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Posted at 11:15 AM ET, 08/23/2011

Hurricane Irene may hug East Coast rather than track inland


Hurricane Irene at 10:15 a.m. EDT (NASA)

Since yesterday, Hurricane Irene has grown to a Category 2 hurricane, and presently contains maximum sustained winds of 100 mph. Situated about 70 miles south of Grand Turk Island, hurricane warnings are in effect for all of the Bahamas and the Turks and Caicos islands. Marching west-northwestward at 12 mph, the storm is very likely to intensify into a major hurricane (category 3 or higher) and head towards the U.S. East Coast, but the exact track and specific impacts remain fuzzy. The latest guidance suggests the storm could remain over the ocean, battering the coast, rather than coming inland.

Irene’s current presentation

Irene exhibits a rather ominous presentation in the satellite imagery. The latest observations indicate that long-lived deep convection (tall thunderstorms) now sits atop the center of circulation – with reconnaissance aircraft (hurricane hunters) recently reporting a closed eyewall.

This is a sign that Irene may intensify quickly and very soon become a major hurricane, category 3 or higher. High-altitude outflow is solidly established in most directions, with evidence of a clockwise (anticyclonic) turning in the cirrus cloud streams near the edges of the circulation. Maximum sustained winds are estimated to be near 100 mph, with additional strengthening expected as it continues to move over 85+F water and encounters little environmental wind shear.

Forecast

The National Hurricane Center’s (NHC) official forecast brings Irene through the Bahamas on a northwest track during the next three days, eventually reaching the dangerous category 4 status on Thursday.


Latest track guidance from computer models (University Center for Atmospheric Research)
The million dollar question is whether or not Irene will make a U.S. landfall. The answer is … though it is still too early to tell with a high degree of certainty one way or the other, it’s looking more than likely it will, but barely.

It’s important to keep in mind that the average track error at day 4, according to NHC, is 200 miles. For a storm like Irene whose radius of hurricane force winds is 45 miles, this means that a 4-day forecast is basically unable to clarify whether or not one should expect eyewall conditions or instead a showery and breezy afternoon with partial sunshine.

With that said, here are some pieces of the puzzle to look at:

The track guidance (shown to the right) is consistent in the idea that Irene will curve northward in a couple of days after it plows through the Bahamas over 100 miles east of Florida’s east coast.

But as we head toward the weekend, with Irene propagating northward or north-northeastward toward the Carolinas, the situation becomes less clear and more serious.

The experts at NHC appropriately advertise the still considerable uncertainty in the forecast for the weekend.

The basic idea is this:


Upper level winds are likely to allow Irene to approach the East Coast, but how close?
Later this week, as Irene emerges from the tropics on its poleward track east of Florida, an upper trough in the mid-latitude westerlies (outlined in red in the forecast valid early Friday morning to the right) will likely bypass Irene to the north.

It is now pretty well agreed upon by the models that this trough will not be strong enough to pull the storm out to sea. It will probably leave Irene behind in a weak steering pattern for roughly 36-48 hours that will not significantly alter her northward course and unfortunately keep it on a track that aims at eastern North Carolina. In that case, we will have to hope for the next southeastward-moving trough upstream (outlined in blue) to move into a position close enough to shove Irene eastward.

The guidance currently suggests that this may not happen in time to keep Irene completely offshore. In this scenario, where it is understood that there is 4-6 days worth of time for the meteorology to turn out differently, the Outer Banks of North Carolina would suffer a major hurricane landfall around Saturday. Let’s hope this doesn’t happen.

Impacts from Irene

Bahamas

Dangerous conditions are expected to impact a large part of the island chain beginning this afternoon in the southeast Bahamas, spreading to the central Bahamas late tonight into Wednesday and eventually reaching the northwestern Bahamas Thursday. In addition to destructive hurricane-force winds, the National Hurricane Center indicates a storm surge may raise water levels 9 to 13 feet and rainfall totals could reach 5 to 10 inches.

The Bahamas Ministry of Tourism is strongly recommending that those with plans to travel to The Bahamas within the next five days contact their hotel, airline or cruise line to postpone travel.

Florida

With the tropical storm wind radius at 185 miles (for now), this scenario could deliver tropical-storm wind gusts in showers to the southern half of the immediate Florida shoreline late in the week, with the dangerous part of Irene remaining well out over water. The surf would be rough, but at least the mainland would be on the left (weaker) side of the storm and really too far away to absorb a significant punch.

Further north

The entire eastern seaboard will contend with high surf and dangerous rip currents from the south to north Thursday into the weekend. Whether locations like the Outer Banks, the Delmarva Peninsulsa, the Jersey shore and southern New England deal with true hurricane conditions (mandatory evacuations, damaging winds, torrential rain, major flooding, etc), tropical storm conditions or just gusty showers (or something less) depends on the exact track of the storm. At this point, it is premature to forecast specific details.

However, now is the appropriate time to think about preparing for hurricane impacts if you have property from South Carolina to New England, especially if it is near coastal waters (bay, river, or ocean).

Also, if you are planning beach travel this weekend, keep a very close eye on the forecast. It is possible you will need to delay travel. Furthermore, mandatory evacuations may be ordered in some areas.

If landfall occurs in the Outer Banks, it would most likely be on Saturday, with the storm moving to the latitude of the Delmarva Peninsula and Jersey shore Sunday and finally to New England’s latitude Monday. Storm impacts could begin up to 12 hours before the storm reaches a given latitude.

Video update on Hurricane Irene

The rapidly intensifying Hurricane Irene has already cut a destructive path through the Caribbean. It is now the first hurricane to seriously threaten the U.S. in almost three years. (Aug. 23) (/The Associated Press)

By and  |  11:15 AM ET, 08/23/2011

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