BFR FLT AT NGT, PLT RCVD 2 WX BRIEFINGS & FILED VFR FLT PLN. LOW CIGS PREVAILED ALONG RTE. ABT 7 MIN AFTER TKOF, PLT OPENED FLT PLN & RCVD VFR ADZY FM OAKLAND CTR WITH HANDOFF TO TRAVIS RAPCON. AFTER DISCUSSION ABT RTE, RADAR CTLR PROVIDED VECTOR OF 050 DEG. AT 2216:15, CTLR TOLD PLT RADAR CTC WAS LOST DUE TO ACFT'S LOW ALT & SUGGESTED A CLIMB 'AT LEAST ABOVE 3000 FT.' AT ABT 2216:55, RADAR CTC WAS REACQUIRED & CTLR TOLD PLT HE WAS 8 MI NORTH OF NAPA; TO THIS, THE PLT TRANSMITTED, 'OK, (PAUSE) ITS ALL FLAT COUNTRY OUT HERE I THINK ISN'T IT' & THE CTLR REPLIED 'YEA.' AT 2218:19, CTLR ISSUED HDG OF 080 DEG & ASKED ABT ACFT'S ALT. WHEN PLT SAID 2500', CTLR ADZD 2900' MTN WAS ABT 1 MI RGT OF COURSE. AT 2219:45, CTLR ADZD MTN PEAK WAS LESS THAN 1 MI NE OF ACFT. PLT INDCD HE WOULD TURN LEFT, BUT SOON THEREAFTER, ACFT CRASHED NR TOP OF A RIDGE AT 2450 FT MSL. APRX 12 MI ESE AT TRAVIS AFB (ELEV 62 FT), 2155 PST WX WAS IN PART: 1200' SCT, 2500' SCT, 5000' OVC, & VIS 7 MI WITH LGT RAIN.