Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 347 AM EDT Sat Oct 27 2018 Valid 12Z Sat Oct 27 2018 - 12Z Tue Oct 30 2018 Days 1 through 3... ...New England and Upstate New York... The developing nor'easter near the Mid-Atlantic/southern New England coast will result in widespread precipitation advancing northward across Upstate New York and interior New England throughout the day on Saturday, as a result of isentropic lift north of the occluding surface low and left exit region dynamics from the upper level jet. With a cold surface high initially in place across the northeast U.S., temperatures will be marginally cold enough to support a brief period of snow for the Catskills, Adirondacks, and extending eastward to include the higher elevations of central and northern New England, mainly above 1500 feet. Snowfall rates should be high enough within the first six hours of precipitation to support some 2 to 4 inch amounts, with isolated totals in excess of 4 inches. By later in the day Saturday, strong warm air advection in the 850-700 mb layer is expected to result in a transition to mixed precipitation and then light freezing rain, particularly for interior valley locations. Ice accretion on the order of one to two tenths of an inch is likely from northern Vermont to northwest Maine where the subfreezing surface-925 mb layer will persist the longest. WPC probabilities indicate a low probability of 0.25 inch of ice accretion for day 1, along with a small moderate probability area over northwest Maine. Locations farther south should have a brief period of freezing rain before a change to all rain commences Saturday night. ...Cascades and Northern Rockies... A strong shortwave tracking towards the Pacific Northwest on Sunday morning will lead to height falls and strong onshore flow, along with a strong upper level jet. Moisture surging northward ahead of the impulse will be lifted due to positive vorticity advection and left exit dynamics from the upper level jet. Steep lapse rates follow with the passage of the cold front, with snow levels falling to around 5000 feet, and some upslope enhancement is likely across the Washington and Oregon Cascades. As the moisture plume reaches the northern Rockies, several inches of snow is likely from the Absaroka Mountains to the Bitterroots with decent lift within the dendritic growth zone. WPC probabilities are moderate for 4 inches of snow for the higher mountains of the Cascades, and above 7000 feet for the northern Rockies. The probability for significant icing (0.25 inches or more) is less than 10 percent all 3 days outside of New England. D. Hamrick