During this time, normal daytime operations will be affected. When arrivals are from the west (over London), only the southern runway will be used for arrivals, and the northern runway for departures. They will not switch at 3pm as usual.
Whilst this will no doubt cause a lack of noise respite for those living under the flightpath, it is also something for spotters to note when planning where to spot at Heathrow.
The full BAA news release can be found here