Go to http://www.wunderground.com/ and look at the top menu bar. There is one called "More" that has a submenu of "Historical Weather". You can then input your city and date of interest. To see a month at a time, click on the custom tab, and input the date range of interest. You can search weather data all the way back into the 1940s. At the bottom of the page, there is a link to output a comma delimited file, that can be imported into a spreadsheet for analysis.

For what it is worth, the home page now has a 10 day forecast graph that includes a forecast of the barometric pressure change for your location.