WEATHER WATCH COLUMNS:  OCTOBER 2002 ISSUE

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Weather Watch: How Forecasts Are Made

Producing weather forecasts depends upon knowing the present state of
the atmosphere as determined from surface observing stations,
instrumented weather balloons, Doppler radars, and satellites.
Computers programmed with a set of physical laws for the atmosphere
(called models) then advance the initial state forward in time,
resulting in possible scenarios for the future.  The output from the
models is then interpreted by meteorologists, producing the worded
public forecasts, watches, warnings, and other products.

Of course, everyone has been disappointed when a weather forecast
turns out wrong.  Why does that happen?  One reason is that the
current state of the atmosphere may not be adequately specified.  The
lack of weather observations over oceans and other sparsely populated
areas may give inaccurate depictions of what is actually occurring.
Small errors in the specification of the current state of the
atmosphere can lead to big errors in future states derived from
computer forecast models.  Another reason is that our approximations
of the physical laws of the atmosphere are imperfect, and these affect
the accuracy of the computer forecasts.  While further advances in
forecasting can be expected due to our expanding knowledge and
increases in computer power, it is likely there will always be
uncertainty in weather forecasts.

<-David Schultz, NOAA/National Severe Storms Laboratory>