Been shopping for a mortgage rate? You may want to lock something down. Tomorrow morning, mortgage rates are expected to change. Unfortunately, we don’t know in which direction they’ll move.

It’s a risky time for home buyers to be without a locked mortgage rate.

The action begins at 8:30 A.M. ET Friday. This is when the government’s Bureau of Labor Statistics releases its April Non-Farm Payrolls report.

The monthly Non-Farm Payrolls report is more commonly known as “the jobs report” and provides a sector-by-sector breakdown of the U.S. employment situation, including changes in the Unemployment Rate.

In March 2012, the government reported 120,000 net new jobs created — half the number created during the month prior, and the third straight month of declining job creation. The Unemployment Rate fell one-tenth of one percent to 8.2%.

For April, economists expect to see 160,000 net new jobs created, and no change in the national Unemployment Rate.

Based on the accuracy of those predictions, mortgage rates are subject to change. If the actual number of jobs created in April exceeds economist expectations, mortgage rates should rise. Conversely, if the actual number of jobs created falls short, mortgage rates should drop.

Job growth’s link to mortgage rates is straight-forward. Jobs are an economic growth engine and mortgage rates are based economic expectation. Therefore, as the number of people entering the U.S. workforce increases, so do Wall Street’s growth projections for the economy. When that happens — especially in a recovering economy such as this one — mortgage rates tend to rise.

So, for today’s rate shoppers, Friday’s job report represents a risk. The economy has created jobs for 18 straight months, a winning streak that has added 2.9 million people to the U.S. workforce. If that winning streak continues and expectations are beat, mortgage rates are likely to rise off their all-time lows, harming home affordability.