Observing the sky requires clear skies. We base our decision to open on weather forecasts, which by their nature can be inaccurate, therefore our decision to open gives no guarantee it will be clear; we take a risk.
The observatory may be advertised as open but then postponed close to opening times if there is a late moment change in the forecast or some unfortunate event that prevents us from opening. It is best to check before travelling to ensure that a late moment decision has not been made to remain closed.
Forecasting the weather is not easy, which is why forcasts often differ. And then there are local variations - in time and region - something that no forecaster can address.
Most people have thier own sources, radio, television, newspaper, website etc and for the latter there are the following below, centered on Kenley in South London (which may imply accuracy but is really over a larger area):
Needless to say these should be regarded only as a guide, and the inclusion of these websites rather than any other does not reflect any bias of the society; they are likely to be no better or worse than others.
The following is not so much a forecast service but shows the weather at the current time. It is useful to see if the sky is actually clear and if any clear patches or clouds are arriving: