Chicago has one of the most unpredictable weather forecasts in the country

We all know weather is unpredictable.

The big picture: In Chicago, we count on our weather forecasts to alert us when the weather gets nasty, and sometimes dangerous.

Yes, but: Even though these forecasts are literally lifesavers, there are times they are plain wrong.

• Hint: It's always the computer's fault.

Case in point: Chicago forecasts called for thunderstorms and severe weather during last weekend's Lollapalooza.

• There were storms, but much lighter than predicted.
• Still, the predictions changed the course of what people wore to the festival and caused early cancellations of outdoor events across town.

Zoom in: A recent Washington Post project mapped just how accurate the nation's weather forecasts are, from city to city.

Methodology: The Post used recent data released by the National Weather Service to assess how many days into the future the Weather Service's forecast was accurate.

The verdict: In Chicago, the forecast is accurate only two days out.

• The study also concludes that cooler months are easier to predict than warmer months.

Reality check: Compared to other regions, the Midwest is pretty lousy.

• The Southwest and parts of California are accurate 5-6 days out.
• Florida's forecasts are spot on a full week out!

The bottom line: That five-day forecast you are glued to on your phone may not be the weather bible you were hoping it was.

Go deeper: Check out the entire study here .