The Collaborative International Dictionary of English v.0.48:

Squall \Squall\ (skw[add]l), n. [Cf. Sw. sqval an impetuous
   running of water, sqvalregn a violent shower of rain, sqvala
   to stream, to gush.]
   A sudden and violent gust of wind often attended with rain or
   snow.
   [1913 Webster]

         The gray skirts of a lifting squall.     --Tennyson.
   [1913 Webster]

   Black squall, a squall attended with dark, heavy clouds.

   Thick squall, a black squall accompanied by rain, hail,
      sleet, or snow. --Totten.

   White squall, a squall which comes unexpectedly, without
      being marked in its approach by the clouds. --Totten.
      [1913 Webster]