Freddie Mac released its weekly update on national mortgage rates this morning, showing rates heading lower with the start of the spring homebuying season.

The mortgage company's March outlook calls for 30-year fixed-rate mortgages to stay below 4% throughout the year.

In the most recent week, 30-year fixed rate mortgages (FRM) dropped back down after last week's spike, falling nine basis points to land at 3.54%. Shorter-term 15-year FRMs tracked closely with them, falling seven basis points to 2.72%. Fifteen-year mortgages are now at their lowest levels since late January.

In contrast, adjustable-rate mortgages showed little or no week-to-week improvement: 5/1 ARMs held steady at 2.61%. One-year ARMs dropped a single basis point, to 2.63%.

Commenting on the numbers, Freddie Mac Vice President and Chief Economist Frank Nothaft attributed the FRM downdraft to "low and stable inflation ... placing downward pressure on fixed mortgage rates." As Nothaft pointed out, "annual growth in the consumer price index has remained at or below 2 percent for the past four months, and for the producer price index even lower. This, in part, is why the Federal Reserve monetary policy committee on March 20th lowered the upper end of its inflation forecast for 2013."

Nothaft said "our March Outlook calls for 30-year fixed mortgage rates to remain below 4 percent throughout this year." As of this week, the 30-year fixed rate has remained below 4% for a year, according to Freddie Mac.

link