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.
Follow the link so see a more extended explanation with graphics.