It is almost definitely the weather, i live in NH with similiar conditions and when it is below freezing its almost impossible for me to get over 50, but as soon as its in the 40's mine pops back up, trying increasing your tire pressure and if you have been stop starting your car early (really really hurts the mpg)

4/27/08 612.1 miles @ 62.1 MPG with 110,000 miles on the Civic