We don't know the flight path, only the two observation points. Typically monarchs on their Fall migration will fly South or Southwest from the Eastern United States. Much is still unknown in the West although some tests were observed with farmed monarchs (captive monarch butterflies bred for sale) that were moved around the Western states and released. However, there were not any controls with wild monarchs nor records of wind direction and weather conditions at the time.
From earlier studies about migrating monarchs we do know that they are very susceptible to winds. Dr. Chip Taylor of Monarch Watch observed, "Our tagging records are full of monarchs that have been pushed off course - one by as much as 800 miles. Remember the posting (D-plex) recently on monarchs from the Dakotas, MN, IA with many pushed to the SE?" Furthermore, studies have also documented that monarchs are resilient and can reorient their flight pattern within a week after they are displaced.
|Purple arrow indicates wind direction|
The truth of the matter is we just don't know. We do know that this year the seasonal monsoon wind pattern was more entrenched than earlier years until late September. We can't say for certain that the winds blew this monarch to California, but we also can't say given the winds that week, that they didn't.