"Gary Drescher" wrote

Yup, legality and safety are not synonymous. Still, I think it would be
safe
to fly IFR through a thin cloud layer (with plenty of room above and
below)
even if there's a forecast for occasional moderate icing in clouds. And
according to the AIM's current definition of "known icing conditions",
that
would be legal (for Part 91), as long as there are no PIREPs that confirm
the forecast.

Section 91.527: Operating in icing conditions.

(b) Except for an airplane that has ice protection provisions that meet the
requirements in section 34 of Special Federal Aviation Regulation No. 23, or
those for transport category airplane type certification, no pilot may fly—

(1) Under IFR into known or forecast moderate icing conditions; or

(2) Under VFR into known light or moderate icing conditions unless the
aircraft has functioning de-icing or anti-icing equipment protecting each
propeller, windshield, wing, stabilizing or control surface, and each
airspeed, altimeter, rate of climb, or flight attitude instrument system.

(c) Except for an airplane that has ice protection provisions that meet the
requirements in section 34 of Special Federal Aviation Regulation No. 23, or
those for transport category airplane type certification, no pilot may fly
an airplane into known or forecast severe icing conditions.

(d) If current weather reports and briefing information relied upon by the
pilot in command indicate that the forecast icing conditions that would
otherwise prohibit the flight will not be encountered during the flight
because of changed weather conditions since the forecast, the restrictions
in paragraphs (b) and (c) of this section based on forecast conditions do
not apply.

It appears that for purposes of an IFR flight the rules do not rely on any
definition of "known" versus "forecast" - they're both covered right in the
reg. Paragraph (d) appears to allow a pirep of no icing to supercede the
forcast.