Oops! Forgot to pour the yeast from fermenter to mason jar

I just kegged a hefeweizen that fermented in a 6.5 gallon bucket on a wlp380. That was Monday...

I just realized that I put the lid with airlock back on the bucket and put it in the garage and left it there! I was planning on using it for another hef in the next month or so. I'm in the L.A. area so the daytime temps have been in the 60s and 70s - overnight lows in the garage probably in the low 50s. Is this yeast cake still worth saving?

If the container is closed, and you are confident your transfer to the keg was sanitary, why not? If you were sloppy in the transfer you might have problems. Maybe try a small/cheap beer if you're worried.

From what I have read and heard, hefeweizen yeast strains lose their viability rather quickly with storage. I don't think a couple days at 70 degrees would be too detrimental but storage at refrigerator temperatures would be best. I recommended making a starter with some of your harvested yeast. If the starter ferments normally and the starter beer tastes good, I would go ahead and ferment a full scale batch with it.

I would definitely make a starter prior for a full size batch, though. If what I've heard about hefe yeasts is true, your yeast slurry from the fermenter will lose viability quickly. In this case, it would be better to make a starter with a small portion of the yeast slurry to grow fresh yeast, rather than pitch a weeks-old slurry that would likely contain a lot of dead or dying yeast cells.