P1.7
Inland tropical cyclone wind forecasts for peninsular Florida
Scott M. Spratt, NOAA/NWS, Melbourne, FL; and R. F. Morales
The National Hurricane Center (NHC) prepares and distributes tropical cyclone (TC) advisories for the general public and other users. The TC advisories contain direct or indirect measurements of the current maximum sustained wind (MSW), defined as the "one-minute average at an elevation of 10 meters with an unobstructed exposure", and forecasts through 72 hours. In addition, the advisories indicate the current and forecast 34, 50, and 64 kt MSW radii for each quadrant of the TC, defined as the "largest radii of that wind speed found in that quadrant."
Capitalizing on the expertise of the NHC forecasters and to ensure forecast consistency, these advisories are heavily utilized by individual National Weather Service Forecast Offices (WFOs) to produce local public, marine, and aviation forecasts, as well as weather statements for TC-related hazards. While the NHC wind guidance (MSW and radii) is sufficient for marine and coastal exposures, the wind fields are often biased for inland areas owing to the lack of compensation for surface roughness (i.e. "obstructed" flow). However, to remain consistent with NHC forecasts and absent a proper inland wind reduction factor, Florida WFOs routinely use the wind guidance unmodified to generate local forecasts, often leading to an over-statement of inland wind speeds and duration.
A recent illustration of this situation occurred during Tropical Storm Gabrielle (2001). Archived high temporal resolution surface wind data were obtained for Gabrielle's passage across central Florida. By accounting for documented obstructions upwind from each surface observation site, estimates were made of the surface roughness for each wind direction. The observed winds were then adjusted and found to compare more favorably with NHC forecasts. Using these surface roughness estimates ascertained for 16 locations across central Florida, a range of reduction factors were determined. Although the reductions were generally conservative (most surface observing sites were located on airport grounds and therefore were only marginally "obstructed"), applying such modifications to the wind guidance should allow for a better representation of the actual wind across interior peninsular Florida. Such modifications could be used by the WFOs as a starting point to provide more accurate local wind forecasts for interior locations while still remaining consistent with NHC advisories.
The poster will illustrate the strategies employed for the surface roughness determinations, and will compare NHC (wind) forecasts during TC Gabrielle with the modified forecasts and observations. The resultant range of reduction factors for interior peninsular Florida will be shown. Finally, local forecast examples (textual and graphical) will be used to depict the impact such modifications would have made in realtime during Gabrielle.
Poster Session 1, Tropical Cyclones, Large-scale Dynamics and Convection
Monday, 29 April 2002, 11:00 AM-12:30 PM
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