Weather Contingency Planning

Definition

Weather Contingency Planning is the deliberate treatment of weather as a scheduled, quantifiable risk rather than an uncontrollable act of God. It combines historical climate data, activity-specific weather sensitivity, and predefined trigger actions to protect the programme, the works and the workforce. Done well, it reduces argument during delay analysis and prevents the panic decisions that produce the worst incidents on site.

The Three Layers

• Baseline weather allowance — historical days-lost per month per activity type, embedded into the baseline schedule.
• Rolling forecast integration — 7 to 14 day forecasts overlaid onto the look-ahead schedule each week.
• Trigger-based response — pre-agreed actions tied to specific thresholds (wind above 32 knots, temperature below 5 °C, rainfall above 20 mm/day).

Why It Matters

Contracts distinguish between "normal" weather (contractor's risk) and "exceptional" weather (relievable event). Without a historical baseline you cannot argue where the line sits. Without a rolling forecast you make decisions the morning of, which is always too late for a concrete pour, a heavy lift, or a roofing sequence. Without triggers, individual supervisors make individual calls, and any two of them will disagree. Weather contingency planning is what stops those disagreements from becoming incidents or claims.

Real-World Example

On a wind-farm foundations package in the North Sea coastal zone, the crane erection sequence was originally programmed with a flat 2-day-per-week weather allowance. The controls team pulled 10 years of local data from the Met Office and re-modelled activity by activity. Crane lifts above 45 m were disallowed above 12 m/s sustained wind; the historical exceedance in October was 41% of daylight hours, not the assumed 25%. The schedule was replanned with October crane work reduced to two priority picks and non-lifting activities front-loaded. The result was a programme that survived the first autumn intact; the previous project on the adjacent site had lost 19 crane days in October alone and spent the winter arguing about who owned the delay.

Practical Lessons Learned

• Use 10 years of local data, not general climate averages. Microclimate matters — a coastal site and an inland site 40 km apart can differ by 30% in wind exceedance.
• Weather-sensitivity by activity, not by project. A concrete pour cares about temperature and rain; a lift cares about wind; a paint application cares about humidity. A single blanket allowance is worthless.
• Publish the trigger table. When wind reaches 25 knots at 07:00, the crane goes into hold — everyone knows before they arrive on site.
• Record actual weather daily, not just when it disrupts. The day the rain didn't come is also part of the record.
• Reforecast weekly using the current 14-day outlook against the remaining critical activities.

Common Mistakes

• Treating weather as a lump-sum contingency at the end of the schedule rather than embedded in each affected activity.
• Using national climate averages instead of site-specific historical data.
• No documented weather log, so exceptional-event claims have no evidence base.
• Decisions made verbally at 07:00 with no written trigger — usually made under pressure to keep pouring or lifting.
• Ignoring the forecast for the 24 hours after a critical activity (a slab poured before a cold snap is a warranty problem for years).
• Assuming a covered site removes all weather risk — hoarding wind loads, hoarding ventilation and internal condensation all remain.
• Publishing a weather allowance in the tender and losing it during optimisation without telling the site team.

Expert Tips

• Buy the local data. Met Office / BOM / NOAA station data for the nearest station is inexpensive and transforms the credibility of the analysis.
• Overlay forecast onto the look-ahead each Friday. The Monday shift starts with the weather decision already made.
• Interlock with the concrete pour card. Weather is a release criterion, not an afterthought.
• Photograph the weather — a phone photo of the site at 09:00 in horizontal rain is what wins a delay-event claim.
• Track cumulative weather-lost days per activity family, not just per project. The mix tells you which activities to protect first next season.

Key Takeaways

• Weather is a scheduled, quantifiable risk — treat it as such from the baseline onwards.
• Historical local data plus rolling forecast plus published triggers = defensible decisions.
• Sensitivity is activity-specific; blanket allowances are worthless.
• A daily weather log is the single most valuable artefact for delay-event claims.
• Trigger-based response prevents the panic decisions that cause weather-related incidents.

Related Concepts

Interlocks with Baseline Schedule, Look-Ahead Schedule, Delay Analysis, Extension of Time, and Concrete Pour Card. Weather log templates and trigger tables at PMMilestone.org.