Deciphering NCDC data
The documentation linked from the datasets page states:


  Air Temperature (all units in Fahrenheit on PDF monthly form and tenths of degrees Celsius on CSV or text)
  EMNT - Extreme minimum temperature *
  EMXT - Extreme maximum temperature *  


The Petersburg data looks plausible under this interpretation (EMXT −3.9°C to 33.9°C over the year).