#331 Postby HURAKAN » Thu Sep 07, 2006 4:22 am
Hurricane Erin
1 - 15 September 2001
Richard J. Pasch and Daniel P. Brown
National Hurricane Center
20 November 2001
Revised: 25 January 2002
Erin was the third of a series of four "interrupted track" tropical cyclones during the 2001 Atlantic hurricane season. After re-forming, it strengthened to a category three hurricane on the Saffir-Simpson Hurricane Scale, just to the east of Bermuda.
a. Synoptic History
Erin can be traced back to a tropical wave that emerged from western Africa on 30 August. The system almost immediately showed signs of tropical cyclone formation, with curvature in the bands of associated deep convection. Dvorak classifications commenced at 1800 UTC on the 30th. There was little change in the system's organization, and deep convection was sporadic for the next day or so. On 1 September the cloud pattern began to become better organized, and based on the satellite presentation as well as drifting buoy data that showed a definite closed surface circulation, it is estimated that a tropical depression formed by 1800 UTC that day, located about 600 n mi west-southwest of the Cape Verde Islands. With a mid-tropospheric ridge in place to its north, the tropical cyclone moved on a west to west-northwestward heading at 14-18 kt over the following three days.
At first, there was weak to moderate vertical shear over the area, and the cyclone was able to strengthen into Tropical Storm Erin by 0600 UTC 2 September. Erin's maximum winds increased to near 50 knots by 0600 UTC 3 September. Later on the 3rd, vertical shear associated with an upper-level low to the northwest caused the low-level center of the storm to become exposed to the southwest of the main area of deep convection, indicating that Erin had weakened. On the 4th, the shear appeared to lessen somewhat, and Erin re-strengthened slightly. However the system failed to become much better organized, and was soon on a weakening trend. By 5 September, southwesterly shear caused the tropical cyclone to degenerate into an area of disturbed weather.
About a day later, a surface circulation re-developed in the northern part of the area of disturbed weather that was associated with Erin. The re-generated tropical depression moved north-northeastward, then north-northwestward, and regained tropical storm strength about 550 n mi north-northeast of the northern Leeward Islands at 1800 UTC 7 September. After a mid-level ridge to the north of Erin was temporarily weakened by a passing trough, the ridge re-built, moving the storm toward the northwest and north-northwest. Erin strengthened into a hurricane late on the 8th. While passing east of Bermuda on the 9th, Erin continued to strengthen, and it reached its peak intensity of 105 kt around 1800 UTC that day. A few hours later, the eye of the hurricane passed within about 90 n mi east-northeast of Bermuda, which was Erin's point of closest approach to the island.
After brushing Bermuda, the hurricane continued to move mainly toward the north-northwest. On 10 September, Erin began to weaken, however the weakening was slower than usual over the ensuing days, due in part to slightly warmer than normal waters over the western subtropical Atlantic. A series of short-wave troughs weakened the western portion of the Atlantic subtropical ridge. This caused the motion of the hurricane to turn toward the right, with a decrease in forward speed, on the 11th. Erin's heading veered toward the east-northeast and east on the 12th. Then, a broad, amplifying mid- to upper-level trough over eastern Canada accelerated Erin toward the northeast. The center passed just east of Cape Race, Newfoundland at 0000 UTC, while the system was weakening to just below hurricane strength. Then, Erin lost its tropical characteristics. The extratropical storm accelerated north-northeastward and passed over southern Greenland on 16 September, and merged with high-latitude cyclonic flow over eastern Greenland on the 17th.
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Florence is having almost the same fate as Erin. Of course, the shear is not forecasted to be strong enough to kill Florence, but both systems where affected by UL shear coming from an ULL to its northwest. The important lesson here is that after the shear relaxes, significant strenghtening can be expected, Erin style!!!
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