CITATION FLT CREW HAS AN ALTITUDE DEVIATION WHILE ENCOUNTERING UNFORECAST DESTINATION WEATHER ACCOMPANIED BY FMS ISSUES AND DIVERSION AIRPORT PROBLEMS. SECOND ALTERNATE RECOMMENDED BY ATC PROVES TO BE THE ANSWER TO A VERY BUSY FLIGHT.

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I FLY SIC FOR A PART 91 CPR FLT DEPT. WE WERE FLYING INTO ZZZ. THIS ARPT SITS IN A PLATEAU/VALLEY BTWN SEVERAL MOUNTAIN RANGES. TO THE E AND N ARE THE MOUNTAINS; AND TO THE W ARE MORE MOUNTAINS. ALSO; ON APCH TO RWY; THERE IS A LARGE RIDGE LINE ABOUT 1/4 MI BEFORE THE THRESHOLD; WITH THE ARPT ON A PLATEAU ON TOP OF THE RIDGE. AS PNF; WE BEGAN DSCNT INTO ZZZ1 OVER THE E/N MOUNTAINS. BEFORE THE TOP OF THE DSCNT; THE WX AT ZZZ1 WAS HIGH CLOUDS (10000-ISH FT); WINDS WNW 31 DEGS GUSTING TO 38 KTS. OTHER AREA ARPTS WERE RPTING LOWER CEILINGS; 2500 FT SCATTERED-BROKEN; SOME HAD THE SAME HIGH WINDS; AND SOME WERE WNW AT 10 KTS. THE DRASTIC DIFFERENCES IN AREA WX WERE CONFUSING; AND WE DISCUSSED THIS DURING DSCNT. WE ALSO STARTED PICKING UP ICE (LIGHT RIME) IN THE DSCNT. ALTHOUGH THE ACFT HAS HEATED WINGS; I DO NOT HAVE VERY MUCH EXPERIENCE IN ICING AND THIS DISTR MY ATTN. AS WE DSNDED; WE CHKED THE WX AGAIN; WHICH HAD DROPPED TO 2500 FT SCATTERED; 4500 FT BROKEN. BECAUSE OF THIS; THE PF DECIDED TO BRIEF THE APCH; NDB/GPS 31. AS WE BEGAN TO SET UP FOR THE APCH; WE NOTICED THAT OUR FMS DID NOT LIST THE IAF AS A WAYPOINT ON THE DIRECT-TO LIST AFTER LOADING IT. INSTEAD; THE FIRST FIX WHICH CAME UP ON THE LIST WAS ABC01; A GPS WAYPOINT INBOUND ON THE APCH. WE TRIED LOADING THE APCH MULTIPLE TIMES; BUT IT WOULD NOT LET US FLY TO THE IAF; AND THUS; WOULD NOT ARM THE APCH. THE PF CONTINUED TO FLY THE ACFT WHILE I TRIED TO SORT OUT THE PROB. AS WE DSNDED THROUGH THE MID 10;000'S; WE ENTERED CONTINUOUS MODERATE TURB FROM THE WINDS AND MOUNTAINS. AS WE GOT CLOSE TO THE IAF; WE NOTICED THAT THE WX E (AND BELOW) THE RIDGE LINE WAS VMC AND W OF IT (ABOVE IT; WHERE THE ARPT IS) WAS LOW IMC IN SNOW. WHEN WE GOT TO THE IAF; I HAD NOT YET WORKED OUT THE PROB WITH THE FMS; SO THE PF QUICKLY FLIPPED TO RADIO NAV AND BEGAN TO FLY THE APCH THAT WAY. I THEN TRIED TO LOAD THE APCH AS A VECTORS TO FINAL; SO WE COULD FLY THE OUTBOUND AND PROC TURN MANUALLY; THEN FLY INBOUND WITH THE FMS. HOWEVER; AFTER TURNING AROUND; WE COULD SEE THAT THE ARPT WAS VERY LOW IMC; AND THAT WE WOULD NOT BE ABLE TO LAND. WE DECIDED THAT; INSTEAD OF ENTERING IMC AND ICING AND FLYING A MISSED APCH WITH TERRAIN NEARBY; WE WOULD ABANDON THE APCH AND STAY VMC. THE PF DECIDED TO DIVERT TO ZZZ2; AN ARPT 15 MI AWAY. THIS HAD NOT BEEN BRIEFED PRIOR TO THE APCH AND GOT US OFF OUR IFR CLRNC UNTIL WE COORDINATED WITH CTR. AT THIS POINT; WE WERE SKIMMING THE EDGE OF IMC; IN TURB; TRYING TO RUN MULTIPLE CHKLISTS; SETTING UP OUR NAV; AND TRYING TO BRIEF OURSELVES FOR THE NEW ARPT. DUE TO THIS; THE PF FLEW THE ACFT THROUGH OUR ASSIGNED ALT OF 11000 FT TO ABOUT 11500 FT. HE QUICKLY CAUGHT THE DEV AND CORRECTED. MEANWHILE; CTR INFORMED US OF SEVERAL NOTAMS AT THE DIVERSION ARPT; INCLUDING CLOSED RWYS. AT THIS; WE DECIDED TO DIVERT TO ZZZ3. BY THE TIME WE WERE OUT OF THE TURB AND ABOVE THE TERRAIN; WE HAD CAUGHT BACK UP WITH ALL CHKLISTS AND ATC COMS. THE TIME FROM FIRST STARTING THE APCH TO FINALLY DIVERTING TO ZZZ3 COULD NOT HAVE BEEN MUCH MORE THAN 4 OR 5 MINS. AFTER LNDG IN ZZZ3; THE PF AND I DEBRIEFED. WE BOTH LEARNED A LOT FROM THIS FLT AND WE CAME TO SEVERAL CONCLUSIONS: 1) WITH THE WX AS UNPREDICTABLE AS IT IS IN THE MOUNTAINS; AND WITH A LARGE DISCREPANCY IN THE RPTED WX IN THE AREA; WE SHOULD HAVE BRIEFED THE APCH INTO ZZZ1 MUCH EARLIER; IN CRUISE. THIS WOULD HAVE 1 LESS MAJOR DISTR WHILE FLYING IN ICING AND TURB; AND WOULD HAVE ALLOWED US MUCH MORE TIME TO FIGURE OUT WHY THE FMS WOULD NOT NAVIGATE TO WHERE WE WANTED IT TO. 2) IN ADDITION TO BRIEFING THE APCH; A FEW DIFFERENT POSSIBLE COURSES OF ACTION AFTER A MISSED APCH SHOULD BE COVERED; SO AS NOT TO SURPRISE EITHER PLT WHEN THINGS ARE GOING DOWNHILL. 3) THE CAUSE OF THE FMS ISSUE WAS A LOADING ERROR. OUR FMS REQUIRES SOME SORT OF TRANSITION RTE TO THE APCH. IN THE CASE OF THE NDB/GPS APCH AT ZZZ1; THE ONLY TRANSITION RTE IS FROM ZZZ1; A VOR SW OF THE FIELD. THIS WAS THE OPPOSITE DIRECTION OF OUR ARR; SO WE SIMPLY SKIPPED THIS STEP. ASSUCH; THE FMS ONLY LOADED HALF OF THE APCH; THE INBOUND PORTION. 4) THE SAFETY OF OUR FLT WAS NEVER IN JEOPARDY. EVEN THOUGH WE WERE BOTH VERY BUSY DOING SEPARATE TASKS; OUR DECISION TO STAY IN VMC KEPT US IN VISUAL CONTACT OF THE TERRAIN UNTIL WE GOT A NEW CLRNC. 5) ATC WAS VERY HELPFUL AND PROFESSIONAL. THEY GAVE US THEIR FULL ATTN; WERE GIVING US SUGGESTIONS; ALL THE HELP WE ASKED FOR; AND KEPT US FROM LNDG AT A CLOSED ARPT. WE BELIEVE THAT; DESPITE OUR ERRORS; WE WORKED VERY WELL AS A CREW AND HANDLED THE SITUATION AS WELL AS ANY FLT CREW COULD HAVE. EACH OF US PERFORMED OUR OWN SEPARATE DUTIES AND TRIED TO KEEP THE OTHER AS INFORMED AS POSSIBLE. BTWN THE TERRAIN; ICING; CHANGING WX; HIGH WINDS; TURB; FMS PROBS; FLYING AN NDB APCH AND THE SUBSEQUENT MISSED; DIVERTING TWICE; ATC COORD; RUNNING CHKLISTS AND BRIEFINGS; SETTING UP AND CLEANING UP THE ACFT; AND MY OWN INEXPERIENCE; WE SIMPLY GOT OVERWHELMED. ON THEIR OWN; NONE OF THESE WERE MUCH OF AN ISSUE; BUT WITH ALL OF THEM LUMPED ON US AT ONCE; IT MADE FOR A VERY BUSY FLT.