Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 344 AM EST Sun Jan 5 2025 Valid 12Z Sun Jan 05 2025 - 12Z Wed Jan 08 2025 ...Central Plains through the Mid-Atlantic... Days 1-2... ...Major winter storm to impact areas from the Central Plains and Ohio Valley today through the Mid-Atlantic on Monday... The first major storm of 2025 is currently organizing over the Central Plains this morning as an amplifying mid- level trough ejects from the Central Rockies and should close off over Kansas this morning. Latest radar, surface observations, and lightning networks indicate moderate to heavy snow and mixed precipitation with convective elements are already occurring over parts of KS and MO. This aforementioned closed low will then continue to deepen as it pivots eastward, reflected by NAEFS 500mb-700mb height anomalies falling to as low as the bottom 2.5th percentile of the CFSR climatology. This closed feature will continue to deepen gradually as it moves into the OH/TN Valleys Monday, before slowly opening into a sharp trough across the Mid- Atlantic Monday night. Through this evolution, a strengthening jet streak will pivot around the base of the trough, reaching as high as 150kts, and overlapping effectively with the greatest height falls/PVA to cause intense deep layer ascent. Beneath this synoptic lift, a surface cyclone currently strengthening in the southern High Plains will then move nearly due east with subtle deepening until it exits into the Atlantic on Tuesday. As this surface low deepens and move east, impressive moisture will be drawn northward from the western Gulf of Mexico within a corridor of intensifying isentropic ascent, especially within the 290-295K surfaces. Here, mixing ratios climb to 4-6 g/kg, reflective of the impressive moist driven by the warm advection, further evidenced by PW anomalies surging to above the 97th percentile according to NAEFS. The accompanying theta-e ridge is then progged to lift cyclonically into a TROWAL surrounding this deepening low, and pivot back into KS/MO later today. This is likely to cause an expanding area of precipitation, with the accompanying elevated instability driving more intense precipitation rates through D1. The pattern conceptually matches one that will support both an intense laterally-translating band of heavy precipitation east and downstream of the surface low, with a secondary, likely more intense, pivoting deformation band to the NW of the low. Locations that receive both of these bands, and stay all snow, will receive the heaviest accumulations. As far as snowfall amounts, they are likely to be quite impressive, especially across the Central Plains and into the Ohio Valley. Across the Plains, the lead WAA band will support snowfall rates of 1-2"/hr, and these types of bands can cause long duration of snow in narrow corridors. While the guidance has come into much better agreement with the placement of this band, there is still some uncertainty as to the exact location, especially due to latitudinal variations in the warm nose placement which will wreak havoc on amounts due to mixing with sleet and freezing rain, as well as much lower SLR (heavy-wet snow). As the low begins to push east, an intense deformation axis will develop across central/eastern KS and into NW MO. The guidance is quite aggressive with this axis, and it appears probable that a pivoting band with convective snow rates of 2-3"/hr is likely as folded theta-e and -EPV overlap in cross- sections suggesting upright (CI) convection potential. WPC's snowband probability tracker using the 00z HREF confirms this potential for 2-3"/hr snowfall rates between 12z this morning and 21z this afternoon across northern KS into MO. During this time the column also cools significantly, DGZ depth increases, and winds intensify, leading to blizzard and near blizzard conditions. It is this region that will likely experience the most significant snowfall, reflected by WPC probabilities that are high (70-80%) for 12+ inches in N-Central KS. A broader swath of 8+ inches is likely (70-90%) encompassing this region and spanning into northern MO. Farther downstream, the system begins to move more quickly across the OH VLY and into the Mid-Atlantic, which will limit the duration of heavy snow moreso than in points west and reflected in the lower probabilities for 8-12"+. Additionally, the weakening upper low will cause the deformation axis to weaken resulting in less intense snowfall rates. However, impressive rates that will still exceed 1"/hr are expected within a WAA band from Illinois through Ohio, and then continue across portions of the Mid-Atlantic where some uncertainty still exists. Additionally, guidance has become more excited about a secondary shortwave digging through the opening longwave trough, leading to an inverted trough NW of the departing low Monday, with secondary low development also possible east of the DelMarVa. This will, at the very least, lengthen the duration of light to moderate snow from OH to DE, with potentially a secondary axis of heavy snow along this trough adding to snow totals. At this time, despite some latitudinal uncertainty, WPC probabilities are high (>70%) for 6+ inches of snow on D1 from roughly Indianapolis to the higher terrain of western MD and WV. Chances for 6+ inches are only slightly lower across the DMV, Delmarva, and much of MD, with WPC probs at 50-70%. Locally as much as 12" of snow is possible across the lower OH Valley and central Appalachians, with lesser totals expected across the Mid- Atlantic, although more uncertainty exists there. Not to be outdone, this system will also pose a significant to damaging ice threat within the warm nose south of the heaviest snow. Intense WAA over-topping the cold surface layer accompanied by persistent E/NE dry-bulb advection is a classic setup for damaging freezing rain. Although there is some uncertainty into how efficiently the ice can accrete at times due to what should be intense precipitation rates, particularly from southern IL eastward through southern IN and KY, the threat for damaging ice remains from eastern KS through southern MO and into central KY. Here, WPC probabilities for more than 0.25" of ice are high (70-80%) on D1, with even some 20-40% chance for greater than 0.5" of ice in central KY. This could result in downed limbs and power lines leading to widespread long-lasting power outages and impossible travel. It is worth noting that following the storm's exit on Monday, temperatures will remain very cold across the Ohio Valley and Central Plains with low temperatures dipping into the single digits and below zero where snowpack exists. Farther east across the Central Appalachians and surrounding foothills, WPC probabilities for 0.25" of ice peak around 20-40%. Finally, between the heavy snow and heavy icing, the forecast profiles indicate a narrow stripe with cold layer depths greater than 1000m, or above the 90th percentile for freezing rain in the south-central US. This suggests sleet, especially in the presence of a relatively weak warm nose aloft at around 750-800mb and the aforementioned persistent dry advection at the surface. Heavy accumulations of sleet are difficult to achieve, but in this setup it is possible that 1-2" of sleet will accumulate causing tremendous driving impacts. The greatest risk for the major sleet accumulations are from near St. Louis southeast to southern IN, but fluctuations in placement are still possible. This system has prompted the issuance of key messages which are linked at the bottom of the discussion. ...Western U.S.... Days 1-3... A shortwave trough riding over a ridge axis centered off the Pacific Northwest crosses the Northern Rockies today before being absorbed into the trough over the Rockies that trails the low developing over the Central Plains. Moisture streams in ahead of this trough, making for enough precipitation to reach inland to bring moderate snows to the Bitterroots and ranges around Glacier NP and Yellowstone today with snow levels rising to around 4000ft. Day 1 WPC snow probabilities for >4" are 50-80% over the greater Idaho, western MT, and western WY ranges. The next stronger shortwave trough rides over the ridge tonight, diving across the OR Coast and becoming very positively- tilted over the the Desert Southwest on Tuesday. High initial snow levels around 7000ft over the OR Cascades only drop to around 6000ft under the trough axis, so only the highest OR Cascades, well above pass level will receive heavy snow. Snow levels will be closer to 5000ft over the Great Basin and 4000ft over the northern Rockies. Day 2 snow probabilities for >4" are generally 30-60% over terrain in southern ID, northeast NV, and northern UT. As light to locally moderate precip shifts south with the trough Monday night, snow levels in Utah will be around 5000ft. However, on Tuesday there is a heavy snow potential on the eastern slopes of the central/southern CO Rockies as strong high pressure (central sfc pressure around 1040mb) shifts south down the Dakotas, aiding an easterly low level flow to the moisture coming in from the west. Day 3 snow probs for >4" are 20-50% for southern portions of the Front Range and much of the Sangre de Cristos. ...Lake Effect Snow - Lake Ontario... Day 1... Low pressure centered over James Bay weakens today as a rapidly developing low off Nova Scotia becomes the dominant surface low in a broad gyre over southeast Canada. As a result, the great pressure gradient between James Bay and a 1044mb high over Manitoba will weaken and the strong WNW flow over central Ontario will gradually diminish. Sufficient WNW flow will persist over eastern Lake Superior, northern Lake Huron, southern Georgian Bay, Lake Simcoe, and Lake Ontario to provide maximum moisture fetch for LES single- banding to persist through midday today between the Tug Hill and Syracuse before drifting south and weakening this. Day 1 snow probabilities for an additional >6" are 40-60% on an axis from Syracuse west-northwest toward the shores of Lake Ontario. Weiss/Snell ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png