The tropical transition of hurricane Alex (2004): An observational perspective

A low-level (925-850 hPa) vorticity seedling that became associated with the incipient disturbance that became Hurricane Alex (2004) was first evident in the 1.0 degree National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analyses as part of an inverted trough over the southern Bahamas at 0000 UTC 29 July 2004. The National Hurricane Center (NHC) began tracking the tropical depression that was to become Alex at 1800 UTC 30 July 2004. Hurricane Alex reached maximum intensity as a Category 3 storm (105 kt) at 0000 UTC 5 August.2004. A noteworthy aspect of Alex was that it achieved hurricane intensity contrary to the expectations of forecasters and the available operational numerical and statistical guidance. The purpose of this presentation is to give an overview of the analyzed development of Alex and to discuss some of the forecast implications of its development. In particular we will show that Alex was another example of a tropical transition (TT), a category of tropical cyclones that seems especially difficult for numerical models to predict.

Extended analyses of potential temperature on the dynamical tropopause and potential temperature, winds and vorticity in the lower troposphere constructed from the NCEP GFS 1.0 degree gridded datasets revealed that the non-tropical development of Alex was influenced by 1) a westward-moving potential vorticity (PV) filament, 2) a digging upstream midlatitude trough, and 3) a local maximum in lower-level shear vorticity. Cyclogenesis occurred on 29 July in association with large-scale environmental ascent to the east of two merging PV filaments, a rapid reduction in shear (and stability) during the merger of these two PV filaments, and the production of mesoscale cyclonic vorticity by dynamically forced and convectively driven ascent. TT occurred on 30-31 July as deep convection created a warm core and maintained weak shear in conjunction with the redistribution of PV in the vertical. Alex gained tropical storm status on 1-2 August as both PV anomalies interacted with one another in the presence of a narrow coastal ridge that enveloped Alex from the north. Alex rapidly achieved hurricane intensity by 1200 UTC 3 August as a second midlatitude PV anomaly approached the storm from the northwest. The forecast implications of these multiple PV interactions will be discussed.