Although forecasting tropical cyclone tracks has improved over the last five years, the intensity errors remain large. Operationally, forecasters at Joint Typhoon Warning Center depend on the Dvorak technique to estimate the intensity values. They use the intensity estimate, the intensity trend, and the large-scale flow to help forecast the intensity over the next 72 hours. How useful are these large-scale fields to the forecast process? In particular, can we use the large-scale fields to estimate the vertical wind shear in the vicinity of a tropical cyclone? Can we develop a technique to improve the intensity forecast?

Using the Northeast Pacific best track data (from the coast of North America to 180°) and the NCEP/NCAR Reanalysis data for the period of 1975 - 1999, the effects of vertical wind shear on hurricane intensity are examined. On a 2.5° x 2.5° grid, the reanalysis data uses satellite-derived winds from the water vapor and the visible channels to represent the large-scale flow at the upper and lower levels, respectively. The vertical wind shear was therefore calculated between 200 mb and 850 mb. To isolate the effects of the vertical wind shear, we have stratified the data. In our sample, hurricanes were limited to westward or west-northwest tracks to minimize thermodynamic effects since the sea surface temperature isotherms run essentially east to west in the Northeast Pacific between approximately 5°N and 20°N. Using the intensity from the best track data, the hurricanes' life cycles were divided into intensifying, steady state, and decaying stages. Multiple regions around the hurricanes were examined to determine which regions and what shear magnitudes have the greatest affect on the hurricane's intensity during the various stages.

Preliminary results show that the hurricanes are able to maintain their intensity in shears of 20 to 30 knots within 7º of the circulation center, while 35 and 40 knots shear can trigger the decay stage within 12 to 24 hours. During the intensifying and steady state stages, the shear generally remained below 35 knots. A few exceptions occurred under unidirectional flow. This may suggest that the flow relative to the circulation center plays a part in determining a hurricane's intensity under the same shear values. These initial results suggest shear diagnosed from the large-scale fields does have an impact on a hurricane's intensity.