 It is different for places with a Mediterranean climate . In the western United States , during the cold season from September – May , extratropical cyclones from the Pacific Ocean move inland into the region due to a southward migration of the jet stream during the cold season . This shift in the jet stream brings much of the annual precipitation to the region , and sometimes also brings heavy rain and strong low pressure systems . The peninsula of Italy has weather very similar to the western United States in this regard . 
