11:06 p.m. on November 18, 2008 (EST)
In some sense, the bike trip I described comes somewhat close to matching your scenario, except that you can cover 20 miles on bike on good roads much faster than you can hike that distance on trails, and the snowfall was about 6 inches. We were much ok for clothing, given that April is generally pretty reasonable temperature in New England. The blizzard was unique in that the dividing line was so sharp (couple miles from blizzard, though no whiteout, to warm 70 deg and muggy) and so great a difference from one side to the other.
But for early spring in the Sierra, Cascades, or Rockies, I would not venture 15-20 miles from the trailhead that lightly equipped. Heavy snowstorms are common in many years as late as mid-April - heavy both in the sense of volume and the snow being very high water content, hence dense. It snows in all those places every month of the year.
Some specific comments -
First, overall - going with that gear in "early spring" in the Western mountains or the NH Whites or Baxter Park is travelling right on the edge (except in the Whites, you can't get 15-20 miles from the nearest trailhead, though you can get severe weather that makes travel difficult to impossible).
1. "15-20 miles" - That's 2 to 3 days hike in good weather for most backpackers, a long way for any type of emergency. So this is not a weekend trip, except for fast ultralight hikers, more like a 4 to 5 day hike. Yes, I have taken older scouts on 20-25 mile 3-day hikes, but in summer, well away from snow likelihood (lower elevations for one thing), and a loop with potential 5 mile or less bailout points.
2. "Backpacking, 3-season tent, 40 deg bag, 3 days of food" - So you are leaving no margin for a hike that is 2-3 days travel from the trailhead. I would also say that a 40 deg bag in areas that get snowstorms in the early spring is really pushing your luck.
3 and 4 are fairly typical for lots of backpackers, and if you stick to the trails, that is ok, unless you get your hypothetical blizzard.
Still, as the incidents that Tom mentions indicate, people do indeed head out at times of year when there are sudden "unexpected" blizzards with about that amount of gear. One of the worst examples was in the fall a few years ago, when 4 or 5 separate parties in different parts of the Sierra were trapped and most needed rescue. In a couple of the cases, the people were car camping way back up forestry roads, with the cars (4WD in at least one of the groups) being finally retrieved the following spring/summer.
But given your scenario, and assuming the people were smarter than their choice of gear for a 15-20 mile back in from the trailhead for a weekend trip in early spring in an area that this surely wasn't the first early spring blizzard would indicate, the best bet, as Tom said, is to pitch your tent in as sheltered spot as possible and start rationing food and fuel immediately. They should stay put and prepare to place very visible signals as soon as the storm lets up (see any good wilderness survival book for appropriate signals, or look in Mountaineering Freedom of the Hills). I would hope that they left an accurate itinerary with someone who is responsible enough to notify the local authorities of the situation.
Also, a emergency signalling device such as SPOT (probably the least expensive option), or other PLB/EPIRB would help searchers find them more quickly (as my upcoming review notes, PLBs and EPIRBS - slight difference between these - transmit on 406 MHz and involve the COSPAR-SARSAT satellites to relay the emergency message, which for some versions includes a GPS-derived position, While SPOT relays a GPS-derived position via a HELP or 911-emergency message via the Globalstar satphone's digital messaging system to your designated contacts (HELP) or an international SAR coordination organization. Of course, just having the position doesn't guarantee the 5-minute response time of urban 911, as in the unfortunate case a few weeks ago in the Sierra, where a blizzard prevented the SAR team from getting to the victim for several days (the autopsy indicated that he died from hypothermia within a few hours of the first 911 message that went through the SPOT Globalstar system).