Fun Weather Facts: Where do Forecasts Come From? (Part Three)

The good news is that computer forecast models now make possible reasonable forecasts out to five days.  The bad news is now that everyone uses the models to make forecasts, if the models goof up, everyone goofs up. 

So ... why do they goof up?  The best way to understand how the models fail is to imagine an archer aiming an arrow at a target.  If he is off a little in his aim, he will miss the bulls-eye by a little bit.  The more he is off in his aim, the more he will miss the target. This is like the models not quite getting what the atmosphere is like when they start running into the future.  There are lots of reasons for that, including errors in measurements and only having measuring stations in certain locations

Additionally, the further away the target from the archer, the more the arrow will miss the bulls-eye.  The distance away from the target is like the number of days into the future the model is predicting.

Now imagine that the arrow has a crooked shaft and faulty feathers. When the archer shoots the arrow, instead of going straight it curves wildly to the right.  Now the error in hitting the target becomes much greater the farther away the target, than if the arrow flew straight. 

Sometimes, the atmosphere behaves like a straight shot arrow and the forecast errors are not so great.  Other times, the atmosphere behaves like a curving arrow and the forecast errors can be great. 

One way forecasters check the possible error in the models is to compare several different models to see how closely they resemble one another.  If the models agree, they are more confident in the forecast. 

Another way is to run the same model several times, but each time to slightly change the atmosphere when the model starts running into the future.  If the forecast doesn’t change much from each model run, then the forecast is more accurate (the straight arrow atmosphere).  If the different model runs are wildly different then the forecast will not be very accurate (the curved arrow atmosphere).

In the last ten years, weather radars and satellite pictures have become a normal part of TV forecasts.  Satellite and radar are invaluable in weather research as they show precipitation and cloud patterns not possible to see from the ground. They save lives because they can track the paths of tornadoes and other severe storms that are too small to forecast accurately by the models.  They can also track bands of heavy rain that also escape the model. 

However both the satellite pictures and radar do not do well at forecasting how the weather will change in the future.  More than one forecaster has been red-faced by the sudden change in direction or intensity of a shower (myself included).  Satellite pictures are also indispensible at tracking hurricanes due to the hurricanes distinctive shape.   They are also indispensible in determining when hurricanes are developing an eye.

Next time I’ll tackle the question (actually a very complicated question) of why, if the air is colder in the mountains, does the warm air from the surface not go up and the colder air in the mountains not come down?