Most Read: Local

Posted at 11:37 AM ET, 09/05/2012

Tropics: Leslie and Michael strengthen, while Isaac may come back for an encore

The extremely active 2012 Atlantic hurricane season continues. Leslie and Michael are swirling in the open sea, while a piece of Isaac’s remnants might regenerate into tropical storm Nadine.

Model forecasts for tropical storm Leslie steer it towards Bermuda Saturday into Sunday
Bermuda, Nova Scotia and Newfoundland need to be on-guard for possible impacts from Leslie. The northern Gulf Coast should keep an eye on the ghost of Isaac.


In the past six days, Leslie has been in a moderate-to-high shear environment, limiting its intensity, but not dismantling it. Now, models are in fairly good agreement that the shear should subside and the storm will finally become a hurricane.

While the track remains far off the U.S. East Coast, Leslie could impact Bermuda later this weekend, and likely as a rather strong hurricane.

The latest suite of model runs keeps a tight cluster centered on the tiny island. At 11 a.m. this morning, Leslie’s maximum sustained winds were 70 mph; it was centered about 470 miles south-southeast of Bermuda and drifting north at 2mph. In the longer term, Nova Scotia and Newfoundland need to be on the lookout.


Michael formed on Monday afternoon as a depression, but was quickly upgraded to the 13th named storm of the season on Tuesday morning.

It’s a very small system, with tropical storm force winds extending just 35 miles from the center (recall Isaac’s typically extended about 200 miles from the center). It is very far from any land, but the best reference point would be the Azores islands, 1155 miles to the northeast.

Michael is a 50 mph tropical storm and is not forecast to change much in the coming days... perhaps gradually strengthening as it meanders generally northward.


Finally, in an unusual fashion, the remnants of Isaac may be making a comeback... over the northern Gulf coast!

Tracing the low-level circulation (850mb vorticity - area of spin about 5,000 feet aloft) over the past week reveals a complex history of what was once Hurricane Isaac. After moving inland across Louisiana, Arkansas, and Missouri, the circulation was distorted and ripped apart by a trough.

I simplified the events that transpired in the crude diagram shown here (to the right). Sometime around Monday, it appears that a part of the circulation split off to the northeast and a part split off to the south. This was not a clean separation, and someone else might analyze the circulation tracks slightly differently. But the basic point is that there is a disturbance re-entering the northern Gulf of Mexico that has some of Isaac in its “genes”. However, should this disturbance become a tropical storm, it would get a new name – Nadine – because there is not enough of Isaac’s circulation in its pedigree. As the National Hurricane Center described on its Facebook page:

There have been quite a few inquiries about whether the name “Isaac” would be given to the area of disturbed weather currently located along the northern coast of the Gulf of Mexico, if it were to develop into a tropical cyclone. The short answer is no, it would get a new name.

As of this morning, the disturbance is certainly active and producing heavy rain (regional radar loop) across parts of LA, MS, AL, and FL, but the bulk of the thunderstorm activity is offshore. For the most part, model guidance suggests that it will continue to drift toward the Gulf, then get nudged back east toward northern Florida... making “landfall” this weekend. Even if it doesn’t get named or develop beyond what it is now, it should still be a big rain maker for the northeast Gulf coast over the next few days.

Seasonal update

As an update to my post on Friday regarding seasonal activity and Accumulated Cyclone Energy (ACE), as of 8 a.m. this morning, ACE stands at 66.3, compared to the average (1981-2010 base) 39.7 by the beginning of September 5 ---- a whopping 167% of average for this date.

We’re also already on the 13th named storm as of September 4th, which isn’t a record, but it’s really close. The only years to beat that date are 2005 and 2011 when the 13th named storm formed on September 2nd. Since records began 160 years ago, only about 8% of years even reach the 13th named storm by the END of the season, let alone prior to the peak.

But, in terms of major hurricanes (Category 3+), this season is definitely lagging behind its peers. By this date in 2005, we already had three major hurricanes (Dennis, Emily, and Katrina), and by this date in 2011, we had one major hurricane (Katia). This year, we have had none.

* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

By Brian McNoldy  |  11:37 AM ET, 09/05/2012

Categories:  Latest, Tropical Weather

Read what others are saying

    © 2011 The Washington Post Company