Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 417 AM EST Fri Dec 23 2022 Valid 12Z Fri Dec 23 2022 - 12Z Mon Dec 26 2022 ...Great Lakes through the Interior Northeast... Days 1-3... ...Powerful blizzard with extremely cold temperatures, dangerously frigid wind chills, and a flash freeze likely... A deep and expansive cold core low will continue to develop as Arctic-sourced air wraps around the system with the surface low continuing rapid deepening as it lifts from Lower Michigan through Ontario, reaching 969mb by this afternoon. The Arctic cold front sweeps east across the Northeast through this evening. The pressure gradient between the deep low and a 1050mb high drifting east from the northern High Plains will continue to drive intense NW winds across the eastern CONUS. The heaviest snowfall through the Holiday Weekend will be from lake enhanced and effects from the Great Lakes. Additional heavy snow today will be localized at high elevations in the Northeast on warm air advection from the southern stream wave phasing into the primary low along with interior Northern Mid-Atlantic snow behind the cold front and under the upper low. Day 1 snow probabilities for over 4 inches are moderately high over central Upstate NY down into east-central PA as well as over the Adirondacks, Greens, and Whites. Cyclonic flow over the entirety of the Great Lakes provides yet another significant lake effect event for the main snow belts off each Great Lake. However, boundary layer flow of 40 to 50kt and near surface DGZs will make for such turbulence that SLRs should be somewhat suppressed placing a little limit on this very cold air over currently ice free lake surfaces. Sufficient instability and ideal trajectories will result in heavy LES continuing for all Great Lakes through Saturday night, lingering off Lakes Erie and Ontario through Sunday night. Snowfall rates of 2+"/hr will almost certainly occur which will yield event total snowfall of 3-4 feet in some areas downwind of Superior, Erie, and Ontario with widespread risk for an additional foot off Lake Michigan through Saturday night. ...Pacific Northwest... Days 1-3... ...Significant to damaging freezing rain event continues through Saturday... Arctic air that has spread down to the PacNW coast will be overridden by a slow moving warm front advancing eastward from the Pacific Ocean. This front will be driven to the east ahead of a modest mid-level shortwave today, but will encounter shortwave ridging to the east and in-between mid-level confluence will reinforce the surface high. This indicates that while moisture will steadily stream eastward into the region, the high pressure will be extremely slow to retreat, especially as precipitation falling into it helps to reinforced the cold surface air. However, robust WAA invof the elevated warm front will surge more quickly on 30-40 kts of 850-700mb wind, driving a warm nose above +0C inland. This will help raise snow levels, and continue a p-type transition from snow to sleet to freezing rain, and eventually rain near the coast. Moisture will be plentiful as an atmospheric river with IVT >250 kg/m/s shifts onshore coincident with the WAA, resulting in a long duration of precipitation. With the warm nose surging inland but surface temps remaining quite cold, this will likely result in significant freezing rain. Continued feeding of cold air through the Columbia Gorge looks to maintain this freezing rain threat, at least in the Gorge itself and in the Columbia Basin. Precip will become mostly ZR for the Willamette Valley, Columbia River Gorge, coastal ranges, and lowlands from Portland, OR through Seattle, WA this morning as the warm nose spreads inland. In this area, there should also be sufficient easterly/dry winds to help offset the latent heat release of freezing inherent to limiting freezing rain accretions, resulting in what could be significant to damaging ice in some areas. Where freezing rain occurs the longest and is able to most efficiently accrete due to cold temps and gusty winds with just modest rain rates, there is high confidence in greater than 0.25" of additional ice after 12Z as reflected by Day 1 WPC probabilities of 60-90% east of Portland through the Columbia Gorge and closer to 50% for the Seattle-Tacoma metro. There is also a 10% risk for an additional 0.5" ice within the Gorge. Both Seattle and Portland could experience around 0.25" of freezing rain, resulting in dangerous commuting and scattered power outages today. The warm front eventually shifts east to the Cascades by tonight. However, significant freezing rain continues in and near the WA/northern OR Cascades and the Columbia Gorge through Saturday with Day 2 probabilities for an additional 0.25" low to moderate. A warmer system shifts inland Saturday night with snow levels rising farther and freezing rain limited to the Columbia Basin. Snow levels linger around 6000 ft today/tonight, then surge up to around 8000 ft Saturday with the next wave. Days 1 and 2 snow probabilities are both moderate for 8 or more inches in the higher WA Cascades. Jackson Key Messages for Blizzard --Blizzard Conditions from Strong Winds and Heavy Snow A powerful winter storm will continue to produce snow through the Great Lakes and interior Northeast. The heaviest snowfall is likely in the Great Lakes where Lake Effect Snow will aid in producing 2-4 foot totals. Even where lesser accumulations occur, heavy snow rates combined with wind gusts of 40 MPH will result in blizzard conditions. --Widespread Potentially Damaging Winds & Ground Blizzards Even away from the heavy snow, this powerful storm will bring widespread wind gusts over 40 MPH to a large portion of the Central and Eastern U.S. into Sunday. In some spots, the wind gusts could approach or exceed 60 MPH resulting in damage and power outages. These winds atop existing snow cover will produce ground blizzards. --Extremely Dangerous Travel with Whiteout Conditions Where blizzard conditions occur, expect periodic whiteouts with near zero visibility and considerable blowing and drifting of snow. Traveling in these conditions will be extremely dangerous, to at times impossible. --Life-threatening Cold Temperatures Dangerous wind chills will create a potentially life-threatening hazard for travelers that become stranded. If you must travel, prepare for extreme cold. In some areas, being outdoors could lead to frostbite in minutes. Ensure outdoor animals and livestock have sufficient shelter. --Coastal and Inland Flooding Threat for the Northeast Heavy rain falling onto a melting snowpack is forecast to cause flooding impacts in urban areas, and along rivers and streams today. Minor to isolated major coastal flooding is possible due to onshore winds. Key Messages for Cold Front and associated Snow Squalls/Flash Freeze --Dangerous Cold for Much of Country Frigid temperatures and gusty winds behind a powerful Arctic cold front will continue to bring dangerously cold conditions across most of the country into or through this weekend. --Increased Danger if Power Outages Occur The danger of the cold will be increased if any power outages occur due to the widespread gusty winds around the winter storm system. --Rapid Temperature Drops with Front Temperatures have already been observed to drop more than 20 degrees in an hour or less with the front. Expect sudden changes in conditions. --Considerable Travel Impacts Possible Snow squalls, or a brief burst of moderate to heavy snow (1 to 2 hours), are expected immediately behind the Arctic cold front across portions of the Mid-Atlantic and Northeast. -Snow will be occur with gusts around or over 40 MPH, creating sudden whiteouts and zero visibility, and a danger of pile-ups on roadways. -Snow will coincide with rapid temperature drops, which will likely lead to flash freezing and icy spots developing on roadways. Any flooding in the Northeast may also re-freeze.