Why is the weather so unpredictable in the Spring?

Question

Why is the weather so unpredictable in the Spring?

Answer 1

Especially in early Spring, warm and cold air clash causing the atmosphere to become unstable more often.

Answer 2

Because of the high contrast temperature differences throughout the United States.  

Most computer models have high resolutions now, but slight inaccuracies in the analysis of the atmosphere are higher due to the wide gap between observations, and the tighter contrasts of temperatures.

As far as forecasting thunderstorms, that is always difficult for anybody.  So many factors that are very hard to analyze.  Thunderstorms can form in areas where there are no weather stations to gather weather data, so cannot predict very easily.  That is why you hear the "% chance of rain" in all of the forecasts.