Okay, I found this under our long-overturned (to keep water out) wheelbarrow. I flipped the wheelbarrow over, three very large centipedes (I counted two not four legs per segment) scurried out, and then I spotted this momma. I believe it is the momma: there was a much smaller identically colored beastie nearby, probably the male.
What is it? A false black widow is my best guess.