Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Mon Oct 15 2018 Valid 00Z Tue Oct 16 2018 - 00Z Fri Oct 19 2018 Days 1 through 3... ...Southwest/Rockies/Great Basin... Short wave energy ejected from a closing mid level low over the Southwest state interacts with moisture to produce heavy snowfall over portions of AZ during Day 1, and southwest CO on Day 2. Colder air over west TX and southeast NM leads to ice accumulations during Day 1 and 2. The closed low evolves into a positively long wave trough on Day 3, as snowfall is relegated to the higher terrain of the Rockies. Since there was good model agreement with the synoptic scale systems, the thermal portion of the forecast was based on a multi model blend. The QPF portion of the forecast was based on the most recent WPC QPF. Day 1... Colder air driving south into southeast NM and far west TX undercuts warmer air aloft, resulting in a broad area of overrunning precipitation. Model soundings across this area showed the shallow cold air remaining wedged against the terrain (generally less than 200 meters deep), resulting in an area of freezing rain. Boundary layer temperatures rise only slowly after the wet bulb temperature is reached, so a large area of 0.01 to 0.10 inches of icing was placed over southeast NM and much of west TX (south to the Rio Grande River portion of the Big Bend). The 12z NAM/09z SREF showed pockets of 0.10 to 0.25 inches of icing over west TX (north of KMAF), but it is possible these solutions may be too cold, producing icing amounts that could be too high. In any event, the threat for icing is expected through 11/12z, after which time the threat wanes as boundary layer temperatures warm, especially over west TX. Further west, a closing mid level low over southern CA/southwest AZ ejects short wave energy over AZ/NM that interacts with moisture returning to the region around surface high pressure over the Southern Plains. Lowering heights produce steepening lapse rates, which supports an area of stratiform precipitation with embedded convective elements, focused on the terrain. Lowering heights also results in lowering slow levels, which drop to between 7000/8000 feet across the Mogollon Rim. As the snow levels fall, rain changes to snow above these levels, and there is a multi model signal (led by the 12z HREF mean) for a stripe of 4 to 8 inches over the higher terrain. These amounts are supported by a few members of the 00z ECMWF ensemble output showing 8+ inch snowfall amounts here. Day 2... The closed mid level tracks from central AZ into southwest UT, as height slowly rise. The low level southwest flow (around the surface high over the Southern Plains) becomes upslope across southeast UT and southwest CO. The best lift associated with the closed mid low level coincides with the best upslope over the southwest CO to result in an area of 8 to 12 inches of snowfall over the San Juan Range in southwest CO. These amounts are well supported by QPF amounts from the 12z NAM/GFS (though the 12z GFS appears to place the QPF outside of the area of best upslope), as well as a few members of the 00z ECMWF ensemble output showing 8+ inch snowfall amounts here. Further east, the cold air remains locked in place across southeast NM as the surface high over the southern Plains continue to supply shallow Arctic air to the region. Model soundings once again support icing, with profiles showing near freezing air less than 200 meters deep. There is a multi model signal (including the 12z NBM) indicating local 0.10 inch icing amounts south central NM. Across west TX, the low level cold air erodes, allowing the freezing rain threat to diminish during Day 2. Day 3... The closed mid level low evolves into a positively tilted long wave trough extending from WY southwest into AZ. Heights slowly rise as the evolution occurs, which allows snow levels to rise as well. This should relegate 2 to 4 inch snowfall amounts to the higher terrain of the San Juan Mountains in southwest CO and the eastern Wasatch Mountains in UT. The probability of significant icing (0.25 inches or more) is less than 10 percent on Days 2 and 3. ...Great Lakes/New England... Short waves moving through a deepening and sharpening long wave trough over the Great Lakes and New England during Day 1 through 3 will provide synoptic scale ascent for some lake effect snowfall across the Great Lakes and Northern New England on Days 2 and 3. There was very good model agreement concerning the overall synoptic pattern, so the thermal portion of the forecast was based on a multi model blend. The QPF portion of the forecast was based on the most recent WPC QPF. Day 2... As the long wave trough strengthens over the Great Lakes on Day 2, short wave energy tracking through the trough provides synoptic scale lift over the Upper Great Lakes. The column is fairly dry, resulting in little in the way of synoptic scale snowfall. However, model soundings showed the inversion deepening, reaching between 800 and 775 mb before 18/00z. The boundary layer northwest is favorable for lake effect precipitation across Lake Superior, and as low level lapse rates steepen, lake effect bands are expected to impact the UP of MI. Snow levels eventually drop below 300 meters, but this only allows the higher terrain to receive an inch or two of lake effect snow during Day 2. Further east across the lower Great Lakes, the column is generally too warm for snow, even as the lake effect circulations develop. For now, an inch or two of snow was placed over the higher terrain east of Lake Ontario for Day 2. Day 3... The main focus for lake effect shifts to the lower Great Lakes, but as was the case on Day 2, the column remains too warm for snowfall, except across the higher terrain east and southeast of Lake Ontario. Further east, as colder air filters into the higher terrain of western and northern ME, snowfall amounts of 1 to 3 inches of snowfall is possible, with the highest amounts over the higher terrain of far western ME. The probability of significant icing (0.25 inches or more) is less than 10 percent. Hayes