This series of Haby Hints investigates problems that cause a forecast to bust. A bust occurs when a certain weather parameter is expected but one or more factors cause the forecast to be wrong. This particular Haby Hint will focus on how wind speed and direction causes forecast problems. The wind speed will play a key role on the surface temperature in situations where there is a strong temperature change with height in the boundary layer. In particular, a strong temperature lapse rate in the portion of the boundary layer nearest the surface. The earth is heated and cooled from the ground. It is the wind that mixes this air at ground level with air higher aloft. During the day when winds are light and the skies are clear, heat will build at the surface. Temperature in this case will tend to be warmer than if the wind speeds were stronger. This is because stronger winds will mix the warm air near the surface with cooler air aloft. On a clear night the opposite is the case. Light winds at night will allow cool air to build at the surface. If winds are stronger than expected then the surface temperature will be warmer since the wind will mix warmer air aloft with the shallow cool air building at the surface. The wind speed is also important in determining the rate at which thermal advection will take place. Temperature forecasts will be off if either CAA or WAA are stronger or weaker than expected. Wind direction shifts can dramatically alter the forecast. It is important for the forecaster to know where significant water bodies are in relation to the forecast area. It is also important to know where significant elevation changes occur in relation to the forecast area. Some wind directions will bring in more moisture while others will bring in drier air. Some wind directions will promote upslope flow while others will promote downslope flow. Precipitation chances will generally be enhanced with a flow that is from a moisture source and/or the flow produces an upslope flow. Precipitation chances will generally be reduced with a flow that is from a dry source and/or the flow produces a downslope flow. If the wind direction ends up being different than forecasted, certain forecast areas are very likely to have a busted forecast. This is especially true for forecasts made out several days into the future. Wind direction is very critical during lake-effect snow events. Just a minor shift in wind direction will alter the snow forecast by several inches or even feet over time! |