If one is enroute during a VFR cross country and encounters weather, one need not land and file a flight plan. A "pop-up" clearance is effective and done often. I've done it many times myself. Including while flying a heavy tanker.
My firefighting experience is limited to heavy tankers, single engine tankers, air attack, fire patrol, etc, as well as a number of years of firefighting on the ground doing wildland, and structural. This isn't particularly relevant to what happened to Tom and company, though I will say I flew with Tom and knew him well. You asked.
A tanker crew is not exempt from being required to file when ferrying in instrument conditions (which they obvoiously were)
No, they obviously were not. They were making a VFR cross country, and obviously performed continued VFR into IMC conditions. Not ferrying in instrument conditions. They attempted to stay out of instrument conditions, initially.
I know the area where it ended, intimately. I owned a house there, lived near the base of the hill where they were killed. I dropped retardant on the same hillside where they died. I had a fire contract there for several years. I've flown that area in all conditions, including low visibility on fires. I've done it in high winds, and extreme turbulence. I've been on search teams on the ground for children lost in the old mines. I know those hills about as well as I know any place. Personally, I wouldn't have made the decision to do as they did.
Tom and I disagreed on many occasions. I have always known Tom Risk to make decisons that were considerably more conservative, and safer than the other guy. I've known Tom to make mistakes, too; he was human. I've been known to make mistakes, being human, too.
I don't presume to know why Tom elected to permit flight at that location. I don't presume to know the specifics involved, and I don't assume. Others here do, and rush to judgment.
The story here isn't new. The P3 in Missoula, the P2 at San Bernadino, and so on. Continued VFR into IMC has long been, and will continue to be a slippery-slope killer. In the fire business, we regularly operate in low visibility under VFR...very often in IMC while operating under VFR. It's frequently the nature of firefighting. I've been there in single engine tankers, and heavy tankers. Nearly always in mountainous terrain, and often as not in turbulent conditions. Operating in low visibility in close proximity to terrain is very nearly second nature, as is operating close to the ground.
That this familiarity, far more than most pilots would likely understand, may have played a factor in the decision to go where this flight went, is a possibility. Not a certainty. The only certainty here is that the airplane hit the hillside, and that everyone is dead.
One can play guesswork as to what was referenced with the color yellow, whether it was the garmin terrain feature or xm weather. One can play guesswork as to what was being seen in the cockpit. One can play guesswork as to the assertiveness of each crewmember, or the lack thereof. One can play guesswork as to where they thought they were (I can tell you from the transcript that they were not where they thought they were, which very likely played a big role in the event...and that if they'd been where they indicated that they thought they were, it wouldn't have been a problem). In the end, guesswork proves nothing more than nothing.
The comments from certain here, however, are nearly laughable...as said before, some of you have spent too much time watching "Always." Great movie, but far from reality.