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John Wayne Airport (SNA) is the closest midsize airport to Temecula (1.25-1.5 hours away) with direct flights from O'Hare (for you Chicago folk). And you'll fly into lovely Orange County.
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If you're more inclined to fly with Southwest or from Midway (again for you Chicago folk), there are direct flights to Long Beach Airport (1.25-1.5 hours away from Temecula).
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LAX is the largest airport, albeit a little far from Temecula (1.5-1.75 hours away). If you're hoping to run into someone famous, then LAX is the airport for you.
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If you want to bask pool-side in the sun and soak in 1970s vibes, then consider flying into Palm Springs Airport (1.25-1.5 hours away). Great if you want to explore inland California (fair warning it'll likely be toasty in June)