There are several ways you can get your eTrex 30 to function more as a weather barometer than an altimeter:
You should then calibrate the altimeter based on known altitudes. This will improve the barometric reading.
- Setup > Altimeter > Plot Type = Barometric Pressure
- Elevation Plot > (menu key) > Change Data Fields > (select one of the fields) > Barometer
There is also Setup > Units > Pressure, in case you want to switch between Millibars, Inches of mercury, or Mm of mercury.
More info on using your barometer / altimeter to predict backcountry weather: