This is because the first sentence is affirmative, and the second one is negative.
In the negative form, the article will disappear. This is not restricted to words describing the weather.
Compare:
Il y aura de la neige
Il n’y aura pas de neige
Il y a de la pluie
Il n’y a pas de pluie
J’ai bu de l’eau
Je n’ai pas bu d’eau
You will find some rules and more examples here.
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