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Posted at 11:15 AM ET, 08/29/2010

Hurricane threat to U.S. may increase next week

By Greg Postel

* Heat wave begins: Full Forecast | NatCast | Rip current risk *
* The good, bad & ugly of hurricane forecasting: Part 1 & Part 2 *
* Katrina, 5 years later: Full Coverage | Hurricane Tracking Center *

Satellite image from today showing three areas of storminess in the Atlantic: Hurricane Danielle (farthest left), Tropical Storm Earl (middle), and what may soon become Tropical Depression #8 (bottom-right edge of image). Credit: NOAA.

11:15 a.m. update Sunday (originally posted at 12:55 p.m. Friday): Earl has strengthened into a hurricane and the latest track guidance has shifted slightly to the west -- with the North Carolina outer banks on the periphery. Though Earl will likely stay off the coast , we are watching it carefully and will post a full update tomorrow.

The "Cape Verde" portion of this year's hurricane season has come alive. Thunderstorm clusters associated with tropical weather disturbances moving westward off of Africa -- and close by the Cape Verde Islands, hence the name -- are now showing an ability to remain intact as they trek across the Atlantic.

Now appearing with near regularity, roughly one every 5-6 days, many of the recent systems have been highly suited for further development. In fact, the last couple have evolved into significant tropical cyclones: Hurricane Danielle (category 4 as of Friday morning), Tropical Storm (soon-to-be-Hurricane) Earl, and another disturbance that has just recently moved off the African continent that might very well become Tropical Depression #8 in a matter of hours. And this may just be the beginning...

Upper-level winds as of today. Credit: University of Wyoming.

Fortunately, in the last couple of weeks or so, the global weather pattern has favored the accumulation of upper-level troughing, or a dip in the jet stream, near the East Coast of North America and over the western Atlantic. This resulting tendency toward a counter-clockwise flow along the Eastern Seaboard has created a persistent wind from the southwest at high altitudes over the western Atlantic.

So far these offshore winds (shown to the right), recently penetrating as far south as Florida, have been able to shove approaching storms back out to sea. This will likely be the case with Danielle and Earl, as these systems are projected to remain over the Atlantic and not threaten the mainland U.S.

One of the mechanisms nudging the atmosphere toward this state is the MJO (Madden-Julian Oscillation). The MJO is a tropical weather system the size of an entire ocean basin. It propagates eastward around the tropics and is intricately involved with varying the wind, sea-surface temperatures, cloudiness and rainfall in the hurricane development regions.

Right now the MJO is in a place that tends to encourage East Coast troughing at upper levels of the atmosphere. However, the latest forecasts for the MJO suggest that it will revert back to a configuration that favors the opposite -- East coast upper-level ridging, or a northward displacement of the jet stream, a pattern we've seen many times in the last several months.

Upper-level winds projected for Monday from GFS forecast model. Credit: University of Wyoming.

By Monday, the clockwise flow associated with the East Coast ridging is expected to bring upper-level winds that are blowing toward the Southeast U.S. from the Atlantic Ocean (shown at right). This setup offers less protection to the U.S. coast from tropical cyclone strikes (particularly of the Cape Verde type) by allowing the storms to move farther westward at tropical latitudes, rather than trying to recurve them on the heels of a high-altitude offshore flow.

So if indeed the Cape Verde disturbances continue to show the potential they have recently, an MJO that tries to coax the global weather pattern into East Coast upper-level ridging is something we need to watch carefully.

By Greg Postel  | August 29, 2010; 11:15 AM ET
Categories:  Tropical Weather  
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My hunch...the "Much-needed rain" crowd is eagerly awaiting this MJO shift in order to ensure us another squishy, drippy September.

My advice: Enjoy the nice dry weather while we still have it. Lower humidity will at least keep late-night temperatures bearable. Farmers like dry weather at harvest time since rain keeps the crops out in the field where they may become moldy.

Posted by: Bombo47jea | August 27, 2010 1:32 PM | Report abuse

We've had everything else this year, may as well have a cat 5 hurricane come up the Chesapeake. :P

- Ray

Posted by: rmcazz | August 27, 2010 1:54 PM | Report abuse

As The Great Sahara Desert extends South...

and Desertification approaches Equatorial Africa...

There Will Be NO "Moisture" to Spawn Cape Verde Storms.

And, as the "Hyposaline Conveyor" of the Gulf Stream Loop Current is Diluted By Greenland Icecap Melt....

The Hydrosaline Conveyor... will LOOSE Its SALINE... and STOP!

Colder Europe, No Hurricanes in Colder Atlantic?

Posted by: rmcnicoll | August 27, 2010 2:08 PM | Report abuse

In all likelihood the Sahara will get moister in a warming scenario (e.g.
) The conveyor is driven by the freezing of saltwater, so as long as winters are cold enough to form sufficient amounts of ice, the leftover water will become more saline and therefore denser and will sink and trigger the circulation. It doesn't really matter how much fresh water is added during the summer.

Posted by: eric654 | August 27, 2010 2:38 PM | Report abuse

While I certainly wouldn't welcome a cat 5 coming up the bay, a "squishier" September would help all of the stressed trees and plants, and help me reseed the "sahara" that once was my front yard. While it would help me and many others, I do hope it doesn't create too much of a wrinkle in the "much needed dance" crowds social schedule.

Posted by: ftwash | August 27, 2010 3:35 PM | Report abuse

Hello CWG,

I am traveling to Manteo NC (near Nags Head) on September 1 and staying through the 7th. What are your thoughts on Earl?


Posted by: eor11 | August 27, 2010 3:38 PM | Report abuse

Bring a big wave board and a pair of juevos and you'll be fine for the Outter Banks.

Posted by: dhartogs | August 27, 2010 3:48 PM | Report abuse

Can we be so sure the U.S. won't be impacted by Earl? If the MJO is going to shift by Monday as you say (at which point the path of Earl is forecast to be around the northern edge of the Leeward Islands), isn't there a chance it could continue on a more westerly track? It's already tracking further south than Danielle did.

Posted by: tiggs03 | August 27, 2010 4:15 PM | Report abuse

hi eor11,

you should be fine with Earl. While it's still a long way out (with lots or room for error), the global weather models are turning Earl out to sea. Statistics are also on your side. A hurricane strike is really an unlikely occurance for any given location.


However, keep an eye to the tropics in general. There will surely be more storms to follow behind Earl, and the likelihood for a U.S. landfall (though small) should increase in early September.

Posted by: gregpostel | August 27, 2010 4:35 PM | Report abuse

Hi tiggs03,

You're right, it's at a lower latitude. And nothing is ever for certain.
But there is a growing consensus that Earl will recurve, too.


Posted by: gregpostel | August 27, 2010 4:40 PM | Report abuse

Thanks Greg. The folks on TV have been saying there is a good chance a C-3 hits the Carolina coast.

Dhartogs: I was not worried about the waves. My wife's family is from the island and the roof gets blown off every now and then.

Posted by: eor11 | August 27, 2010 4:50 PM | Report abuse

There is a small chance Earl could hit Carolina, as reflected in the percentages. But it bears watching as most anything can and will happen. At least the coast from Hatteras to cape May will see large swells neat week

Posted by: pvogel88 | August 28, 2010 2:16 PM | Report abuse

What are the legit stats for this season that a hurricane could hit the mid atlantic region?

I saw one site on the internet that put the odds for 2010 at .50, where the usual odds are about .20, but not sure this is reliable.

Posted by: maths1 | August 29, 2010 12:38 PM | Report abuse

We live in Manteo and took the necessary precautions for Earl and the season in general. It is still too early to predict where Earl is headed. Thanks for the article. A good primer for the current situation.

Posted by: laramy1 | August 30, 2010 9:16 AM | Report abuse

Hi CWG. Can you give us an update soon and a comparison to Hurricane Isabel's impact on the region? I may travel from Maryland to GA this weekend and would like to know what to expect.

Posted by: midanae | August 30, 2010 1:31 PM | Report abuse

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