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Robert H. Hopkins

Abstract

On 1 December 1992 a strong downslope windstorm with an atypical damage pattern hit Anchorage. Downslope windstorms are common in Anchorage from September through April. Typically damaging winds from these storms are confined to the foothills of the Chugach Mountains and east Anchorage within 5 km of the edge of the foothills. In the 1 December storm, moderate wind damage occurred across the entire city in areas where wind damage is a rarity during downslope windstorms. Anchorage forecasters use an empirically derived automated technique to forecast downslope windstorms. The technique works very well. It is the purpose of this study to add to this technique so that atypical storms like the 1 December storm can be detected in advance. Historical records were searched and two storms with similar atypical damage patterns were found. In this study the 1 December windstorm is described, the two historical storms with similar atypical damage patterns are described, and a forecast tool is devised using a combination of characteristics common to the three storms that is hopefully unique. A by-product of this study is a synoptic-scale tool for forecasting the onset of maximum winds for the majority of Anchorage downslope windstorms.

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Robert H. Hopkins

Abstract

Evidence linking long-period, large-scale zonal wind anomalies in the tropical upper stratosphere to those in the winter hemisphere is discussed in the light of current thinking on planetary-scale wave propagation and zonal flow interaction in the stratosphere. The evidence appears to confirm the hypothesis that absorption of planetary waves at the zero zonal wind line is the source of easterly momentum for the easterly regime of the semiannual zonal wind oscillation. Harmonic analysis of the zonal wind data for stations ranging from 31S to 64N reveals that the semiannual cycle is almost symmetrical about the equator but that the amplitude is somewhat greater a short distance to the south and the phase somewhat advanced a short distance to the north. These asymmetries can be accounted for by the wave absorption hypothesis.

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