The problem is the elusive nature of icing.
You may fly through an area of forcast icing without picking up
anything. Another aircraft ten minutes ahead or ten minutes behind you
on the same airway and altitude could pick up a bunch.
Another example was here in Central Ohio last Thursday evening.
A friend and I were going to go up practice approaches until we called
for a briefing. An approaching cold front brought with it a forcast for
icing from 4000 to 8000 feet. That wasn't so bad except that the
ceilings were 1500 to 3000 feet throughout the area. Then there was the
Springfield report of 500 feet overcast and light rain with no precip
showing on the radar.
We elected not to fly.

Paul Tomblin wrote:
I cancelled a flight yesterday because on top of strong gusty winds there
was an Airmet Zulu for light to moderate mixed and rime ice, and on top of
that the destination was reporting layers at about 2,000 and 4,000 feet, a
freezing level of about 3,000 feet, with occassional ceilings of 800 feet
and rain. It seemed to me that I could probably fly between or above the
layers en-route, but I was worried about the possibility of having to
descend through two layers of wet (and possibly icy) clouds and maybe have
to do an approach to minimums in very gusty winds.

I know I did the right thing based on my low level of experience, but any
ice tips from the experts, especially up here in the Great Lakes area.