Interesting thoughts on the Jet Stream and Snowy Owl Irruptions. Some interesting things to think about here though that point to other factors causing the Southward migrations. The leading 2 theories seem to be available food source and weather. If food sources are low then why do they migrate in 10-15 year migration cycles all the way down to Washington and even Northern Oregon on the West Coast? Why not the Fraser River Valley in B.C. or larger populations in the Skagit/Stilliguamish Valleys? If weather is the answer then why have populations in Western Washington been around for a month or more and Eastern Washington (which is severely colder this/most years) seems to be building now? Hmmmmmmm? Dunno!
Alaska and Washington have 10 year water temperature cycles that impact fish and duck populations significantly...perhaps this cycle has effects on the Snowy's as it moves up the food chain?
Raptor migrations each year vary significantly as well. One year a counting/banding station will have record low numbers and the one a mere ridge away will have record high numbers. Perhaps the regular mid-west migrations simply venture a little more to the west every few years as conditions become more favorable? Has anybody (if there is even a way to check) ever cross checked mid-west irruptions/migrations of Snowy's with peak West Coast irruptions?
Maybe it is a territorial thing where higher than average breeding numbers have added numbers to an ever decreasing habitat/territory and younger Owls then are pushed out or seek less populated hunting grounds in the winter as other birds migrate South. They may be easily lured by large flocks of ducks/passerines etc. into following a food source South. Much like the wolves/bears/mountain lions did in the plains when the Buffalo migrated?
It puts a HUGE SMILE on my face to think that we have not yet learned everything there is to learn in this world and that nature still controls itself (to some extent). To quote Terry Tempest Williams "man can try to control nature, even contain it, but when men and women venture into wild places, wilderness controls man, nature and weather still rule the Earth".

Bryan
Shoreline, Wa.
jennandbryan at msn.com