Posted by: pyeager | May 27, 2009

Summer 2009 Forecast?

Weather forecasting and jokes go together better than Monday night and football, and while many weather jokes are trite and repetitious, there are times when meteorologists deserve the jokes, especially the “I can forecast the weather, too–by flipping a coin” joke.

One of the best examples is the way in which the Climate Prediction Center (CPC) issues its long-range forecasts. Let’s take at the summer 2009 forecast as an example.

2009 Summer Forecast

Climate Prediction Center Summer 2009 Forecast

Climate Prediction Center Summer 2009 Forecast

At First Glance
Glancing at the maps above, it seems pretty obvious that the forecast is for warmer-than-normal weather in the Desert Southwest and along the Eastern Seaboard for June through August. The only areas with forecasts of higher-than-normal precipitation are Florida the Four Corners region.

A Closer Look
A closer look at the forecast maps, however, reveals much more uncertainty. While the forecast is, indeed, for higher-than-normal temperatures in the areas previously outlined, as well as cooler-than-normal weather for the Northern Plains, a valuable piece of information for each is missing——how much warmer or cooler the CPC expects it to be.

In addition, even where the forecasters are seemingly most confident (the Desert Southwest), the lines on the map indicate that the likelihood of warmer weather is just between 50% and 60%. In other words, there’s a 40% to 50% chance that it won’t be warmer than normal, which presumably includes the possibility of it being cooler than normal. That’s little more than a coin flip.

The remainder of the country is a coin flip, with a forecast of EC (equal chances) of above or below normal temperatures. That’s right–the areas not highlighted as above or below normal are not areas for which the temperature is forecast to be near normal; they are, in fact, places where the temperature has an equal chance of being above normal and below normal, and since the departure from normal is not a part of the forecast scheme at all, this is a forecast of equal chances of temperatures being an unknown amount above or below normal.

(Note: The precipitation maps use the same forecasting schemes and therefore have the same problems.)

Perhaps some of the coin flip jokes are deserved!

What Would Be Better
Here are a few items that would make the forecasts better:

  • Eliminate the probability part of the equation. When forecasting above- or below-normal temperatures (or precipitation), make the forecast without the addition of a percentage chance. It’s a forecast–everyone understands that it might not be right, and adding a percentage chance won’t help with the perception of the forecast if it ends up being incorrect.
  • Characterize the departure from normal for both temperature and precipitation in a meaningful way, either in absolute numbers (the number of degrees above normal/the amount of rain below normal, etc.) or in terms of percent of normal (75% of normal/150% of normal, etc.).
  • Eliminate the EC category completely. Make a forecast for all areas; don’t assign large portions of the country as having equal chances of either above or below normal.

–Paul Yeager


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