Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 356 AM EDT Fri Oct 26 2018 Valid 12Z Fri Oct 26 2018 - 12Z Mon Oct 29 2018 Days 1 through 3... New England ----------- Miller Type A cyclogenesis along the East Coast Friday night and into Saturday will result in a nor'easter for the northern Mid-Atlantic region and extending northward into New England. Isentropic lift north of the surface low within the moist conveyor belt is expected to produce widespread light to moderate stratiform precipitation within the developing comma head region. The northern edge of the precipitation shield will likely involve some winter weather across parts of Upstate New York and extending eastward across interior New England by Saturday morning. A cold surface high is expected to reside across southern Quebec and Maine Friday night, and this will likely supply enough cold and dry air for a brief period of accumulating snow from the Catskills and Adirondacks to central Maine, with amounts on the order of 1 to 3 inches expected during the first six hours of precipitation. By Saturday afternoon and evening, warm air advection centered around 850 to 700 mb is expected to lead to some sleet and freezing rain for central and northern New England, and mainly rain for the coastal plain. WPC probabilities continue to indicate a slight risk for significant ice accumulations of a quarter inch across parts of northern New Hampshire and northwestern Maine, with this mainly for elevated interior valley locations. Actual accretion on the order of 0.05 to 0.2 inch is most likely. 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. WPC probabilities are moderate for 4 inches of snow for the higher mountains of the Cascades, and also for parts of northwestern Montana. The probability for significant icing (0.25 inches or more) is less than 10 percent all 3 days outside of New England. D. Hamrick