B737-200 IS UNABLE TO CLB VFR AS REQUIRED BY THE SID.

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THIS EVENT OCCURRED DURING THE 'BRIDGE DEP' AT THE JNU ARPT; AK. THE 'BRIDGE DEP' REQUIRES YOU TO HAVE A 4500 FT CEILING; 7 MI VISIBILITY. THE SID STATES 'CLB IN VFR CONDITIONS TO CROSS THE DOUGLAS ISLAND BRIDGE AT 4500 FT THENCE CLB ON COURSE.' THE WX AT THE JNU ARPT AT OUR DEP TIME WAS 3000 FT SCATTERED; 5000 FT OVCST; 7 MI VISIBILITY; WINDS 090 DEGS AT 20 KTS GUSTING TO 30 KTS. A COMPANY ACFT ON THIS DEP 30 MINS PRIOR RPTED THE CEILING OVER THE BRIDGE AT APPROX 4000 FT. BY THE TIME WE DEPARTED CONDITIONS HAD APPEARED TO IMPROVE SOMEWHAT IN THE LCL AREA. THE PROB WITH THIS SID IS THAT THE DOUGLAS BRIDGE IS LOCATED ABOUT 7 MI FROM THE JNU ARPT. YOU CAN SEE THE MOUNTAINS ON THE N SIDE OF THE GASTINEAU CHANNEL WHICH THE BRIDGE SPANS; BUT THE MOUNTAINS ON THE S SIDE ARE OBSCURED BY LCL TERRAIN S OF THE FIELD. WX CAN DIFFER LARGELY IN THIS SHORT DISTANCE. WE DEPARTED JNU ON RWY 8; MAKING A CLBING R TURN TO A HDG OF ABOUT 110 DEGS. THIS PUT US ON A COURSE DOWN THE GASTINEAU CHANNEL TO CROSS THE DOUGLAS BRIDGE. LOOKING DOWN THE CHANNEL; VISIBILITY IS GOOD; 15+ MI. WE CAN SEE THE CITY OF JUNEAU; THE BRIDGE AND SURROUNDING HILLS AND SHORELINE. AS WE CONTINUE TO CLB THROUGH; IT BECOMES APPARENT THAT XING THE BRIDGE AT 4500 FT VMC WILL NOT BE POSSIBLE BECAUSE OF LOWER CONDITIONS BTWN US AND THE BRIDGE CLBING THROUGH 2000 FT WE ENCOUNTER LIGHT RAIN AND MIST AND OCCASIONAL STRATUS. OUR FORWARD AND VERT VISIBILITY IS STILL GOOD AND WE'RE ABLE TO SEE THE SHORELINE AND MOUNTAINS AROUND US. BY 3000 FT WE ARE IMC; CLBING AT ABOUT 3000 FPM ON OUR LAST HDG OF ABOUT 110 DEGS. WE CONTINUE TO CLB THROUGH VARIOUS LAYERS TILL 4500 FT; MAKE OUR R TURN TO SSR VOR TO JOIN J541. BY 6000 FT WE'RE SOLID IFR. DUE TO TERRAIN ON BOTH SIDES OF THE FLT PATH DURING DEP I FEEL GOING IMC WHILE CLBING ON A CONSTANT HDG TO 4500 FT WAS THE PRUDENT THING TO DO. ANY ATTEMPT TO MAKE A 180 DEG TURN BACK TO THE ARPT AT THE TIME WE ENCOUNTERED THE CLOUDS WOULD NOT HAVE PROVIDED ADEQUATE TERRAIN SEPARATION. SINCE THIS SID REQUIRES XING A FIX SEVERAL MI FROM THE ARPT THAT CAN'T BE SEEN FROM THE ARPT IT'S OBVIOUS THAT A BETTER WX RPT IS NEEDED FOR THE RTE OF FLT. DUE TO THE STRONG SE WINDS THIS WAS THE ONLY SID THAT WOULD ALLOW US TO DEPART JNU. PRIOR TO DEPARTING WE THOROUGHLY BRIEFED THE SID; FAMILIARIZED OURSELVES WITH THE TERRAIN AROUND THE FLT PATH; AND DISCUSSED WHAT WE WOULD DO IF THE WX WASN'T AS FORECASTED. THIS PRIOR PLANNING WAS INVALUABLE AS WE KNEW WHERE WE WERE; WHAT WAS AROUND US; AND WHERE WE WERE GOING DURING THIS EVENT. IN HINDSIGHT; IN A SIMILAR SIT I WOULD NOT MAKE THIS DEP. EVEN THOUGH ALL YOU NEED IS 4500 FT TO BE LEGAL ON THIS SID; I WILL REQUIRE A HIGHER CEILING AND VISIBILITY BEFORE I AGREE TO DEPART.