Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 349 AM EST Thu Feb 17 2022 Valid 12Z Thu Feb 17 2022 - 12Z Sun Feb 20 2022 ...Southern Rockies... Day 1... A filling mid-level low across the Four Corners will gradually weaken today while ejecting eastward into the Southern Plains this aftn. This wave will interact with a low-level baroclinic boundary as a surface front continues to drag southward across NM, and the overlap of these features will lead to modest waves of low pressure moving eastward today. Mid-level height falls, broad divergence, and low-level upslope flow should produce locally enhanced snowfall in the terrain of the Sangre De Cristos and southern San Juans, which will otherwise be light. WPC probabilities are moderate for 4 inches only in the higher terrain. ...South-central Plains to the Northeast... Days 1-2... A shortwave ejecting from the Four Corners region will become absorbed into the westerlies and shear into confluent mid-level flow towards the Great Lakes today and tonight. This feature will interact with a northern stream shortwave digging out of the Northern Plains to drive an amplified but positively tilted longwave trough across the area, downstream of which an upper level jet streak will intensify leaving increasing RRQ diffluence over the Plains. The overlap of height falls and this upper diffluence will lead to surface pressure falls, and a wave of low pressure is progged to lift rapidly northeast from Oklahoma through the Ohio Valley and then into Maine by Friday morning. This low will move along a southeastward advancing cold front, and it is this front which will help cause precip to change from rain, to a mix, to snow, in a swath from Oklahoma through the Great Lakes and into Northern New England. The guidance has shifted a bit further SE this morning, reflective of a subtly more suppressed and faster moving wave due to less northern stream interaction. However, while synoptic ascent will in general be modest, and the wave is likely to be progressive, intense mid-level fgen and a stretched deformation axis should help produce heavy precipitation rates, at least for a short duration. On the NW side of this area of precip, snowfall rates are likely to exceed 2"/hr as noted by HREF probabilities and the WPC prototype snowband tool as CSI looks favorable through folded theta-e surfaces noted in cross sections on the edge of a blossoming TROWAL. This band of heavy snow rates should translate rapidly northeast, limiting total accumulations. However, WPC probabilities are high for 6 inches from near Wichita, KS through far northern IN (just south of Chicago) into the far northern Adirondacks of New York and far northern Maine. Locally, 12 inches is possible in a few locations as noted by the WSE 90th percentile, but the speed of the system should limit the coverage of these higher amounts. To the south of the heavy snow, a stripe of freezing rain is expected as a warm nose aloft overruns subfreezing temperatures at the surface. While widespread heavy ice accumulations are not expected, WPC probabilities do indicate significant potential for localized accumulations of 0.10 inch or more from portions of the mid Mississippi Valley to the lower Great Lakes region, and northeast into the St. Lawrence Valley and Champlain Valley of NY/VT. Where freezing rain occurs, there is some concern that a rapid deepening of the cold nose could promote more sleet than freezing rain, but even where freezing rain does occur it may not accrete efficiently due to heavy rates, so freezing rain exceeding 0.25" is not likely. ...Northern Plains through the Great Lakes... Days 2-3... A potent shortwave will dive quickly out of Alberta on Friday morning driving a clipper type low towards the Great Lakes and pushing a sharp cold front southeast across the area. This system will be accompanied by weak WAA out ahead of it, followed by impressive CAA in its wake. Modest LFQ diffluence ahead of a polar jet streak will aid in synoptic ascent, which when combined with height falls and the aforementioned WAA will produce a swath of moderate snow across the Great Lakes, including the U.P. and L.P. of Michigan on D2, and then downwind of Lake Erie and Lake Ontario on D3. While some of this snowfall will be driven by the leading WAA, much of the accumulation is likely to occur during CAA fueled LES, and WPC probabilities for more than 4 inches are 30-50% in the eastern U.P. and NW L.P. on D2, shifting into the Tug Hill Plateau on D3. Additionally, as this cold front surges southward across the Northern Plains and Upper Midwest Friday morning through Friday evening, there is an increasing threat for impressive snow squalls accompanying the FROPA. Recent guidance including the RAP, HRRR, and 3kmNAM suggest intense 925mb fgen coupled with 0-2km theta-e values dropping below 0 collocated with low-level CAPE exceeding 100 J/kg. While the coverage and intensity of these squalls is still difficult to discern beyond 24-hrs, there is a high likelihood for at least scattered intense snow showers/squalls from central ND through MN and WI which could produce severely restricted visibility and strong winds, despite accumulations that will likely be less than 1 inch. ...Pacific Northwest... Day 3... A shortwave embedded within confluent mid-level flow will move onshore Washington State Saturday night, accompanied by modest upper level diffluence and moist advection driving PWs to +0.5 standard deviations above the climo mean. This will spread precipitation across WA, OR, ID, and into western MT D3. Snow levels early D3 will be generally 2000-3000 ft, but should climb as warm advection onshore drives precipitation to the east ahead of a surface cold front dropping southward beneath the shortwave. As the front sags southward Saturday morning, snow levels will likely drop once again, and are progged to be as low as 1500 ft in WA, and 2000-2500 ft elsewhere. This cold airmass will allow for much of the precipitation to be snow in the Olympics, Cascades, and Northern Rockies, where WPC probabilities for 8 inches are above 80% in WA, and 20-40% elsewhere, and locally more than 12 inches is likely in the WA Cascades. Additionally, with snow levels likely to be well below pass level, WPC probabilities for 6+ inches is above 90% at Snoqualmie and Stevens Passes, and above 40% at Lookout Pass and Marias Pass as well. Weiss Key Messages for the Feb 17-18 Winter Storm: - A strong winter storm will impact the Central and Northeast U.S. today through Friday morning, producing wintry hazards of heavy snow, sleet, and freezing rain. - A swath of heavy, wet snow is expected across northern Oklahoma, eastern Kansas, much of Missouri and Illinois, northwest Indiana, and southern Michigan through tonight, extending into Upstate New York and northern Maine Friday morning. Snowfall rates are likely to exceed 2 inches per hour at times. - The combination of heavy snow and strong, gusty winds will lead to severely reduced visibility , hazardous travel conditions, and possible power outages today through Friday morning. - Locally heavy freezing rain and sleet are likely just to the south of the heavy snow, with more than 0.1 inches of freezing rain likely in some places. Impacts to travel are possible in these areas as well.