The worst area of drought in California has significantly narrowed to a small region northwest of Los Angeles that has stubbornly failed to benefit from Pacific storms that have drenched much of the state since the fall and were lining up again Wednesday. Just 2.1 percent of the state is now deemed to be in “exceptional drought” — a far cry from a year earlier when that label applied to a vast region stretching from greater Los Angeles hundreds of miles up the state’s core to far northern counties.