INSTRUCTOR PLT OF A C172 OF A DUAL TRAINING FLEW INTO IMC WX CONDITIONS DURING RETURN TO HOME BASE. THE FLT VISIBILITY WAS REDUCED BY A SNOW SHOWER WHICH TOOK THE RPTR BY SURPRISE. HE DID NOT DO A 180 DEG TURN BACK TO VFR WX SINCE THERE WAS A RPTED B727 BEHIND HIM MAKING AN APCH TO THE ADJACENT PARALLEL RWY.

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WHILE CONDUCTING A PRIMARY DUAL INSTRUCTIONAL FLT UNDER VFR AS A CFI; I FLEW INTO AN AREA OF IMC WHILE LNDG AT A TWR-CTLED ARPT. THE ATIS AT BOEING FIELD INDICATED 5 MI VISIBILITY WITH A CEILING OF AT LEAST 2000 FT. HOWEVER; FROM 6 MI TO THE S AS I RADIOED THE TWR PRIOR TO ENTERING CLASS D AIRSPACE; THE FIELD WAS NOT YET VISIBLE. THE FIELD IS LOCATED IN A VALLEY AND WAS OBSCURED BY WHAT APPEARED TO BE MIST. FLT VISIBILITY IN ALL DIRECTIONS AROUND ME WAS APPROX 5 MI; BUT VISIBILITY N TOWARDS THE FIELD APPEARED TO BE AROUND 3 MI. I WAS TOLD BY THE TWR TO MAKE A STRAIGHT-IN APCH TO RWY 31R. I ALSO HEARD A BOEING JET ON THE ILS BACK COURSE APCH BEING CLRED TO LAND ON RWY 31L. I DID NOT KNOW HOW FAR BEHIND ME THE JET WAS WHEN IT WAS CLRED TO LAND. CLOSER TO THE ARPT; I CALLED FOR AND RECEIVED CLRNC TO LAND ON RWY 31R. AS I PROCEEDED TOWARDS THE RWY; WHICH I STILL COULD NOT SEE; VISIBILITY DETERIORATED TO ABOUT 2 MI IN BLOWING SNOW. I WAS ALREADY AT PATTERN ALT (1000 FT). I FOLLOWED A HWY (I-5) THAT I KNEW FROM EXPERIENCE WOULD LEAD ME DIRECTLY TO THE RWY. IT DID; AND I WAS EASILY ABLE TO INTERCEPT THE VASI AT PATTERN ALT AND LAND WITHOUT INCIDENT. I TUNED IN THE ATIS WHILE TAXIING. A 'SPECIAL OBSERVATION' INDICATED THAT THE FIELD WAS IFR WITH VISIBILITY OF 2 MI IN SNOW. THE TIME GIVEN FOR THE 'SPECIAL OBSERVATION' WAS SEVERAL MINS BEFORE I HAD LANDED. APPARENTLY A FAST- MOVING; LOCALIZED; UNFORECAST SNOW SQUALL (IN SEATTLE?!) HAD MOVED IN FROM LAKE WASHINGTON AND OBSCURED ONLY BOEING FIELD. NEARBY RENTON AND SEA-TAC REMAINED VFR. ANALYSIS: MY DECISION TO CONTINUE VFR IN CLASS D AIRSPACE TOWARDS A RWY I COULD NOT SEE WAS CLRLY AN ERROR. WHEN I REQUESTED AND WAS GRANTED CLRNC TO LAND FROM 3 MI AWAY; I STILL COULD NOT SEE THE RWY. I KNEW I WAS MAKING A DECISION; AND I KNEW THAT IT WAS PROBABLY THE WRONG ONE. HOWEVER; SOMEWHAT INCREDIBLY; IN THE HEAT OF THE MOMENT; I DID NOT IMMEDIATELY THINK OF WHAT THE RIGHT DECISION MIGHT BE; SO I BLUNDERED FORWARD. WHAT I SHOULD HAVE DONE: ON MY INITIAL CALL-UP FROM 6 MI AWAY; I SHOULD HAVE INDICATED TO THE TWR THAT THE FIELD DID NOT APPEAR TO BE VFR; AND TO PLEASE ADVISE. AT THAT TIME; I COULD EASILY HAVE REQUESTED A SPECIAL VFR. CERTAINLY UPON REQUESTING MY LNDG CLRNC; I SHOULD HAVE TOLD THE TWR THAT THE FIELD DID NOT APPEAR TO BE VFR. BY THAT POINT; ALREADY IN CLASS D; I COULD NOT HAVE OBTAINED A SPECIAL VFR CLRNC; BUT SOME SORT OF ARRANGEMENTS COULD HAVE BEEN MADE. IN ANY EVENT; I SHOULD HAVE EXECUTED A 180 DEG TURN UPON ENTERING IMC; SINCE I KNEW THAT VFR CONDITIONS EXISTED BEHIND ME; AND I DID NOT KNOW HOW BAD THE VISIBILITY WAS IN FRONT OF ME. IN CONDITIONS OF BLOWING SNOW; IT COULD EASILY HAVE DETERIORATED TO 1 MI OR LESS. IN GENERAL; MORE COM EARLIER ON WITH THE TWR WOULD HAVE PREVENTED ME FROM GETTING INTO THIS SIT. REASONS I DID NOT MAKE THE PROPER DECISIONS: THERE WAS NO PUSH-TO-TALK BUTTON ON THE R-HAND YOKE; FORCING ME TO USE THE HAND-HELD MIKE. THE MIKE HAD FALLEN TO THE FLOOR IN FRONT OF ME; AND; AS VISIBILITY DETERIORATED; I WAS CONCENTRATING ON MY FLYING AND HESITATED TO REACH DOWN AND PICK IT UP. MOREOVER; THE TWR HAD TOLD ME THAT THEY HAD DIFFICULTY HEARING MY XMISSIONS FROM THE MIKE (AND FROM THE CRUMMY ACFT RADIOS). I HESITATED TO EXECUTE A 180 DEG TURN OUT OF IMC BECAUSE I DID NOT KNOW THE POS OF THE BOEING JET BEHIND ME. IT MIGHT HAVE BEEN RIGHT ON MY TAIL; AND A BLIND 180 DEG TURN TOWARDS IT COULD HAVE BEEN DISASTROUS. MY STUDENT HAD TOLD ME THAT SHE NEEDED TO BE ON THE GND BY XA00 PM; AND TIME WAS RUNNING SHORT. AS A FAIRLY NEW CFI WITH A FAIRLY NEW STUDENT; I DID NOT WANT TO APPEAR THAT I WAS UNSURE OF WHAT TO DO. (HOWEVER; IN PREVIOUS SITS; THIS HAD NEVER BEEN A PROB FOR ME.) AFTER THIS FLT; MY STUDENT LEFT QUICKLY TO GET TO AN APPOINTMENT. BEFORE OUR LESSON THE NEXT DAY; HOWEVER; I GAVE HER A LENGTHY AND CANDID EVALUATION OF THE ERRORS IN JUDGEMENT I HAD MADE; AND WE HAD A VERY PRODUCTIVE DISCUSSION OF HUMAN FACTORS AND AVIATION DECISION-MAKING IN GENERAL.