storm Leslie. Kirk was a small hurricane, but it appeared powerful on satellite. Kirk had a central pressure of 970 mb and sustained winds of 105 mph on Friday, but was down to 75 mph and 989 mb as of Saturday morning and has currently dissipated.
Some people have asked why Isaac had a central pressure of 968 mb but sustained winds of “only” 80 mph (easy to say “only” when your not being affected with 24 hours straight). The answer lies in the structure of the two Hurricanes. Isaac had a large circulation and a large wind field. Instead of developing an inner core around its eye (a classic eyewall) Isaac dispersed its energy over a wide area. Strong winds were evident at the 850 mb level and these were tapped into by the stronger outer bands. Kirk is a small hurricane with a tight core albeit much smaller wind field. Interesting hypothetical question; would you rather deal with Kirk (small, quick moving, stronger winds at center) or Isaac (large, slow moving, strong wind field)?
Kirk is not going to bother anyone on this side of the Atlantic, but may lash the United Kingdom/Scandinavia as an extratropical gale. Leslie is a more interesting storm. Here is the spaghetti plot from Dr. Ryan Maue of Weatherbell.com
The models tried to do the same thing with Isaac, with the exception of the ECMWF (euro). The ECMWF took Isaac into the Gulf of Mexico from the get go. Interestingly the euro took Leslie into Southern New England with its 00 UTC model run yesterday. Therefore we will watch Leslie and the upper air pattern to see what happens. Most of the guidance stall Leslie south of Bermuda in the 3-6 day range. This looks likely as the semi-permanent subtropical Atlantic high pressure is forecast to weaken, then re-strengthen to help move her along. The second player will be a trough diving across the central and eastern United States.
Guidance is backing off on a rapidly intensifying system and this makes sense. There hasn’t been a single Hurricane in the main development region in 2012. Currently wind shear holding Leslie back. I don’t expect Leslie to become a hurricane until it nears 30 N. The latest GFS just came out and it brings Leslie to close to New England to write it off yet. The EURO backed off an east coast track and now favors out to sea.
Have a great Labor Day weekend! And Happy meteorological fall!