The decision to go or wait rarely rests on whether the forecast is good. It rests on how confident that forecast is and how long the conditions need to hold.
Forecast confidence decays with distance
Weather models diverge as they project further ahead, and beyond a few days the range of possible outcomes widens faster than most users expect.
Operators therefore treat a five-day forecast as a planning input and a one-day forecast as a decision input, using them for entirely different purposes.
Comparing several models is standard practice, because agreement between independent forecasts is a better indicator of confidence than the appearance of any single one.
The required window is longer than the attempt
A route that takes eight hours needs conditions to hold for considerably longer, since the party must also return or reach shelter afterwards.
The relevant question is when conditions deteriorate rather than when they improve, and the closing edge of the window governs the decision.
This is why attempts often start in darkness. Beginning early moves the whole effort earlier relative to the forecast change, not relative to daylight.
Local effects override the regional picture
Mountains, coasts and valleys generate their own conditions, and a regional forecast can be accurate while being wrong about a specific face or channel.
Guides build this knowledge over seasons, learning which aspects hold cloud, where wind accelerates and which valleys clear first after a front passes.
That local correction is a substantial part of what a guide provides, and it is not available from any published forecast regardless of quality.
Retreat time defines acceptable risk
A route that can be abandoned quickly permits an attempt on a marginal forecast, because the cost of being wrong is a few hours and some disappointment.
A committing route with no easy exit requires much higher confidence, since a deterioration partway through cannot be answered by turning around.
Operators consequently apply different forecast thresholds to different objectives, which is why one group departs while another waits on the same morning.
Waiting has its own costs
Sitting out marginal days consumes supplies, guide time and the group's remaining schedule, so the decision is never simply between safety and risk.
As the trip's end approaches, the pressure to attempt something rises, and experienced operators plan for this by defining acceptable alternatives in advance.
Having a smaller objective already agreed removes the temptation to accept a poorer window for the main one, which is where most poor decisions originate.