Keyword is "rough guide"...there isn't a definitive set of criteria for determining whether it will snow in the lowlands, although we can arrive at guidelines for a rough 50/50 odds of snow vs rain. While 0c H8 temp (850 mb temp) and 540 thickness are generally the rough values for areas east of the Rockies due to the maritime influence dominant in the PNW those same values (0c and 540) generally equate to a snow level around 3,000 feet in the Cascades and temps closer to 45-50 along the I-5 cor