Low humidity and warming temperatures will present an elevated potential for fires in most of Massachusetts on Sunday, the National Weather Service in Taunton said. “The current forecast is on the cusp or below the cusp of fire conditions,” meteorologist Benjamin Sipprell said Saturday. High temperatures are expected in the mid- to upper-50s Sunday, with southwest winds gusting at 25 to 30 miles per hour, and relative humidity of about 30 percent. Similar conditions on Saturday prompted the weather service to issue a red flag warning for most of the state — its first of the year, according to meteorologist Bill Simpson, who said it has not rained since April 1. More warnings are possible on Monday and Tuesday, with highs in the upper 60s forecast for both days, with a 30 percent chance of rain on Monday evening.