 The county has a humid continental climate ("Dfa"/"Dfb").Average temperatures in Sunbury range from 27.3 °F in January to 72.7 °F in July, while in Mount Carmel they range from 25.0 °F in January to 70.2 °F in July.The main river in Northumberland County is the Susquehanna River and the divergence of the long river into its two branches of navigable river and former divisions of the Pennsylvania Canal System.