We were doing an alpine scrambling trip of Mt Phelps. There had been precip on 4/24-4/25, and warm temps on 4/26-4/27. Our day was forecast as mostly cloudy and cooler, but actual weather was a little warmer & sunnier than forecast (~50F). We selected a route to cut across Mt Phelps' S-facing basin very low (4200'-4300'), specifically because of what we felt was remote chance of a wet-loose avalanche initiating from cliffs at the top of the basin at ~5,000. Our spot at 4300 was fairly thick old-growth timber. At 11:30 AM we encountered a debris field of a previous slide that had penetrated the timber and come to rest at 4300' - we examined it, and (perhaps incorrectly) interpreted it to be not recent.
We traversed a short distance beyond that debris field to discuss the situation when we heard a avalanche moving slowly thru the timber above us. We got a brief glance at it thru the trees - it was moving quite slowly and not an immediate threat, but we turned around without hesitation and booked it back down the mountain. Due to the thick timber, we could not see where it initiated, how far it traveled, what speed it had reached, or how wide of a fan it produced. I characterized it as "S1" because that's the amount we could see, but it was possibly larger, as it penetrated the mature forest.
It was unnerving to be in thick timber, hear an avalanche slowly approaching, but not immediately be able to see it - made it difficult to know which way and how far to run to evade it. It struck me as surprising for a natural avalanche to be triggered so early in the day at such a moderate temperature - did not take much to trigger it.
The photo attached is of the debris field we encountered prior to witnessing the avalanche. If there are any features of the debris field to indicate it was from a recent slide and should have been a "red flag", that would be helpful info to learn for the future.