AN EMS HELI FLC FINDS THEMSELVES OVER A RAPIDLY THICKENING UNDERCAST IN AN ACFT THIS IS NOT; APPARENTLY; CERTIFIED FOR IFR. THEY HAVE TO DECLARE AN EMER TO MAKE AN ILS APCH TO THEIR HOME BASE.

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DEPARTED THE HOSPITAL IN TOWN AT XA33 LCL IN JAN 1997. THE WX WAS THIN SCATTERED WITH ABOUT 6 MI VISIBILITY AND MY BASE WAS RPTING CLR AND 5 MI VISIBILITY; TEMP 16 DEGS C; DEWPOINT 15 DEGS C; AND WIND 200 DEGS AT 10 KTS. MY INITIAL PLAN WAS TO STAY BELOW THE SCATTERED LAYER WHICH WAS ABOUT 1500 FT MSL AND RETURN TO MY BASE. I EXPECTED THE WX TO IMPROVE AS WE FLEW E. IT DID NOT; SO I TURNED S TO FOLLOW LOWER TERRAIN OUT OF THE TOWN'S AREA. I BEGAN TO ENCOUNTER THIN SCATTERED CLOUDS AT 1200 FT MSL. WITH THE WX AT MY BASE STILL RPTING CLR; I ELECTED TO CLB THROUGH THE THIN SCATTERED LAYER OF CLOUDS RATHER THAN CONTINUE LOW LEVEL. THE SCATTERED LAYER BASES WERE AT 1500 FT AND TOPS AT 1800 FT. I CLBED TO 3500 FT MSL AND FLEW E TOWARDS MY BASE AND CONTINUED TO CHK ON THE WX BELOW. IT WAS THIN SCATTERED WITH LIGHTS VISIBLE THROUGH IT. AT ABOUT 22 NM (9 MINS) OUT; THE LEVEL BELOW TURNED SOLID TO BROKEN. I LOOKED BEHIND ME AND THAT WAS SOLID; TOO. I CALLED APCH AND THE TWR SAID MY BASE WAS STILL CLR. AT 8 MI OUT I ASKED FOR THE WX AGAIN BECAUSE I COULD ONLY SEE A GLOW AND OCCASIONALLY SOME GND LIGHTS. THE TWR RPTED THE WX AS STILL CLR THEN STOPPED AND SAID THAT WX WAS MOVING IN FROM THE W RAPIDLY. THE WX THEN WENT TO 700 FT BROKEN AND 5 MI IN MIST. I CONTINUED TO OCCASIONALLY SEE GND LIGHTS THROUGH THE LAYER. I WAS CONCERNED THE WX WOULD DETERIORATE FURTHER. I DECLARED AN EMER AND WAS CLRED FOR AN ILS TO RWY 4R. AFTER REQUESTING APCH INFO; I ASKED TO BE VECTORED BACK OUT TO REINTERCEPT THE LOC DUE TO HIGH GND SPD AND NOT COMFORTABLE WITH THE APCH. I ENTERED THE CLOUD LAYER AT ABOUT 1800 FT AND BROKE OUT AT ABOUT 800 FT MSL. THE CABIN CREW RPTED GND LIGHTS VISIBLE THROUGHOUT THE DSCNT. I STAYED ON THE INSTS IN ORDER TO NOT BECOME DISORIENTED OR GET VERTIGO. IN RETROSPECT; I SHOULD HAVE STAYED BELOW THE CLOUD LAYER AND CONTINUED TO MY BASE; EVEN THOUGH THE RPTED AND FORECASTED WX THERE INDICATED THE CLB WOULD BE A BETTER COURSE OF ACTION.