FLC OF A DC9 TOOK EVASIVE ACTION AND MADE A GAR AFTER A TCASII ALERT AND VISUAL SIGHTING OF A PIPER CHEROKEE PASSING 100 FT BELOW THEIR ACFT ON A 2 MI FINAL.

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ACTUAL INFLT VISIBILITY 1-1.5 MI; EXTREMELY MARGINAL VFR AT BEST. MKE APCH CTL CLRED US FOR THE ILS APCH RWY 19R AND HANDED US OVER TO THE TWR. UPON INITIAL CALL; THE TWR RESPONDED WITH 'VFR TFC ABOUT 3 MI NE OF THE ARPT LNDG RWY 19; CLRED TO LAND RWY 19R.' AT APPROX 1100 FT AGL TCASII TFC APPEARED AT 11 O'CLOCK LESS THAN 1/2 MI MOVING FROM L TO R; 100 FT BELOW OUR ALT WITH A SIMULTANEOUS AURAL TCASII 'TA.' VISUAL CONTACT WAS MADE WITH THE TFC AND EVASIVE ACTION IMMEDIATELY TAKEN IN THE FORM OF A SMOOTH; POSITIVE GAR. NOW THAT WE WERE ESTABLISHED IN THE CLB; WE RECEIVED A 'CLB' RA AS THE PIPER CHEROKEE PASSED BENEATH US BY SEVERAL HUNDRED FEET; STILL ON A WESTERLY HEADING. I SHOULD NOTE THAT AFTER I HAD MADE THE DECISION TO GO AROUND; THE TWR STILL HAD NOT UPDATED US ON THE POS OF THE VFR TFC AND WAS QUERYING THE CHEROKEE TO VERIFY THAT HE HAD VISUAL CONTACT WITH US; THERE WAS NO RESPONSE. WE ADVISED THE TWR THAT WE HAD ALREADY BEGAN A GAR. THE TWR THEN; AFTER THE FACT; ADVISED US TO GO AROUND. CLR OF CONFLICT. I FEEL THAT DUE TO THE RESTR VISIBILITY THE CHEROKEE DID NOT HAVE VISUAL CONTACT WITH RWY 19L; OR US AND EXTENDED ITS BASE LEG THROUGH THE RWY 19R CTRLINE INTERSECTING OUR FLT PATH. THIS HAVING OCCURRED AT LOW ALT INHIBITED OUR TCASII TO A CERTAIN EXTENT; AND THE FACT THAT THE TWR LACKED CONCERN FOR A POTENTIAL CONFLICT; PLT ACTION HAD TO BE TAKEN TO AVOID A NEAR-MISS. HAD WE CONTINUED ON THE ILS FOR SEVERAL MORE SECONDS A COLLISION WITH THIS ACFT WAS IMMINENT WITHOUT EVEN RECEIVING AN RA. CONTRIBUTING FACTORS: 1) THE CHEROKEE PLT FAILED TO PROPERLY NAV HIS ACFT IN REDUCED VISIBILITY. 2) DUE TO FREQ CONGESTION AND ACCOMPLISHING THE 'BEFORE LNDG CHKLIST' WE DID NOT HAVE TIME TO PLACE AN INQUIRY AS TO THE EXACT POS OF THE TFC ONCE WE WERE LESS THAN 3 MI OUT. 3) THE TWR DID NOT KEEP US UPDATED ON THE VFR TFC AND DID NOT PROVIDE ADEQUATE VFR/IFR SEPARATION. 4) OUR TCASII IS DESIGNED TO ELIMINATE RA'S BELOW APPROX 900 FT AGL IN A DSCNT AND 1100 FT IN A CLB; HENCE THE DELAYED RA. 5) THE ATIS; ALTHOUGH RPTING GND VISIBILITY; WAS STILL NOT ACCURATELY REFLECTING ACTUAL EXISTING CONDITIONS. THIS ARPT HAS THE CONSISTENT; TERRIBLE HABIT OF NOT UPDATING ATIS WX WHEN THERE IS A SIGNIFICANT CHANGE; IE; CEILING; VISIBILITY; WINDS OR HVY RAIN ON THE ARPT WHEN THERE IS CONVECTIVE ACTIVITY IN THE AREA. AS A PLT; YOU HAVE TO DRAG THIS INFO OUT OF THEM (IF THERE'S TIME). WHAT YOU SEE MAY NOT BE WHAT YOU GET. A VERY UNSAFE PRACTICE.