Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 406 PM EST Fri Dec 13 2024 Valid 00Z Sat Dec 14 2024 - 00Z Tue Dec 17 2024 ...Lake Ontario... Day 1... Lingering single band in westerly flow over Lake Ontario will persist into this evening for the Tug Hill area before diminishing around midnight. A few more inches are likely after 00Z. ...Western U.S... A powerful shortwave trough rounding a low centered west of WA will shift over the CA coast early Saturday before tracking over the northern Rockies through Sunday. The next plume of moisture arrives Sunday night into the PacNW Coast ahead of a weakening low off Vancouver Island. A less potent onshore flow then expands over the Northwest through Monday. California... Day 1... A moderately strong atmospheric river will continue to stream in over far northern California tonight before a potent and digging vort lobe crosses the SF Bay area around 15Z Saturday. This sharp trough will provide an intense focus for lift and orthogonal flow to the Sierra Nevada on Saturday with a quick end Saturday evening behind the trough axis. Snow levels linger around 5000ft over the CA Cascades and Sierra Nevada through tonight before rising perhaps 500ft just ahead of the wave Saturday. Heavy snow through synoptic and upslope flow will extend along the length of the Sierra with high rates persisting longest over the northern half. Per 12Z HREF, hourly rates exceeding 1"/hr resume around 08Z for the northern Sierra where they persist until about 22Z with the southern Sierra seeing heavy rates from about 18Z to 00Z. Max rates exceed 2"/hr for a few hours as the max precip works its way south down the High Sierra 14Z to 22Z. The additional snowfall maxima for Day 1 alone is around 36" in the highest western slopes of the northern and central Sierra. Pacific Northwest through Northern Rockies... Days 1-2... Farther north, the closed mid-level low pushes ENE over Vancouver Island late tonight. Persistent moist advection into the Pacific Northwest occurs through Saturday night as an additional reinforcing shortwave trough swings through on Sunday driving widespread heavy precipitation into the Northern Rockies through Sunday. Snow levels will fluctuate through the period, but generally remain around 4000-5000 ft. However, strong ascent and convection could result in locally lower snow levels during heavier activity. This configuration of back-to-back waves consolidating over the NW focuses the inland heavy precip on central ID terrain. Day 1.5 WPC probabilities for >8" are moderately high (50-80%) along the WA/OR Cascades and the Olympics and high (>80%) over the , Salmon River/Sawtooths in ID along with the Wallowa Mtns in northeast OR. This then expands to western WY down to the Wasatch where Day 3 snow probs for >8" are 50-80%, highest in the Tetons. Expect considerable impacts in the passes here. ...Pacific Northwest... Day 3... The next wave has an initial burst along the Cascades early Monday with snow levels around 2500ft in WA and 3500ft in OR. However, the weakening low and diminishing moisture influx means more moderate rates through the rest of Monday as snow levels slowly rise. Day 3 snow probs for >8" are 30-60% for the Cascades above those snow levels. ...Central Plains into the Upper Midwest... Days 1-2... A trough currently ejecting from the Central Rockies will amplify into a deep low over KS tonight before shifting over northern IN through Sunday. Height falls downstream of this developing low will combine with weak but coupled jet streaks aloft and a residual low-level baroclinic gradient to drive surface cyclogenesis over KS by this evening. As this low progresses ENE, increased Gulf moisture will be drawn north and around the low and over the I-80 corridor around IA which has been under and Arctic airmass the past couple days, resulting in a wintry mix. Despite the fairly strong warm air advection, the plain rain line looks to only modestly work northward through Saturday night as precip continues over the Upper Midwest. It will take some time for the DGZ to saturate as the antecedent air is quite dry, so this setup is better for freezing drizzle turning into freezing rain/sleet, and snow on the northern end. Day 1 ice probs for >0.25" are 20-40% over both western and eastern IA with coverage of >0.1" ice extending from eastern Neb to just beyond the northern and eastern IA borders. ...Central Appalachians... Days 2/3... The same high pressure retreating eastward from the Upper Midwest will drift over New England Saturday through Sunday, allowing a strong Cold Air Damming (CAD) wedge to setup east of the Appalachians over the Mid-Atlantic. Moisture from low pressure moving into the Great Lakes shifts eastward over the central Apps Saturday night. Air will initially be cold enough for snow, especially in the higher elevations, before the robust WAA brings a wintry mix that lingers over terrain and preferred higher central App valleys into or through Sunday night. WPC probabilities for >0.1" of freezing rain are highest 12Z Sun-12Z Mon (Day 2.5) with 70-80% over the Laurels of PA down into the MD Panhandle with >20% probs extending from north-central PA through southern WV. ...Northern Plains... Days 2/3... The potent trough axis crossing SF Bay early Saturday lifts ENE over the northern Rockies early Sunday with further development over ND on Sunday. Overrunning flow ahead of the wave brings a light freezing rain threat to northeastern MT Saturday night into Sunday morning with the Day 2 ice probs for >0.1" 20-60%. Snow bands then develop north of the low with Day 3 snow probs for >4" around 20% for the north-central border of ND. Jackson