A bizarre weather phenomenon known as a fire cloud is likely to form over the fire raging between Lithgow and Bilpin if weather conditions deteriorate from Tuesday, as predicted, say fire analysts.

The unique weather phenomenon - also called a pyrocumulus - occurs when a large fire is coupled with an unstable atmosphere.

Rural Fire Service spokeswoman Natalie Sanders said the State Mine fire, which stretches across more than 42,000 hectares, had generated a pyrocumulus last Thursday and similar, unstable conditions were forecast for Wednesday.

''If these fires are still going strong, there's a potential for that to happen again,'' she said.