Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 337 AM EST Wed Dec 12 2018 Valid 12Z Wed Dec 12 2018 - 12Z Sat Dec 15 2018 ...Pacific Northwest through the Central Rockies... Days 1-3... Two distinct shortwaves, the first today, and the second on Friday, will bring heavy mountain snows to the Mountain West this period. The first impulse will push inland from the Washington coast today along with a surface cold front and a plume of significant moisture. Westerly 850-700mb wind combined with jet level diffluence will drive ascent into the Washington Cascades, with subsequent moisture spillover into the other west mountain ranges being squeezed out as snow through height falls and easterly advecting jet diffluence. WPC probabilities are high for 8 inches of snow in the Washington Cascades, with high probabilities for 4 inches in the Bitterroots, Tetons, and Colorado Rockies. The highest snow totals will be confined above 6000 feet. After a brief lull, the second significant shortwave will move into Washington state on Friday. Another push of Pacific moisture will saturate the column across Washington and Oregon, with pronounced lift within the Pacific jet producing snow across the Cascades of Oregon and Washington, as well as the northern Rockies into Montana. The trough axis will not move onshore until late on day 3, so warm advection ahead of this feature will raise snow levels back to 5-6 kft before crashing Friday night into Saturday. This will keep the heaviest snow above these levels, and WPC probabilities show a high risk for 4 inches of snow only in the highest terrain of the Cascades. ...Southern Plains... Day 2... A northern stream shortwave trough will dig from the Pacific Northwest while deepening into the Southern Plains Thursday morning. This feature is likely to close off as it becomes anomalously deep, as evidenced by forecast 500mb heights of -4 standard deviations below climo. Beneath this feature, a surface low pressure will move east across Texas, with the system becoming vertically stacked late on Day 2. The guidance continues to insist that an area of heavy snow will develop west of the upper low, but timing and placement details persist a lower than average confidence forecast. Although column temperatures will be marginal, rain will likely transition to snow as an area of 850-600mb frontogenesis coincident with a well defined TROWAL produces strong ascent, aided by theta-e lapse rates dropping below 0 in the vicinity of the saturated DGZ. Despite cold advection at 850mb, warm advection within the TROWAL wrapping cyclonically around the upper low will limit lapse rate steepening, important since dynamic cooling due to strong snowfall rates will be required to produce significant accumulations. Still, there is good model consensus among the NAM/GFS/ECMWF and their ensembles that more than 4 inches of snow will occur in a north-south oriented stripe across north-central Texas. WPC probabilities echo this despite likely low SLRs, and a few places may exceed 6 inches if the frontogenetical band becomes intense enough. The probability of significant icing (amounts of 0.25 inch or more) is less than 10 percent. Weiss