|FORECAST BUST: WIND SPEED AND DIRECTION
METEOROLOGIST JEFF HABY
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
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!