The last paragraph of clause 60.1 (13) determines the criteria for assessment of a weather compensation event, as the difference between actual weather conditions and up to the 1 in 10 year average. This is where the fun commences though as this appears pretty open ended with regard to how you assess the effect.
The compensation event is apparent when the officially recognised data is obtained, although it actually occurs when the ‘tipping point’ of the 1 in 10 year average is exceeded, which could be on the last day of the month. My view, however, is that you treat the entire month as the compensation event period for assessment, because that is the period of time that clause 60.1 (13) relates to and is averaged out across.
Personally I would take the Accepted Programme prior to the applicable month and update to the beginning of the month. Weather conditions are one of the instances where I believe the Accepted Programme is not actually updated to the point when the compensation event occurred, as it is assessed over the entire month. Assess the forecast progress, obviously including time risk allowances, including for weather conditions which are not a compensation event, and compare with the actual progress for that month.
Although this ‘re-measure’ approach includes an element of your own risks which do not relate to weather conditions, including Equipment breakdowns etc, it does at least provide a starting point for an assessment. Agree the programme effects with the Project Manager and then confirm any agreement in a formal communication.
The quotation assessment would also factor in other consequent measures, such as any additional de-watering equipment, temporary surfacing to access roads, additional excavation support and weather protection etc, which are a direct consequence of the compensation event,. Hopefully an early warning was previously notified which discussed the use of such measures anyway, so the Project Manager would have been aware of what was proposed.