BE20 DESCRIBED WX ENCOUNTER IN ZZZ AIRSPACE AT VARIOUS ALTS.

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I WAS CAPT ON AN FAR PART 135 FLT. DUE TO WX CONCERNS; I HAD FILED IFR WITH INTENTIONS OF BEING VECTORED S THEN N. THE WX CONCERNS WERE NOTED ON OUR FLT PLAN UNDER 'REMARKS.' WE RECEIVE THE USUAL AREA ROUTING INSTEAD. APCHING ZZZ1 FROM THE E; ATC ADVISED THAT APCH WAS NO LONGER ACCEPTING TFC INTO THE AREA. WE CHOSE TO DIVERT TO ZZZ2. ONCE ON OUR WAY THERE; ATC SAID TFC WAS NOW BEING ACCEPTED AND ASKED IF WE WANTED TO CONTINUE TO DEST. I ASKED ABOUT ANY RIDE RPTS AND WAS TOLD A DASH 8 RPTED MODERATE TURB WHEN HE WENT THROUGH THE AREA OF WX ALONG OUR RTE. ATC'S TONE OF VOICE MADE IT SEEM LIKE IT WAS NO PROB NOW AND WE DECIDED TO ACCEPT THE OFFER TO CONTINUE TO OUR ORIGINAL DEST. ONCE ESTABLISHED AGAIN; WE SAW A SOLID LINE OF WX JUST TO OUR R. WE COULDN'T PAINT GND THROUGH IT WITH OUR WX RADAR. VERY SOON AFTER THIS; THE TURB INCREASED TO CONSTANT MODERATE TO OCCASIONALLY SEVERE; WITH OBJECTS BEING LIFTED OUT OF PLACE AND TOSSED ABOUT. ATC THEN ISSUED A HDG THAT WOULD TAKE US DIRECTLY INTO THE HEAVIEST WX WE COULDN'T PAINT THROUGH. I REFUSED CITING THE WX WE WERE EXPERIENCING. ATC BECAME ADAMANT TO THE POINT WE HAD TO DECLARE AN EMER DUE TO WX. WE ADVISED WE WERE FLYING OUR OWN HDGS FROM THAT POINT TO AVOID THE SEVERE WX. WE WERE ABLE TO LAND SAFELY AT DEST. AFTER LNDG; WE LOOKED AT WX RADAR FOR THE AREA AND FOUND 'MESO' ACTIVITY IN THE AREA WE WERE FLYING IN. IT SEEMS THE ATC ROUTINGS ARE TOO RIGID IN THE NE. I BELIEVE THIS SIT WOULD HAVE BEEN AVOIDED IF WE HAD BEEN ISSUED OUR FILED ROUTING. ALSO; ATC NEEDS TO BE MORE HELPFUL AVOIDING SEVERE WX SINCE OUR WX RADAR IS QUITE LIMITED IN WHAT IT SEES. WE DON'T HAVE THE GND BASED NEXRAD THAT ATC SHOULD HAVE ACCESS TO.