What are the thunder and lightning surrounding the witches in Act 1 scene 1 in Macbeth meant to represent?

In Shakespeare's Macbeth, the
"Thunder and lightning" in the opening stage directions of the play
don't, in themselves, "represent" anything.  In other words, they're not symbols in
themselves.  They are more indicative of other aspects of the
play; they contribute to other
aspects. 

Together with the witches, the weather creates an
eerie atmosphere that will pervade the play, and the First Witch, of course, references
the weather, and particularly the weather to come, in her lines that open the
play:

When
shall we three meet again?

In thunder, lightning, or in
rain?

This draws attention to
the weather, and the weather is, or at least soon will be, highly suggestive.  The day
is stormy, as is the state of affairs in Scotland.  The witches close the opening seen
by again referencing the weather, but also by connecting the foul weather with the
political affairs in Scotland, though this is only implied by this
point:

Fair is
foul, and foul is fair,

Hover through the fog and filthy
air. 

Macbeth unkowingly
mimics the witches, due to the weather, in Act 1.3.39 when he enters the stage with
Banquo:

So
foul and fair a day I have not
seen.

This connects Macbeth
to the witches, and foreshadows things to come.

During the
course of the play, foul weather will contribute to the multiple meanings of the fair
and foul statements (thanes that are assumed to be loyal are not, Lady Macbeth wants to
be a man, Malcolm suspects Macduff of being a traitor when he is not, etc.), and become
indicative of the unnatural way in which Macbeth gains power and keeps
it. 

The weather, then, sets the stage for things to come. 
It contributes to theme and is indicative of the political situation in Scotland.  It's
difficult to see the witches meeting Macbeth and serving as catalysts for the murder of
a king, the murder of Banquo, and the slaughter of Macduff's family, in bright, cheery
sunshine.