The ADS-B data shows that the pilot had indeed intercepted the 101 freeway out near Woodland Hills, which despite its name, is fairly flat and densely populated. He intercepted the 101 from a southwesterly track because ATC had routed him north of the Burbank and Van Nuys airports, to avoid some IFR departures which were coming off to the south. He then tracks the 101 dead on all the way into the Calabasas area, which is several miles, so the indications are that he has visibility below the aircraft at this point. He’s flying at 1250 ft calibrated altitude and 130 knot ground speed heading into this area, but Calabasas is where the 101 starts winding through the hills. After the first couple of turns of the 101, the aircraft starts gaining altitude, and the pilot informs ATC that he is climbing to avoid a cloud deck. One eyewitness describes this area as “a bowl, with its own microclimates”. Tracking a ground reference, like the 101 freeway, at low altitude, high speed, and winding through hills, it may have been impossible to see an approaching decrease in the cloud ceiling until after coming around the bend. Heck, I’ve done it in a car in the mountains, on a windy road, and that’s at low speed. Came around the bend, and suddenly I’m in the soup. The climb and the initial speed decrease to 110 knots was most likely the best response if this was the case, but why the aircraft subsequently turns left and descends rapidly, will be the key for investigators to determine (if possible).