The Rapid Collapse of Hurricane Felicia (2009) near Hawaii

In early August of 2009 Hurricane Felicia threatened the Hawaiian Islands. With an interest to improve the track forecast, the Central Pacific Hurricane Center in Honolulu requested NOAA to conduct synoptic scale surveillance missions around the category 4 hurricane. The NOAA G-IV ferried out to the islands on August 7th, then conducted two circumnavigations, approximately 3 degrees latitude from the center of Felicia, on the 8th and 9th. During the ferry and the subsequent circumnavigations the G-IV crew deployed 72 Global Positioning System dropwindsondes (GPS sondes) from above 200 hPa. Over these 3 sampling days Felicia weakened rapidly, with a minimum central pressure rising from 955 to 995 hPa. Investigation of Felicia's collapse with the G-IV dataset is noteworthy because there have been only three other G-IV deployments in the eastern Pacific basin and no other G-IV mission has been as comprehensive as those conducted around Felicia. The GPS sondes jettisoned from above 200 hPa provide an opportunity to investigate the role of environmental factors that impact hurricane intensity that include the vertical shear of the horizontal wind (VWS) and the presence of dry air in the midlevels. Near the Hawaiian Islands during the summer there is a tropical upper tropospheric trough (TUTT) which alters the location and strength of the subtropical jet stream (STJ). The STJ contributes to strong VWS located near or over the islands, and is thought of as the primary “defense” against strong landfalling hurricanes approaching from the east. The sea surface temperature (SST) gradients are aligned north-south and thus have far less impact on intensity than is commonly thought, especially for those hurricanes moving due west. The GPS sondes are used to establish the location and evolution of the TUTT, the STJ, and the conditions around the hurricane periphery. Storm relative radial flow at 250 hPa from the 8th and 9th of August indicates that a substantial intrusion of the STJ into the core of the hurricane occurs in the northwest (NW) quadrant that doubles the deep layer VWS. The SST, derived from satellite-borne sensors, decreases from 28.5°C to below 25.5°C from August 6th through 8th. Decreasing SST appears to be the initial factor in Felicia's weakening, given the motion of Felicia to the NW. Skew-T profiles throughout the depth of the troposphere indicate that convective available potential energy exists only in the SE sector of the circumnavigation around the hurricane. Analysis of equivalent potential temperature in unison with relative radial flow at 700 hPa supports the contention that Felicia is not adversely affected by the importation of dry air. We will use satellite IR and visible imagery as well maximum potential and actual intensity estimates to highlight the evolution of the hurricane. Our evidence supports the argument that a hurricane is primarily impacted by fluxes in the boundary layer and interaction with upper level winds. This investigation of Felicia is an opportunity to determine the value of the G-IV reconnaissance flights for forecasts of intensity change, in addition to their proven value for track forecasts.