What causes the 'calm before the storm'?
Quoting from Is there really a "calm before the storm"? at howstuffworks.com this sometimes happens ("Other storms skip the calm and proudly announce their presence by instantly unleashing bad weather").


  As warm, moist air is pulled into a storm system, it leaves a
  low-pressure vacuum in its wake. The air travels up through the storm
  cloud and helps to fuel it. The updrafts in the storm, however,
  quickly carry the air upward, and when it reaches the top of the cloud
  mass, this warm moist air gets spit out at the top. This air is sent
  rolling out over the big, anvil-shaped head of the thunderclouds or
  the roiling arms of hurricanes. From there, the air descends -- drawn
  back toward lower altitudes by the very vacuum its departure created
  in the first place.


That descending air becomes warmer and drier which is relatively stable, and once it blankets a region, it stabilizes that air in turn. This causes the calm before a storm.

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