"Checking the weather at the time" may not come up with the answer either, checking the weather for that day and even the day before would possibly help more. Once delays begin to amount the consequences could last much longer than the problem itself. If the weather at Heathrow or inside the London TMA was bad in the morning, there could still be cancellations throughout the afternoon to get the operation back on schedule, surprising how much a brief regulation can affect an entire operation.