27th Conference on Hurricanes and Tropical Meteorology


The extratropical transitions of eastern Pacific Hurricane Lester (1992) and Atlantic Hurricane Andrew (1992): A comparison

Michael Dickinson, Accurate Environmental Forecasting Inc., Narragansett, RI; and L. F. Bosart and K. L. Corbosiero

Considerable attention has been given to the transition of tropical cyclones to extratropical cyclones in the recent literature. This process is referred to as extratropical transition (ET). A review of the literature reveals that ET occurs in the Atlantic, western Pacific, and Australian tropical cyclone basins. The only region to experience frequent tropical cyclones and not experience ET is the eastern Pacific. It has been argued that the synoptic-scale environment is not conducive to the ET process in this region.

This work will show a bonafide example of an eastern Pacific ET event. Hurricane Lester (1992) underwent ET as it moved across the desert southwest. Lester's transition was soon followed by the ET of Hurricane Andrew near the Tennessee Valley.

Both events are examined using the European Center for Medium Range Weather Forecasts (ECMWF) Re-Analysis (ERA) dataset available from the National Center for Atmospheric Research (NCAR) on an equivalent resolution of 1.125 x 1.125 and daily total precipitation data (from 12 UTC to 12 UTC) obtained from the National Centers for Environmental Prediction/Climate Prediction Center (NCEP/CPC) Unified Precipitation Dataset (UPD).

Hurricane Lester (1992) made landfall on the Baja Peninsula and northwestern Mexico near 0000 UTC 24 August ahead of a broad, synoptic-scale, positively tilted trough. The Lester circulation continued to move north-northeastward ahead of the approaching trough. Transition occurred as Lester became embedded within the 1000-200 hPa thickness gradient, indicating that Lester no longer maintained a warm core structure. While the remnants of Lester did not re-intensify, the ET process resulted in considerable precipitation (in excess of 60 mm) from Arizona/New Mexico, northeastward through Colorado and Nebraska. Upwards of 100 cm of snow fell on Mt. Harvard in Colorado during this event.

At this same time, powerful Hurricane Andrew devastated south Florida and made a second landfall along the central LA coast. Andrew was steered poleward ahead of the same baroclinic system responsible for the ET of Hurricane Lester just days prior. This interaction led to a more "classic" transition event where the trough became cyclonically wrapped (LC1) and the low-level vorticity remnants of Andrew reintensified.

One of the interesting aspects of this study is that one trough was responsible for the transition of two storms. The upscale effects of the ET process were considerably different in each case. The difference in low-level and upscale responses to each transition suggests that there are many flavors of ET.


Session 3A, Extratropical Transition of Tropical Cyclones I
Monday, 24 April 2006, 1:30 PM-3:00 PM, Big Sur

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