M wrote:
BTW, FAR 91.144 restricts flight operations when barometric pressure
exceeds 31 inch mercury. Any idea why? (note this restriction applies
to VFR traffic a well).

The "why" is that the altimeter setting range on most altimeters doesn't
go past 31.00". Does anyone have an example of a NOTAM under 91.144?
(how restrictive is it).

The rules that apply in Canada are a bit more explicit:
http://www.tc.gc.ca/CivilAviation/pu...12-1.htm#12-12

Basically it says
- for enroute, set the altimeter to 31"
- for IFR approaches, set the altimeter to the actual altimeter setting
if able, otherwise set it to 31" and adjust the altitude minima (add
100' and 1/4 SM per 0.1" above 31.00")
- aerodromes which cannot report the actual altimeter setting are
restricted to VFR.
-"For aircraft operating VFR, no additional restrictions apply; however,
extra diligence in flight planning and in operating in these conditions
is essential. "