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Earl maintaining Category 4 strength; hurricane watches for North Carolina

By: Dr. Jeff Masters, 9:44 PM GMT on August 31, 2010

Hurricane watches are now posted for coastal North Carolina, as Hurricane Earl continues towards the west-northwest at 13 mph with little change in strength. Currently, the outer reaches of the storm are affecting the Turks and Caicos Islands, and NOAA's Wavewatch III model is predicting 15 - 20 foot waves are affecting these islands. Early this morning, waves at buoy 41043 offshore of Puerto Rico reached 31 feet. Winds at Providenciales in the Turks were only 20 - 25 mph this afternoon, though Cockburn Town and Balfour Town in the extreme eastern Turks may be seeing winds near tropical storm force--40 mph.

Figure 1. Afternoon satellite image of Earl.

Intensity forecast for Earl
Recent visible satellite loops show that the southwest side of Earl is getting eaten into by wind shear and dry air. The storm is no longer as symmetrical as 24 hours ago, and the spiral band on Earl's west side has been destroyed by dry air. Wind shear as diagnosed by the latest SHIPS model forecast shows a moderate 10 - 20 knots of wind shear, due to upper level winds out of the southwest from a trough of low pressure to Earl's west. This moderate shear is predicted to continue through Friday. Water vapor satellite loops show some moderately dry air to Earl's southwest, and very dry air to the northwest. Once Earl works its way farther to the northwest, it is possible that the persistent moderate wind shear will be able to drive this dry air deep enough into Earl's west side to significantly disrupt it. However, none of the models is predicting this, and Earl is probably large and strong enough to fend off this sort of assault. I give a 30% chance that a major dry air intrusion will significantly disrupt Earl between now and Thursday, weakening it to a Category 2 hurricane. Ocean temperatures are a near-record 29.5 - 30°C, and very warm waters extend to great depth, resulting in a total ocean heat content favorable for intensification. Earl is undergoing an eyewall replacement cycle, which may diminish its winds by 10 - 20 mph for a day or so. There is no evidence that Earl has weakened, though, based on the latest pressure of 940 mb from the Hurricane Hunters. It is likely Earl will be a major Category 3 or 4 hurricane at its closest approach to North Carolina Thursday night and Friday morning. By Friday night, when Earl will be making its closest approach to New England, wind shear will rise to a high 20 - 30 knots and ocean temperatures will plunge to 20°C, resulting in considerable weakening. Earl will still probably be a Category 2 hurricane on Friday night, when it could potentially make landfall in Massachusetts. Earl is more likely to be a Category 1 hurricane on Saturday morning, when it could potentially make landfall in Maine or Nova Scotia, Canada.

Track forecast for Earl
The latest set of computer models runs from 8am EDT (12Z) are pretty unified in taking Earl 100 - 200 miles off the coast of North Carolina, then 150 - 300 miles off the coast of Southeast Massachusetts, then into Nova Scotia. The high degree of model unity gives confidence that this is the correct solution, but it is good to keep in mind that the average error in a 48-hour NHC forecast is about 125 miles. It is likely that Earl will bring a 15-hour period of heavy rain and tropical storm force winds of 39+ mph to North Carolina's Outer Banks, beginning on Thursday evening. NHC is giving Cape Hatteras a 15% chance of receiving hurricane force winds. By Friday evening, Southeast Massachusetts can expect a 6 - 8 hour period of heavy rain and tropical storm force winds of 39+ mph. NHC is giving Nantucket a 13% chance of receiving hurricane force winds. These odds are 5% for Boston, 6% for Providence, 8% for Eastport, Maine, and 14% for Yarmouth, Nova Scotia. The main determinant of whether Earl hits the U.S. or not is a strong trough of low pressure predicted to move off the U.S. East Coast Friday. This trough, if it develops as predicted, should be strong enough to recurve Earl out to sea. Keep in mind that the average error in position for a 3-day NHC forecast is 185 miles.

Regardless of Earl's exact track, the U.S. East Coast can expect a long period of high waves beginning on Thursday. Significant beach erosion and dangerous rip currents will be the rule, due to waves that will reach 10 - 15 feet in offshore waters. Waves from Hurricane Danielle killed two swimmers in the U.S. over the weekend and forced hundreds of water rescues along the U.S. East Coast. Earl's waves will be worse, and will likely cause millions of dollars in beach erosion damage.

Next update
Dr. Rob Carver is planning an update late tonight, and I'll have a full update in the morning.

Jeff Masters


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