Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 417 PM EST Thu Jan 20 2022 Valid 00Z Fri Jan 21 2022 - 00Z Mon Jan 24 2022 ...Western U.S... Days 1-2... A well-defined mid-level shortwave trough currently moving through the Pacific Northwest will dig south-southeastward through the Great Basin and gradually close off as it drops down into the Southwest U.S. in the vicinity of western AZ by Friday night. On Saturday and Sunday, the evolving mid-level low center is forecast to continue south into areas of far northwest Mexico. Going through Friday, there should be sufficient moisture and forcing, especially with the aid of orographics, to favor locally heavy snowfall accumulations for areas of the Sawtooth/Bitterroots and down into the Absaroka Range of the northern Rockies with as much as 6 to 12 inches of new snowfall and isolated heavier amounts. By late Friday and through early Saturday, the advance of mid-level moisture should advance down into the central Rockies with the Colorado high country seeing several inches of accumulating snow. Overall, the Sangre De Cristo range will see the heaviest totals across this region with multi-day totals over the higher terrain perhaps locally exceeding 1 foot. Sufficient forcing with the low center may also allow for some light snowfall accumulations to occur over the Mogollon Rim of AZ. ...South Texas/Gulf Coast to the Southern Mid Atlantic... Days 1-2... An arctic cold front will continue to drop southeast and well south of the Gulf Coast region this evening and through the Southeast overnight as cold surface high pressure advances east from the Midwest to the Ohio Valley and Northeast while also nosing down through the southern Plains. Concurrently, a positively-tilted mid-level trough will slowly amplify from the Midwest to the Mid-Atlantic states with the energy then advancing offshore on Saturday. Farther west, a separate shortwave currently pivoting through the Southwest will eject east and arrive out across the Rio Grande Valley and South Texas on Friday, supporting a wintry mix across portions of the Texas Hill Country and Deep South Texas tonight into Friday morning. Areas of sleet and freezing rain are expected with sufficient shallow low-level cold air for ice accretions that may reach as much as .05" to .10". Isolated heavier ice amounts will be possible going through especially mid-morning Friday before the low levels begin to moderate a bit and edge back above freezing. Farther east as this system edges away from South Texas, it is also possible some very spotty light freezing rain could occur across areas of the central Gulf Coast, with a focus perhaps on parts of south-central LA. A larger freezing rain threat though will develop over central/eastern North and South Carolina behind the arctic front where isentropic ascent and favorable upper-level jet dynamics ahead of the approaching upstream trough will set up later tonight and extend through Friday night. Locally significant ice accumulations are expected, with the heaviest concerns for freezing rain Friday night across the immediate coastal plain of eastern NC and northeast SC as low pressure begins to organize just offshore of the Mid-Atlantic and stronger forcing arrives in conjunction with colder low-level air from the aforementioned high pressure evolution to the north and west. Though the model guidance has trended more suppressed and east with the system overall, this particular area of the coastal plain is still expected to see a swath of 0.10" to 0.25+" ice accumulations with locally heavier amounts (especially over southeast NC) going through very early Saturday. The colder air arriving from the north will also support high potential for snow and sleet across areas of central to northeast NC and southeast VA. As the upstream energy arrives, and low pressure deepens offshore, there will be a period of moderate to locally heavy snow that should occur. Right now the heaviest accumulations should be generally from northeast NC into far southeast VA near the Hampton Roads area where some 4 to 6+ inch amounts will be possible by early Saturday. Some of this snowfall potential will include some smaller scale banding/ocean-effect snow enhancement as influenced by a developing moist north-northeast low-level fetch off the Atlantic Ocean. ...Upper Midwest/Great Lakes... Days 2-3... Northern stream shortwave energy just atop the Northwest system day 1 will dip through the northern Plains and Upper Midwest by Friday night and cross into the Great Lakes on Saturday. A cold front will have some moisture in its warm advection side to support a couple inches of snow locally across Minnesota, but waning as the front crosses the region. However, broader cold cyclonic flow over the Great Lakes arriving with the mid-level trough and especially with the stronger cold air advection behind the frontal passage will set the stage for additional heavy lake effect snow. This will especially be the case downwind of Lakes Erie and Ontario by early Sunday with at least several inches of snowfall likely as a favorable west-southwest fetch of cold air down the long axis of the lakes facilitates well-organized and intense lake effect snowband/squall activity. Orrison ~~~ Key Messages for Winter Storm Impacting Portions of the South and Mid-Atlantic Jan 20-22 ~~~ -- A wintry mix, including freezing rain, will impact areas as far south as South Texas and the central Gulf Coast later today into Friday. This could result in dangerous travel conditions. -- Low pressure is expected to develop over the Southeast today spreading snow, sleet, and freezing rain from the Southeast into the Mid-Atlantic states. -- Significant ice accumulations are likely in the Carolinas, resulting in dangerous travel conditions and scattered power outages. -- Moderate to heavy snow is possible from central North Carolina through the DelMarVa Peninsula. -- Changes in the track and strength of this system can have significant impacts on precipitation types and amounts.