Weather patterns have a huge impact on retail sales. By using weather data to analyze the impact and correlation to sales, you can identify the most important weather parameters and trigger points for your business. A useful source of weather data is Windows Azure Marketplace: https://datamarket.azure.com/
It has several weather data sources available, including data from Weather Trends International, UK Open Data Program, Weather Services International … This data can be easily consumed using Microsoft Powerpivot for Excel. Here is an article by Chris Webb on Importing UK Weather Data from Azure Marketplace into PowerPivot. Once the weather data is imported into Excel, you can then import your sales data into excel and build a cube with the dates linked.
Here are some recent tweets via @MSRetail wrt Weather Patterns and Retail:
Weather Patterns & #Retail: People don't buy based on the calendar, they buy based on how they feel http://yhoo.it/Z3viZS
Weather Patterns & #Retail: Prolonged Winter Puts Sales in Deep Freeze http://bit.ly/Z3uhkg
Also very interesting around storm planning and retail is the Home Depot story that featured on Fortune Magazine in 2010: http://money.cnn.com/galleries/2010/fortune/1008/gallery.home_depot_hurricane.fortune/index.html