Fixed and adjustable mortgage rates hit their lows of 2010 in the week ending May 20, according to Freddie Mac's weekly survey. 

The 30-year fixed rates averaged 4.84%, its lowest level since mid-December, down nine basis points from a week ago. 

Meanwhile, the 15-year fixed rates fell six basis points to 4.24%, which the GSE said "has not been lower since Freddie Mac started tracking the 15-year [fixed-rate mortgage] in August of 1991."

On the adjustable side, 5/1 hybrid ARMs is at a record low of 3.91%, down four basis points from last week. The GSE began tracking the rate in January 2005.  Finally, one-year ARM rates slipped two basis points to 4% — its lowest since late October 2004.

While refinancing activity should be stimulated by the improved rates, Credit Suisse analysts said 4.50% and 4.25% mortgage rates are the "key thresholds for prepay and supply spikes in a full-blown flight-to-quality rally scenario taking 10s below 3.00%."

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