According to http://www.somervillema.gov/snow :
During snow emergencies, parking is allowed on the odd-numbered side of the street only (unless otherwise posted). Cars not moved from the even-numbered side of the street by 11:30 p.m. on Saturday evening may be subject to ticketing and towing.
Residents unable to find a parking space on the odd-numbered side of the street may park in any municipal lot or in designated school parking lots.
Updated information can always be found on this page and in the following locations:
Twitter: @SomervilleCity www.twitter.com/SomervilleCity
Twitter: @311Somerville www.twitter.com/311Somerville