Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 424 PM EST Thu Dec 22 2022 Valid 00Z Fri Dec 23 2022 - 00Z Mon Dec 26 2022 ...Midwest through the Interior Northeast... Days 1-3... ...Powerful blizzard with extremely cold temperatures, dangerously frigid wind chills, and a flash freeze likely... An exceptionally deep closed mid-level low characterized by 850-700mb heights approaching -5 standard deviations below the climatological mean according to the NAEFS ensemble tables will shift across the Great Lakes tonight into Friday, then pivot northward over New England Friday night into Saturday, before deepening even further and spinning in place across Ontario/Quebec into Sunday. The excessive amplitude of this system will keep it moving very slowly, and when accompanied by strong height falls and PVA, and a 170+ kt jet streak arcing around it, will result in widespread significant winter weather across the eastern half of the country. Beneath this slow moving closed low, a surface low will move from Lower Michigan into Ontario while deepening explosively, becoming around 965mb Friday night. The pressure gradient between this and a strong high to its west will help drive intense NW winds across the eastern CONUS, following an arctic cold front draped from this low which will traverse the entire east coast through Saturday morning. While the heaviest snowfall the next few days is likely in the Great Lakes and Northeast, anafrontal precip behind this arctic front could result in brief periods of very heavy snowfall from the mid-Mississippi River Valley eastward through the TN and OH Valleys and maybe even into the Mid-Atlantic and Northeast during D1. The greatest risk for rapidly accumulating snow appears to be across the TN/OH Valleys where some fgen and correlated residual theta-e lapse rates <0C will support CSI and very heavy snow rates which the WPC snowband tool indicates could reach 2"/hr. Total accumulations behind the front should be less than 2 inches, but could locally exceed 4" as shown by an increase in WPC probabilities reaching 20-40% from northern TN through western OH. Regardless of amounts, greatly reduced visibility and strong winds will make travel treacherous for a few hours. More significant snowfall is likely D1 across the Great Lakes, northern OH Valley, and into the higher terrain of the interior Northeast where the best overlap of moisture and deep layer synoptic ascent will occur. While areas of New England will feature lower SLRs due to stronger WAA with snow changing to rain and then back to snow, exceedingly high SLR approaching 20:1 is likely across the Great Lakes due to the very cold airmass, only being limited by the strong winds below the DGZ which will somewhat fracture dendrites as they fall. Still, sufficient moisture and fluffy SLRs will likely result in snowfall that will exceed 6 inches across much of the Great Lakes, with additional enhancement occurring in the favored N/W snow belts as impressive LES develops late D1 and continues into D2. LES development could be nearly ideal in some areas, although the very strong winds could limit parcel residence times across the Lakes. Nevertheless, sufficient instability and ideal trajectories will result in heavy LES continuing through D3, although the most intense LES should gradually shift from Superior and Michigan D1-2 to Erie and Ontario D2-3. Snowfall rates of 2-3"/hr will almost certainly occur which will yield event total snowfall of 3-4 feet in some areas downwind of Superior, Erie, and Ontario. ...Pacific Northwest... Days 1-3... ...Significant to damaging freezing rain event likely Thursday night through Friday... Arctic high pressure entrenched east of the Cascades and expanding eastward through the Plains will be impinged upon 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, 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 cause 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 after an initial burst of snow and sleet. The guidance has continued to get just a bit colder for this event, likely due to just how impressively cold the antecedent airmass is. This could result in a longer period of sleet as the cold depth beneath the warm nose is 150-200mb deep for a time on Friday. However, it is likely that precip will become mostly ZR for the Willamette Valley, Columbia River Gorge, coastal ranges, and lowlands from Portland, OR through Seattle, WA. 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 ice as reflected by WPC probabilities of 50-70% for these areas on D1, and again D2 primarily for the Gorge. Locally more than 0.5" is becoming likely, especially within the Gorge, but also possible for the OR coastal ranges and Willamette Valley. Both Seattle and Portland could experience around 0.25" of freezing rain as shown by recent WSE plumes and NBM probabilities, resulting in dangerous commuting and scattered power outages. The worst of the freezing rain for these areas is expected D1, with significant freezing rain continuing into D2 across the Gorge, before expanding eastward into the Columbia Basin and across the WA Palouse, although with likely lesser intensity and accretions on D3 as a secondary wave of moisture surges onshore. While freezing rain is likely the most notable hazard from this event, with snow levels slowly climbing to 6000 ft, there is likely to be heavy snow even at the Cascade Passes of Snoqualmie and especially Stevens D1-2 where WPC probabilities for 6 inches are moderate to high, and storm total snow could exceed 24 inches in some of the highest terrain. WPC probabilities also indicate a moderate risk for more than 6 inches of snow D1-2 across the Bitterroots and Northern Rockies. Weiss Key Messages for Blizzard --Blizzard Conditions from Strong Winds and Heavy Snow A powerful winter storm will continue to produce snow from the Midwest and Ohio Valley 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. 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 bring dangerously cold conditions across most of the country this week, and to the Eastern U.S. by weekâ€s end. --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, and have been observed, immediately behind the Arctic cold front across the Central Plains, Mid-South, Ohio Valley, 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.