[pct-l] Snow impasses?

[jmhow01 at yahoo.com]

I've read a lot of blogs throughout the past couple years talking about weather conditions in the sierras creating a temporary impasse (hikers turn back and wait it out or flip flop), but I never really get an idea as to what conditions make it an impasse. Obviously snow is the culprit, but what about the snow makes things impassable? Navigation difficulties? Snow covering the trail? Snow depth? Lack of proper gear (ropes, crampons, axes)?  Ice slides?

I'm here admitting my naivety, so be gentle with me. But it seems that crossing the sierras, in mid April before the melt, is less a risk than, say, trekking through the snowy conditions of Tibet.  According to those I speak with in my community, it takes crazy conditions to cause an impasse on those trails where snow is very common. So if there is an ice slide covering the trail, for example, couldn't you chop foot holes with an ice ax and if it's too icy, then wait till late afternoon to be able to chop through the melted ice? 

I'd almost prefer to cross the sierras in mid to late April to avoid dangerous river crossings and for other reasons. So, does anyone have any words of advice regarding early season Sierra trekking, and what conditions I should be expecting that will possibly cause me to want to turn around? 


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