«فَلَا تَثِقْ بِوَمِيضٍ مِنْ بَرْقِهِ فَهُوَ خُلَّبُ»
Do not trust the flash of its lightning, for it may be deceptive.
Wamīḍ is a glimmer; barq is lightning; khullab describes a deceptive flash that fails to yield the expected rain. The image is not simply beautiful weather but the difference between promise and fulfilment.
Lā tathiq is a prohibition. The warning targets trust placed in attractive appearances without reliable consequences. It does not claim that every promising sign must be false.
Application: the flash is observed; the certainty of rain is an unsupported inference.