The forecasts around here ( Calgary Alberta ) are notoriously unreliable. Part of the problem is that we are so close to the Rockies, a very small shift in wind direction gives a marked change in weather. The other is the cheapskate bureaucrats don't update the forecast from the raw data often enough.
One morning i checked, and the forecast was for a chill, but dry day, not a word of any precipitation. Not 20 minutes later, riding into work, "What the hell is this white stuff!??" had an inch of snow before reaching work, and about 6 by home time.