Weather worse than the extremes already predicted, combined with large bushfires burning out of control, and the real potential for others to spark up, have prompted a very grim warning.

''This will be as bad as it gets,'' Rural Fire Service Commissioner Shane Fitzsimmons said. ''On days like [this], there's a very real potential for more loss of homes and life.''

The RFS had warned since the weekend about the potential devastation Wednesday's weather could cause but the revised forecast on Tuesday was even more worrying, Mr Fitzsimmons said.

Wednesday would be hotter, drier and windier than the dire predictions previously forecast, posing a grave challenge to one of the largest firefighting contingents assembled in NSW history. They would have to battle an active 1500-kilometre fire edge in the Blue Mountains where three major fires, one at Bilpin now linked to the second at Mount Victoria and a third at Springwood, burned out of control.

''It's a difficult, dynamic, dangerous fire-ground situation,'' Mr Fitzsimmons said.

The forecast was for temperatures in the mid to high 30s, very low humidity and wind gusts of between 80 and 100km/h.

Three nursing homes in Springwood were evacuated late on Tuesday as a precaution, with 400 residents moved out of the Blue Mountain into Sydney facilities.

Vivienne Francis, 74, sat quietly in her pink pyjamas and held tightly two paperback books as she watched the commotion around her in the main foyer of Principal Endeavour nursing home.

"Doesn't worry me," Ms Francis said. "Got my pyjamas, a couple of pairs of knickers and two photos of my grandkids.''

The Rural Fire Service said there would not be any forced evacuations but all schools, preschools and daycare centres in the Blue Mountains would be closed on Wednesday and anyone who didn't need to be in the area was being told to keep away.

Nine schools would be closed in the Hawkesbury, as a precaution if the fires spread east. Nine schools would be closed in the southern highlands where a fourth major blaze continued to burn near the townships of Bargo and Balmoral.

Rain that fell on parts of the fire ground through Tuesday had little to no impact. Cloud cover was again expected for Wednesday morning but it would clear and, coupled with strong winds by mid-morning, fire crews were preparing for challenging conditions early.

''Even though we awoke today to grey skies and cooler weather, and even though there is the fact that there is some rain falling, it is not substantial and we shouldn't be lulled into a false sense of security,'' Premier Barry O'Farrell said.

More than 1400 firefighters would be deployed to the Blue Mountains to work alongside the 1000 already there.

Reinforcements had arrived from across the country, with the number of interstate firefighters tripling to 750 on Tuesday.

Mr Fitzsimmons said that, aside from the four big fires, all burning under the second highest alert level of ''watch and act'' on Tuesday night, there was the prospect that new fires could break out throughout Wednesday.

Aside from the big four fires, another 53 are burning across NSW, 13 of which are uncontained.