Posted by: pyeager | August 27, 2009

More on Forecast Accuracy

The publication Smart Money recently wrote an article about the 10 Things that Weather Forecasters Don’t Want You to Know, which is part of their Spending Rip-offs section. We’ve already looked at one that related to forecast accuracy, but let’s look another today:

Forecasters are pretty accurate–as long as the sun in shining

The article states that forecasts are generally accurate when the weather is dry but are much less accurate when showers and thunderstorms are occurring, and the article includes information about how computer forecast models are of no help in these situations.

When dealing with showers and thunderstorms, the statement is largely correct. It has little to do with computer model output, though. It has much more to do with the scattered and imprecise nature of showers and thunderstorms.

Predicting the precise location of a shower or thunderstorm is analogous to predicting which kernel of popcorn is going to pop first. You’ll can accurately predict the general area where thunderstorms will occur, but forecasting the precise location in much more difficult, not something that’s done with great accuracy.

That’s how we end up with one of my meteorological pet peeves–a 50% chance of rain.

Not True In All Instances

Steadier types of precipitation, such as snowstorms and rainstorms, are predicted with much greater accuracy than the convective type of shower and thunderstorm mentioned earlier, and it’s unfortunate that the article turned an accurate specific statement into an inaccurate general statement.

Computer forecast models are of more assistance, but so is the nature of the precipitation. Tracking a large-scale storm as it moves across the country can be done with good accuracy. The storm will change character as it moves across the country, strengthening and weakening depending on upper level changes and available moisture (among other things). It might be difficult to predict the precise amount of precipitation at a given location (for some of the same reasons related to thunderstorms), but the forecast for precipitation is largely accurate.

–Paul Yeager

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