Mt. Whitney – Mountaineers Route

The weather is your entire problem this trip. Today is the only clean window: 27°F at the NWS observation point (figure 10-15°F colder at the 14,505 ft summit with lapse rate applied, so call it 10-15°F at the top), light SW winds 6-18 mph that will roughly double at the ridgeline to 12-36 mph — manageable but expect gusts. Partly sunny with 11% precip. Tonight holds with W winds 10-18 mph and an 18°F low at the valley level, meaning mid-route temperatures well below zero with wind chill. If you're pushing for the summit Friday, go tonight or very early Friday morning to be off the summit and clear of the Mountaineers Route couloir before Saturday's weather arrives.

Saturday is a no-go. 80% precip, SW winds 20-28 mph at the surface — call that 40-56 mph at the ridgeline with potential for higher gusts — and snow showers with patchy blowing snow. That combination creates serious whiteout risk in the upper couloir and active wind loading on the lee aspects you'll be traveling through on descent. Saturday night doubles down: 82% precip, 20-28 mph W, 14°F. Even Sunday only partially clears with 43% chance of snow showers and 20-24 mph winds persisting. Your last viable descent window before conditions deteriorate is Friday afternoon.

Avalanche data is unavailable for this zone, which is not unusual for the Whitney area — it falls outside ESAC and NWAC forecast regions. What the weather data tells you directly: SW winds loading up Friday into Saturday will be actively building wind slabs on the north and east aspects. The upper couloir on the Mountaineers Route sits in exactly that loading zone. If you're on the route Saturday or Sunday, treat the upper couloir and the chutes below the summit notch as loaded wind slab terrain and probe before you commit. The 37-inch snowpack is solid for this time of year and the approach should be well-consolidated, but Saturday's new snow on top of that surface is a fresh slab problem.

Stream crossing data in the briefing reflects gauges well outside the Whitney drainage and is not applicable here. Lone Pine Creek crossings on the approach will be running with afternoon snowmelt — cross early in the day when temperatures are still cold. With 13.5 hours of daylight and sunrise at 6:08, a 3 AM alpine start from basecamp Friday puts you on the summit before 10 AM and back at Guitar Lake or Lower Boy Scout Lake well ahead of any Saturday precip. That's your plan. If you're not moving by Friday afternoon descent, turn around.