Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 348 AM EDT Tue Oct 16 2018 Valid 12Z Tue Oct 16 2018 - 12Z Fri Oct 19 2018 Day 1... ...Southwest and Four Corners... A closed 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. The combination of lowering heights to produce steepening lapse rates, as well as diffluence within an upper jet streaking around the low will produce widespread mountain snows across New Mexico and Arizona, lifting into Utah late on day 1. Increased snowfall due to convective elements and upslope enhancement is likely across the highest terrain of Arizona and New Mexico above 9000 feet where WPC probabilities are high for 4 inches of snow, and moderate for 8 inches or more. Lower elevations down to 7000 feet have the potential to see just a few inches of snow accumulation. ...Davis, Guadalupe, and Sacramento Mountains... Cold air advecting southward as high pressure builds down into the Plains will undercut warm moist flow aloft around the closed low over Arizona. This produces isentropic lift, and light precipitation will overspread this region. Forecast soundings depict a shallow sub-freezing surface layer remaining trapped much of the day supporting freezing rain accretion. The highest WPC probabilities for greater than 0.10 inches of freezing rain will be across the Sacramento Mountains which will see reinforcement of cool/dry boundary layer air through the day. Boundary layer temperatures will gradually begin to warm across West Texas, so much lighter freezing rain accretion is expected there. Day 2... ...Mountains of the Four Corners... 500mb closed low will drift north from Arizona into Utah this period while the 700mb reflection opens to an elongated N-S oriented trough. Veered 850-700mb flow will transport moisture towards the Four Corners as warm advection commences in conjunction with height rises. This leads to increasing temperatures and rising snow levels so that only the peaks above 8000 feet will see significant snow. The strongest lift due to jet diffluence and vort energy rotating around the mid-level low will will coincide spatially with the best upslope on southerly 700mb winds across southern Colorado. This will drive the heaviest snow amounts into the San Juans, where WPC probabilities for 8 inches of snow are high. Lower amounts are expected elsewhere above 7000 feet, with minimal probabilities for 4 inches. ...Great Lakes into Northern New England... Shortwave energy will rotate through the base of a trough centered around an upper low over Hudson Bay to cross the Great Lakes early Wednesday before exiting New England later in the day. Cold advection on NW flow will top the Great Lakes producing instability, which combined with minimal surface-700mb directional shear will produce the potential for lake effect snow, especially downwind of Lake Superior and Huron which will maximize fetch distance. Low-level flow will be westerly early in the day over Lake Erie and Ontario before the trough axis shifts east, and this will support light snows downwind of these lakes, with some local enhancement possible downwind of Lake Ontario where moist air streaming from Lake Huron may combine with the fetch across Lake Ontario. Temperatures will be marginal so accumulations will be light, and likely confined to the locally higher terrain. Although WPC probabilities are high for more than 1 inch of snow, they remain less than 10 percent for significant snow of 4 inches or more. As the shortwave lifts east across northern New England, lift associated with vorticity advection and diffluence within the left exit region of an approaching jet streak will produce light snows in the high terrain of NH and ME. Snow accumulation is expected to be light, with minimal WPC probabilities for more than 2 inches of snow. ...Sacramento Mountains of New Mexico... Warm and moist mid-level flow around closed low across the Desert Southwest will top persistent cool NE winds as high pressure ridges down through the Plains. Temperatures will slowly warm through the day, but soundings suggest enough cold air near the surface that light freezing rain will persist across the Sacramento Mountains on Wednesday. QPF is light, but several ensemble members show a few hundredths of an inch of additional accretion before precipitation changes to rain. Day 3... ...Mountains of the Four Corners... Slow moving closed low over Utah will open and eject northeast. This drives the best moisture and synoptic forcing to the east across southern Colorado as dry advection occurs from the west. Weak synoptic lift and upslope enhancement into southern Colorado will produce light mountain snows across the San Juans where WPC probabilities show a slight risk for 4 inches of snow. The probability of significant icing (0.25 inches or more) is less than 10 percent. Weiss