FLT CREW OF A319 INADVERTENTLY ENCROACHED HOLD SHORT LINE AT TXWY H1 ON RWY 9L AT ORD WHILE REVERSING TAXI COURSE AT GND CTLR'S DIRECTION. SNOW; BLOWING SNOW AND OBSCURED RWY MARKINGS AND ARPT SIGNAGE CONTRIBUTED. ACFT TOOK OFF FROM RWY DURING THE ENCROACHMENT.

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WE WERE CLRED TO PUSH FROM THE GATE. WX CONDITIONS WERE 5000 FT RVR; LIGHT SNOW AND BLOWING SNOW; WINDS 280 DEGS; 10 KTS; 10 DEGS F. ARPT CONDITIONS WERE 1-3 INCHES SNOW ON TXWYS WITH BLOWING SNOW. ACFT WAS GIVEN TAXI INSTRUCTIONS TO MAKE A L TURN OUT; AND TAXI E ON TXWY H; R TURN ON TXWY P; TXWY A BRIDGE FOR RWY 27L. AFTER VERIFYING INSTRUCTIONS ON THE AIRPORT CHART; A TIGHT L TURN WAS INITIATED DUE TO OTHER ACFT AT GATE. ATC THEN CHANGED THE TAXI INSTRUCTIONS TO TAXI W ON TXWYS; H; B; HOLD SHORT OF A6. THE BRAKES WERE SET AS NEW INSTRUCTIONS WERE COPIED BY THE FO AND REVIEWED ON THE AIRPORT CHART BY BOTH PLTS. A SLOW 180 DEG TURN WAS INITIATED IN ORDER TO COMPLY WITH THE INSTRUCTIONS AND A FORWARD TAXI FOLLOWED BY A L TURN ONTO TXWY B WAS PLANNED. DUE TO MARGINAL VISIBILITY THE CAPT ATTEMPTED TO VERIFY POS WITH TAXI SIGNS BUT WAS UNABLE TO SEE ANY IN THE ACFT'S IMMEDIATE VICINITY. AS THE ACFT MOVED FORWARD; A TXWY C TAXI SIGN CAME INTO VIEW ON THE L SIDE OF THE ACFT. SINCE THIS WOULD INDICATE THE ACFT HAD ACTUALLY PASSED TXWY B THE CAPT BROUGHT THE ACFT TO A COMPLETE STOP. SIMULTANEOUSLY; ATC ISSUED A CLRNC TO 'STOP AND HOLD POS.' THE ACTUAL ACFT POS AT THIS TIME WAS ON H1 SHORT OF RWY 9L. AT NO TIME DID EITHER CREW MEMBER SENSE THAT THE ACFT'S POS WAS UNSAFE WITH RELATIONSHIP TO THE RWY. APPROX 5-10 SECONDS AFTER THE ACFT HAD COME TO A COMPLETE STOP; AN RJ DEPARTED ON RWY 9L. RWY HOLD SHORT MARKINGS; INCLUDING RWY EDGE LIGHTS WERE COVERED BY SNOW. THERE WERE NO RWY OR TAXI SIGNS VISIBLE AT THIS INTXN; ALTHOUGH AFTER THE ACFT HAD STOPPED THE FO RECALLED SEEING A PARTIAL RWY SIGN FORWARD OF OUR POS; ON THE R. WE WERE THEN CLRED ONTO RWY 9L AND GIVEN FURTHER TAXI. ONCE ESTABLISHED ON THE RWY CTRLINE LIGHTS WERE VISIBLE; BUT DIM. THE ACFT DEPARTED NORMALLY AND NOTHING WAS SAID TO INDICATE A DEV BY ATC UNTIL ON THE GND AT OUR DEST.