Thursday, 2 August 2001: 10:50 AM
Evolution of Hurricane Michael (2000) from cold- to warm- to cold-core over a 5-day period: Observations and modeling
The development of Hurricane Michael from a baroclinic low and its subsequent interaction with a mid-latitude trough during extratropical transition is analyzed in this study. An upper cold low dropping south along the US East Coast interacted with the southern end of a stationary frontal zone over the Bahamas on 13 October 2000, triggering baroclinic cyclogenesis. On 15 October the surface low deepened to subtropical status offshore from Jacksonville and later in the day to sub-tropical storm status. The system remained stationary over warm SSTs of >28C on the 16th while gradually acquiring a warm core according to AMSU observations. The outer gale region contracted from 220 to 110 km radius according to Quikscat observations. GOES and SSMI images indicated convection became stronger near the vortex center. Dvorak intensity estimates at 00 UTC 17 Oct supported the naming of the system as Tropical Storm Michael. Reconnaissance observations indicated further intensification to hurricane strength by early afternoon of the 17th. During the 18th and early on the 19th, Michael experienced rapid deepening in response to an approaching mid-latitude trough, reaching a minimum surface pressure of 980 mb. Michael accelerated rapidly to the north, on the 19th, deepening further to 965 mb and making landfall in Newfoundland late on the 19th with an eye still visible on radar. Peak winds and minimum pressure remained nearly constant during the transition to an extratropical system late on the 19th and on the 20th over Newfoundland.
Analysis of baroclinic factors leading to initial vortex generation, factors leading to the transition from cold core to warm core and factors associated with subsequent mid-latitude trough interaction and further storm intensification are presented. Observations from Air Force reconnaissance and NOAA research aircraft are used to diagnose the unusual low level asymmetric structure on the 18th as Michael intensified initially. This case represents the first use of AMSU satellite observations by NHC forecasters to designate a storm. The AMSU-derived warm-core profiles were a major contributing factor in naming Michael. The observations from the disparate data platforms are blended with more conventional observations and gridded model-derived datasets to help synthesize a physical picture of the remarkable double transition of Michael. These observations are also being used to initialize an MM5 numerical model simulation of Michaelís evolution.