A Freeze Warning will be in effect from this evening until Friday morning for most of Northern Arizona.  Cities and towns in the warning area include: the Greater Flagstaff area, Williams, the Navajo and Hopi nations, Winslow, Holbrook, the Grand Canyon and the White Mountains.  Forecasters say temperatures could drop into the teens in some places, which would be uncharacteristic for this time of year.  Frost and freeze conditions will kill crops, other sensitive vegetation and possibly damage unprotected outdoor plumbing.  Although some of these areas have already seen temperatures below freezing, this is the first widespread event of the Fall.  Officials say to take steps now to protect tender plants from the cold.  To prevent freezing and possible bursting of outdoor water pipes they should be wrapped, drained, or allowed to drip slowly. Those that have in-ground sprinkler systems should drain them and cover above-ground pipes to protect them from freezing.