On 12-13 November 2003, a mid-latitude cyclone caused a widespread non-convective high wind event across the Great Lakes region. In this paper, we attempt to explain the dynamical cause for these winds using ageostrophic wind theory First, the theoryof ageostrophic winds is explored and related to some conventional rules of thumb for wind forecasting. Next, ageostrophic wind terms relating to the isallobaric wind, horizontal advective processes, vertical advective processes, and friction are calculated from the North American Regional Reanalysis. The aggregate total wind results for each region are compared to observed winds at three different pressure levels: 925, 850, and 700 hPa. The aggregate results are in good agreement with observations, particularly at lower altitudes in the Lake Erie region, lending credence to our approach. The dominant ageostrophic contributor to the high winds for this storm was the isallobaric wind, but all terms played non-negligible roles at one location, level, ortime. Analyses that focus only on one term or do not vectorially combine all geostrophic and ageostrophic contributions are therefore likely to be misleading.
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