For Amateur and Professional Meteorologists - Thunderstorm Forecasting Tip

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Severe thunderstorms began to develop in the same area three hours after this photo was taken.
The clouds you see are called "altocumulus castellanus" (meteorologists call them ACCAS or "ay-cass"). If you see them in the afternoon or evening, they mean thunderstorms will occur within about 80 miles by 7 the next morning, regardless of what the models show. This rule is especially useful in astronomical spring. 

Wednesday night in Kansas City was a perfect example. ACCAS showed up around 8pm and were still there at darkness. No thunderstorms were forecast by the NWS's Storm Prediction Center nor in the local forecast. Yet, thunderstorms (below) developed.

ACCAS do appear differently in different lighting. Here they are at sunset. 
While not perfect, the rule is quite reliable. Today's meteorologists are often reluctant to forecast something not shown by the computers. But, if you pay attention to this rule, you'll be correct more often than not. 

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