That is, a fog with frozen particles of ice. This is a regionalism, specific to the western United States (and possibly also the Rock Mountain provinces of Canada? -- not sure). It comes from Shoshone paγɨnappɨh or pakenappeh, meaning cloud or possibly thunder cloud -- dictionaries are disagreeing here. So no, nothing to do with the Greek root pogon, beard.
Sorry I'm late, the pogonip made route 357 something treacherous.