This amazing feat of crystal ball gazing is based as much on barcodes as barometers; the Meteorological Office has teamed up with another forecaster, market research firm Taylor Nelson AGB, to predict what shoppers will buy under different weather conditions.
This odd form of planning for a rainy day was initiated by the Met Office's Weather Initiative business unit in 1990, but will take a dramatic leap forward this week with the addition of data from Taylor Nelson's "super panel".
The panel gathers information from 10,000 homes equipped with barcode readers that scan all newly purchased food and household goods.
In the past, said David White, Taylor Nelson's commercial director, the Met office has had data about general categories of goods - meat, for example. "Now they'll be able to tell how many pork chops were sold."
While ice creams and sun tan lotion sales are obviously dependent on the weather, so too are more unlikely goods. Britons buy more loaves of bread when the weather is bad, for example, and more buns when it is balmy. The degree of correlation is also important. Beer sales traditionally go up by 10 per cent for every three-degree rise in temperature.
Detailed formulae such as this, combined with accurate forecasts, can make a huge difference to companies. One high street retailer made an extra pounds 200,000 by delaying an autumn sale for a week after the Met Office predicted an unseasonable warm spell.
Another company, which makes liquid petroleum gas, saved pounds 500,000 by slashing its stocks ahead of a prolonged period of good weather.
The push into market research is an important part of the Met Office's drive to become financially independent of the Ministry of Defence. Its first full year as a "trading fund" will be completed at the end of this month, and Met Office secretary Martyn Bittleston says it is on target to meet its projections, although it is keeping those forecasts to itself. Its budget is pounds 150m, but it also has to pay off a loan from the MoD for "tens of millions", said Mr Bittleston.