I had a class in Homelessness and Public Policy from SFSU some years ago. I lived in California a total of 12 it 13 years, iirc. Five and half of those were as a homeless person.
I am author of the blog the San Diego Homeless Survival Guide, which means I hear from both homeless individuals and reporters periodically. I still participate on a homeless discussion forum where it is not uncommon for people to ask where they should move to as a homeless person. I traveled from Georgia to San Diego as a homeless person. I went there in part for the dry, temperate weather.
I recently got off the street in part by traveling to someplace cheaper. It is much wetter here than the places I lived while homeless. Nonetheless, it is a relatively temperate part of Washington and seems to attract a lot of homeless. I am fairly confident the relatively temperate weather is a factor.
Only 10% of total counted reported living outside of California when they became homeless. I can intuitively understand that weather may be a draw, but San Francisco's findings this year, as well as in 2015 don't appear to back that a majority of those experiencing homelessness in San Francisco are travelling there from outside the state.
[1] http://hsh.sfgov.org/wp-content/uploads/2017/06/2017-SF-Poin...
However, the most outwardly visible facet of homelessness is individuals who are unsheltered, i.e. sleeping in public places or vehicles. The states with the highest rates of unsheltered people (out of total of those experiencing homelessness) was in California, Nevada, and Hawaii, Oregon, and Mississippi [1]. On the other hand, the statewide percentages themselves are not particularly insightful as to the magnitude of the problem, as they aren't weighed by the total number of individuals.
Surprisingly, raw population of unsheltered homeless individuals per metropolitan area, or as a share of total population in a metropolitan area, is a metric rarely aggregated in reports, but from digging though the data [2], one can see that the absolute number of unsheltered homeless individuals is indeed highest in Los Angeles (34000+), San Francisco (~9000), Seattle (~5000+), and New York City (4000+). Weather is certainly a factor in outcomes of unsheltered individuals, but I'm not convinced weather is a factor in contributing causes in the particular concentration of people. It appears that New York City seems to muster more assistance to individuals experiencing homelessness than Los Angeles, where in the latter more people end up on the streets, and San Francisco's problem is, in both absolute and relative terms, not as acute as that of Los Angeles, despite public perception occasionally being the contrary.
Sources: [1] https://www.hudexchange.info/resources/documents/2017-AHAR-P... [2] https://endhomelessness.org/resource/homelessness-populous-u...