An EMS helicopter pilot; attempting to salvage a pick up and delivery thwarted by low clouds and visibility; entered IMC inadvertently. Declared an emergency and obtained assistance from ATC until back in VMC.

Narrative

Our EMS arrival; from West to East; was uneventful; except that I noted the weather conditions appeared to be deteriorating somewhat. Still; I was able to maintain an absolute altitude of 800-1000 FT until about a half mile short of the hospital; when I crossed a ridge at 400-500 FT AGL. Ridge tops were visible everywhere except to the East. Visibility was 5-7 miles throughout the event. After occupying the hospital helipad for about 15 minutes; I checked outside and became concerned about lowering conditions. I checked ASOS for a nearby airport by telephone and it reported 1/4 SM; 001 OVC. Some of the ridge tops had also become obscured. We elected to continue the transport by ground and the medical crew departed via an ambulance for the receiving facility. An hour later; I observed clearing to the South with visibility 7 SM underneath and decided to attempt departure. Upon departing the pad; I climbed to 700 FT and observed the valley; 10 miles West; in clear air and turned in that direction. Attempting to cross a ridge; I entered IMC and lost sight of all ground references. I immediately entered a climb; contacted Approach control and declared an emergency. I told them I was inadvertent IMC and advised my position. Approach provided a squawk and gave me a vector toward a close airport at 4000 MSL. No icing was observed; although I did turn on pitot heat and engine anti-ice. 2-3 minutes later I emerged into clear air; canceled IFR and proceeded to the local hospital to pick up the medical crew who had diverted there en route to the Medical Center due to the patient's condition. Approach did not request a report or ask me to call them.