A friend and I started at Onion Valley and continued on the JMT toward Whitney. When we got to Crabtree the ranger said forecasts were in the teens and the wind was increasing in higher elevations. Forecast only three days before was for a full 30 degrees warmer and we had prepared for 10-15 degrees lower, just because... rather than huddle together for warmth and brave the winds we hiked on to Horseshoe. Last day over New Army my friend had every single stitch of clothing on while hiking (I still had my midlayer packed, wore the rain jacket as a windbreaker). We got to the parking lot where it was 40F - felt warm.

It pays to be prepared, and to be willing to alter plans when you are surprised despite your planning. We didn't go out there to suffer. We contemplated getting a day permit and just summiting as a day hike (what's a few more thousand feet of gain after you've done 12,000 cumulative in four days?), but opted to head for home instead - mountain is still there, we'll do it this year.