How accurate is the forecast in Iowa? New data shows how far out you can trust the temps.

A true Iowan knows the forecast is always on the edge of change. But why exactly is it so difficult to accurately predict the weather in the Midwest?

The Washington Post analyzed government data to examine the accuracy of the National Weather Service's official high-temperature forecast. The NWS forecast is used across the country to trigger emergency alert systems.

The Post created a map of the U.S. that shows how many days into the future the Weather Service’s daily high temperature is accurate, which it defined as within 3 degrees.

How predictable is the weather in Iowa?

The Post notes that its study reveals the accuracy of a forecast, not the skill level of particular forecasters. Simply put, some places really are just less predictable than others.

In places like the Midwest, air masses will often converge with one another, The Post found. A warm, moist air mass from the Gulf of Mexico, for example, could move northward and meet a cold, dry mass coming from Canada, causing rapid, unpredictable temperature swings.

For these reasons, the forecast is only accurate two days out in Des Moines, as it is in most places across the Midwest. But, warmer months are easier to predict than cooler months, The Post said. Iowa City's forecast was accurate three days out.

Coastal regions can be easier to forecast, The Post said, due to the ocean acting as a thermal regulator, absorbing the sun’s warmth during the day and releasing it over time. Similarly, desert areas in the southwest are somewhat predictable, due to arid conditions preventing most severe weather systems. But, the middle of the country lacks such factors.

How does NWS forecast temperatures?

The National Weather Service blends data from 44 computer models to produce its official forecasts, The Post said. These models are produced by the US government and meteorological services in Australia, Europe and Canada.

The Weather Service uses “Dogwood,” to process the data, one of the world's fastest supercomputers, located in Manassas, Virginia. Its backup is called “Cactus" and is located in Phoenix.

While computer-generated forecasts have improved, the Weather Service still uses human expertise. With 122 offices in the United States, human forecasters can edit the official forecast based on their knowledge and experience of local conditions, The Post said.

Victoria Reyna-Rodriguez is a general assignment reporter for the Register. Reach her at vreynarodriguez@registermedia.com or follow her on Twitter @VictoriaReynaR.