AIR AMBULANCE LANDS BELOW MINIMUMS AND HAS LOSS OF ACFT CTL ON RWY.

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WAS DISPATCHED FOR A MEDICAL FLT TO SCH AND RETURN WITH PATIENT TO AGC. CONTACTED FLT SVC AND RECEIVED A WX BRIEFING. AT THE TIME OF ARR BACK AT AGC; WX WAS FORECAST TO BE DOWN TO AN OCCASIONAL 200 FT OVCST AND 1/2 MI VISIBILITY IN SNOW. WX AT TKOF TIME OF DEP WAS 800 FT OVCST AND 2 MI IN SNOW. I FELT WE COULD MAKE THE TRIP BEFORE IT GOT THIS BAD. ARRIVED AT SCH AND WX WAS STILL 800 FT OVCST AND 2 AT AGC. RETURNED TO AGC AND AT 20 MI OUT; WX WENT BELOW LNDG MINIMUMS; SO I ELECTED TO DIVERT TO YOUNGSTOWN WHERE WE STAYED FOR APPROX 2 HRS. AGC TWR CALLED US AND STATED THAT THE VISIBILITY HAD GONE UP ABOVE LNDG MINIMUMS. SO WE TOOK OFF AND RETURNED TO AGC. ONCE WE PASSED THE FAF TWR VISIBILITY WENT BELOW MINIMUMS AGAIN. I ELECTED TO CONTINUE THE APCH. I DID THE ILS TO RWY 27. WINDS WERE OUT OF THE N AT APPROX 20 KTS. RWY WAS IN SIGHT ABOUT DECISION HT AND A LNDG WAS MADE. ON TOUCHDOWN THE ACFT BEGAN TO SLIDE SIDEWAYS. DIFFERENTIAL REVERSE WAS USED; BUT NO CHANGE. ACFT DID NOT LEAVE THE RWY AND NO DAMAGE WAS DONE TO THE ACFT AND NO INJURIES TO OCCUPANTS. I FEEL THIS INCIDENT COULD HAVE BEEN PREVENTED IF THE PRESSURE TO GET THE PATIENT BACK TO PITTSBURGH HAD NOT BEEN THERE.