How early can a scrub be decided ?

The status report shows a 60% change of weather not permitting a launch
on saturday and similar stats for sunday.

As I recall, the decision is usually made during the t-9 minutes hold.

Are there rules/procedures that cause NASA to decide on a no-go much
earlier ? For instance, if the weather service predicts a 90% chance of
weather prohibiting launch, would NASA still go ahead with the filling
of the ET (a rather expensive endeavour and one which is stressful to
the aluminium and foam) ?

Or is the only time NASA might look at weather is during the T-9 hold
because ot is only then that weather becomes part of a go/no-go to
proceed with resumption of countdown ?

Also, since the shuttle launch procedures were written back in the late
1970s, would it be fair to assume that weather prediction accuracy has
improved significantly ? Does a "60% chance of weather violating rules"
24 hours before launch in 2006 provide significantly more authoritative
data than back in 1982 ?

If, 3 or 4 days before a launch, they predict a hurricane may hit/skirt
the launch site on the launch day, I take it NASA would postpone the
launch and batten down the hatches to prepare the stack to widthstand
the storm ? Is there a defined "line" above which NASA postpones the
flight, and below wich NASA continues with all preparations in the hopes
that 9 minutes before launch, the clouds might hopefully be far enough
away ?