REGIONAL ACR FLC TAXIED ACROSS A RWY THAT WAS CLOSED FOR SNOW REMOVAL DURING OPS IN LOW VISIBILITY CAUSED BY BLOWING SNOW. ATCT GND CTLR INTERVENED AND ISSUED FURTHER TAXI INSTRUCTIONS.

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THE WX AT PHILADELPHIA WAS DETERIORATING WITH HVY BLOWING SNOW WITH VISIBILITY LIMITED TO 1/4 MI. THE GND CTLR WAS VERY BUSY ISSUING TAXI INSTRUCTIONS TO US AND MANY OTHER ACFT. RWY 9L WAS CLOSED AS PER ATIS AND AS WE APCHED RWY 9L COULD OBSERVE PLOWING AND BROOMING TAKING PLACE. WE SLOWLY TAXIED OUT; MORE CONCERNED ABOUT WATCHING OUT FOR OTHER ACFT. APPROPRIATE TXWY MARKINGS WERE BECOMING MORE DIFFICULT TO SEE IN THE DRIVING SNOW. WE THOUGHT WE WERE CLEARED ACROSS RWY 9L; BUT GND CTL QUICKLY INFORMED US THAT WE WEREN'T AND THEN GAVE US FURTHER INSTRUCTIONS TO THE DEP RWY. WE BOTH KNEW RWY 9L WAS CLOSED WHICH WAS EVIDENT BECAUSE OF THE NUMBER OF PLOW TRUCKS; BUT IT WAS NO EXCUSE FOR XING THIS RWY. THE CAPT AND I BOTH HEARD THE SAME THING; OR THOUGHT WE DID; AND NEVER QUERIED THE CTLR BEFORE XING RWY 9L IN SUCH LOW VISIBILITY CONDITIONS; EVEN IF THE RWY WAS CLOSED. WE WERE CONCERNED ABOUT JUST KEEPING OUR ACFT ON THE CTR OF THE TXWY. THE CTLR'S INSTRUCTIONS TO US WERE MISINTERPRETED. THIS CAN BE AVOIDED NEXT TIME BY ALWAYS CONFIRMING RWY XINGS IN LOW VISIBILITY AND WHEN IN DOUBT; DELAY THE FLT UNTIL CONDITIONS IMPROVE.