Your TPMS (tire pressure monitoring system) may light up in the morning when its colder and go out as you drive, or if cold enough, may stay on all day. Typically they are set to light/warn you when the pressure is 25% higher or lower than the manufacturer recommended pressure (listed on the driver door jam). Having an under or overinflated tire can cause premature wear, higher risk for a blowout and lower your gas milage.
Why the change in tire pressure? The cold weather causes the pressure to drop from 1-2 psi per 10f drop in temperature. So going from 70f to 30f could cause the pressure to drop upwards of 8psi. As always we will check and inflate the tires to correct pressure, so stop up anytime