It depends on what exactly "forseeable" means in this context. That Hitler really hated Jews was obvious to anyone paying attention and not in outright denial about it. That he was going to embark on a campaign of industrialized murder and genocide was not.

Notable is that Hitler constantly referred to Jews as vermin or a disease (in line with his Social Darwinist worldview of peoples/races being literal oranisms) in Mein Kampf, conversations, and speeches. The earliest documented evidence of him advocating for murder in private is from a letter of law student Heinrich Heim from Aug. 12th, 1920 to Fritz von Trützschler about a visit to Hitler. Hitler allegedly stated that the "bacillus needed to be exterminated" and that one shouldn't hesitate to kill Germans that didn't understand that, let alone the "hostile foreign tribe". Though as noted, this was in a private conversation. His speeches at the time were rabidly antisemitic, but not necessarily advocating murder. The second volume of Mein Kampf published in 1926, however, contains a statement where Hitler hypothesizes that World War I could have been won if "twelve to fifteen thousand" Jews/Marxists (Hitler often treats them as synonyms) had been subjected to poison gas as a preemptive measure.

To claim that this is proof that the Nazis planned the gas chambers as early as the 1920s is a stretch though. Persecution of Jews escalated as the Nazis tightened their grip on power and then, after starting World War II, Europe. First Jews were excluded from public life (such as by being barred from work and through restrictions on marriage), then expropriated, terrorized during Reichspogromnacht in 1938 in an effort to encourage emigration, and eventually deported East. Extermination of Jews in areas outside of Germany began as soon as troops marched in. Sometime in the early 1940s, the Nazis elite decided to exterminate all Jews and organized a conference in 1942 to find the most efficient means to do so.

So while a system of camps set up to terrorize and later engage in murder on an industrial scale is not something people should necessarily have seen coming, it should be obvious to anyone paying attention at the time that Hitler really hated Jews and stated explicitely that he wanted them gone and would use violence as a tool to accomplish that goal.

As early as 1942, Poland's government in exile had provided a report to the Allies about Nazi atrocities in their occupied country. So it was somewhat forseeable however given that the camps were still a secret the picture was not 100% crystal clear until 1945 came.