DURING THE WX BRIEFING THE PLT WAS ADVISED THAT THE WX ALONG HIS PROPOSED ROUTE OF FLT WOULD BE GENERALLY GOOD VFR WITH THE EXCEPTION OF POSSIBLE TURBULENCE & THAT BLOWING DUST WOULD BE THE BIG PROBLEM. THERE WERE PIREPS OF 65K WINDS AT 3,000 FT OVER OKLAHOMA CITY WHICH WAS FORECASTING VISIBILITIES DROPPING TO 3-4 MI IN BLOWING DUST BY NOON. THE PLT'S FORMER FLT EXAMINER TRIED TO CONVINCE THE PLT TO POSTPONE THE TRIP DUE TO HIGH WINDS ENROUTE. AT APPROXIMATELY 1015 A GROUND WITNES ON A ROAD NEAR THE LAMAR ARPT STATED HE SAW A DUST CLOUD NORTH OF TOWN RISING TO AN ALTITUDE OF APPROXIMATELY 2,000 FT. BY THE TIME THE WITNESS REACHED THE ARPT VISIBILITY WAS LESS THAN 100 FT WITH AN ESTIMATED WIND OF 60 MPH. ANOTHER WITNESS WHO HEARD THE CRASH SAID IT WAS TOO DIRTY TO SEE ANYTHING. THE LA JUNTA FSS, LOCATED APPROXIMATELY 32 MI WEST OF THE ACCIDENT SITE, 0958 & 1058 OBSERVATIONS INCLUDED SURFACE WINDS GUSTING TO 57K & SKY COVERAGE OF 0.6.