Just checked my pressures, reading 30 all around. This is with ambient temp at ~28F, and after sitting 1.5 hrs post-commute.
Would the low ambient temp cause them to read lower?
The tire label specifies certain max pressures cold, but what is cold?

 

Your tire pressure will go up, or down 1 psi for every 10F ambient change. That being said, you should check and change your tire pressure as the weather changes. Keeping in mind that the pressure will go up 3-4 psi as the tires warm up while driving. Depending on your driving style, of course.

 

i check my tires early morning or late evening giving 4-5hrs cool down time
before i measure. if your car's parked outdoors measure it when the
sun is not out as direct sunlight can alter the tire pressure by 2-3psi
on one side of the car.

i like to run 33psi on my civic and legacy up to +3/-2 psi.
they recommend 30 and 31 psi cold, but i know they go for comfort
more than performance so i use the higher end.

on my MCS which is garaged during the week i check tire pressure
Friday night so that im ready to go on Sat morning. i usually run
about 34 +1/-0 psi. unfortunately due to snow, my MCS will not
be driven for some time...

 

Since you really dont know what the temps are inside the tire are, stone cold in the morning would be the most consistant.