Parts of western Sydney could experience their hottest four-day spell in more than 20 years between Friday and Monday. A slow-moving mass of hot air will sweep over NSW in the coming days, sending the mercury soaring and elevating fire danger ratings. The Rural Fire Service has issued total fire bans in five fire areas on Friday, including the Greater Sydney and Greater Hunter. Penrith is forecast to reach 41-43 degrees during the next four days, which would be its first four-day spell above 40 degrees in 23 years of records. Richmond is also forecast to reach 40 degrees or more from Friday to Monday, although there's a chance it will only hit the high thirties on the weekend. As of Thursday, Richmond had registered seven 40 degree days so far this summer, which is a new record up to this point in the season. The previous record to this date was five 40-degree days in 1957/1958. If Richmond does manage to reach 40 degrees on each of the next four days, the season's tally will reach 11 days and match the current summer record from 2016/2017. Eastern suburbs will also be noticeably warm in the coming days. While sea breezes will keep the mercury much lower than the western suburbs, conditions will become increasingly humid, making it feel warmer than it actually is.