Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 319 AM EDT Wed Mar 19 2025 Valid 00Z Wed Mar 19 2025 - 00Z Sat Mar 22 2025 ...Pacific and Interior Northwest... Days 1-3... The seemingly never-ending training of shortwaves and moisture continues this week across the Northwest CONUS. The first of these will be the most potent, as a strong shortwave trough pushes onshore this evening with a subtle negative tilt. Impressive height falls ahead of this trough will drive ascent, aided by at least weak LFQ diffluence as a streak pivots through the base of this trough. Together, this will produce deep layer lift from the northern Sierra through the Cascades and into the Interior Northwest, supporting heavy precipitation as IVT surges northeast on downstream S/SW flow. Snow levels will gradually rise within the IVT core due to accompanying WAA, before falling again with the passage of a cold front later D1. Despite this, snow levels will generally be quite low through the period as the region remains cold, rising from around 2500 ft to 3500 ft, before crashing again to 2000-2500 ft behind the cold front. This suggests that snow will impact many of the Cascades and northern CA Passes, as well as into the interior NW causing hazardous travel. This first system is progressive, however, and WPC probabilities peak around 70% for 6+ inches in the highest peaks of the WA Cascades and Olympics, with generally 4 inches or less forecast elsewhere. As this first trough shifts eastward, forcing will weaken across the Northwest, but flow across the Pacific becomes increasingly zonal due to squeezing between broad ridging well west off Baja and a modest trough across the Gulf of Alaska. This zonal flow will maintain onshore moisture advection to support rounds of precipitation into the Northwest, with periodic vorticity lobes swinging eastward (although maintaining latitude generally north of OR) to enhance ascent. Snow levels D2-3 will fluctuate generally between 2500ft and 4500ft, with rounds of precipitation leading to periods of heavy snowfall. WPC probabilities both D2 and D3 are similar in magnitude and areal extent, reaching 70% or more for 4+ inches from the Olympics, to the WA/OR Cascades, eastward through the Salmon River/Sawtooth Ranges, and into the Northern Rockies. 2-day snowfall in some of the higher elevations could reach 2-4 feet, with up to 1 foot at the higher passes of the Cascades. ...Central High Plains through the Great Lakes... Days 1-2... ...Significant storm system to bring blizzard conditions to portions of the Central Plains and Upper Midwest. Key Messages are being issued for this system... The low-pressure which has been progged for several days now to deepen in the Central High Plains is taking shape this morning noted by pressure falls and increasing convective elements across CO/KS/NE. The parent PV anomaly is clearly noted in satellite imagery across far SE CO and into the OK Panhandle, which is generally S/SW of all available guidance. This suggests that the E/SE trend that began 24 hours ago is valid, and the event may end up even farther SE than the 00z suite of models. While the exact track will be critical to the placement of the most significant impacts related to this blizzard, a large scale significant event is still likely from the Central High Plains northeast into the Great Lakes. As this low deepens today in response to favorable synoptic and mesoscale ascent, precipitation will expand downstream. Current WV imagery shows both the low and mid level moisture streaming into the developing cyclone, and where isentropic ascent lifts into the intensifying deformation axis, impressive omega will squeeze out all available moisture in the form of heavy rain and snow. The deformation axis on the NW side of this system will likely be quite intense, and will be overlapped by a cyclonically surging TROWAL to enhance both ascent and instability. Where this occurs, any rain will quickly change to snow through dynamic cooling, and then come down extremely heavily, reaching 2-3"/hr at times where CI (thundersnow) can occur. This is supported by HREF 1"/hr and 2"/hr probabilities that reach 90% and 70%, respectively, and by the WPC prototype snowband tool that suggests locally 3"/hr is possible (20%). These snowfall rates will quickly overwhelm hostile antecedent conditions (some places were near 80 degrees Tuesday and will get heavy snow today!), and WPC probabilities have increased for significant snowfall despite the SE shift. The heaviest accumulations are likely from central NE through NW IA where the environment supports a pivoting deformation band, and WPC probabilities are high (>70%) for at least 8 inches. Here, locally 12-15 inches is possible (30%) in some locations. Farther downstream, the band will begin to translate more quickly, limiting the duration of these intense rates, but still heavy snowfall is likely as reflected by WPC probabilities that are high (>70%) for 6+ inches from SE Minnesota through the central U.P. of Michigan. Additionally, strong winds of 50 mph or more will accompany this snowfall, so despite SLRs that will generally be below climo, and will start as a heavy/wet snow, as SLRs climb and aggregates fracture, blizzard conditions are expected which will make travel almost impossible in some areas. Another interesting development that has shown up tonight is for a secondary deformation axis to potentially develop in the vicinity of the primary PV anomaly behind the surface low, bringing some heavy snow rates of around 1"/hr from SE Iowa to the Chicago metro area. While this is not expected to be as impressive as the axis to the NW, moderate snowfall accumulations for which WPC probabilities indicate have low chance (10-30%) of exceeding 4 inches, which when combined with gusty winds could still produce moderate travel impacts. This entire system exits quickly on Thursday, with some residual deformation snow pivoting across lower Michigan, and some northern flow in its wake leading to lake enhanced snowfall across the U.P. and southeast of Lake Michigan. Still, additional snowfall should be modest as reflected by WPC probabilities that are just around 10% for an additional 2 inches of snow in these areas. ...Interior Northeast... Days 2-3... Shortwave energy moving into the Mid-Atlantic will cause a split jet stream (one rotating across the Carolinas while another pivots towards Greenland) resulting in a modestly coupled structure to help deepen a surface low south of New England beginning Thursday night. The guidance has trended maybe just a bit weaker with this low the past 24-hrs, but ensemble low points still agree in a deepening system to around 990mb tracking from norther NJ to Cape Cod and then into the Gulf of Maine by Saturday morning. Uncertainty in the intensity and track remains, but spread is relatively minimal for this time range, and deformation developing on the NW side of this low is still likely to result in an axis of rain changing to heavy snow. The GFS remains the deepest solution and is somewhat discounted based on recent upstream performance, but all the guidance indicates that an axis of sloped 850-700mb fgen will drive omega into the deepening DGZ early Friday through Friday night, which will dynamically cool the column to produce an axis of heavy snowfall rates. The antecedent conditions are not ideal for snow accumulations due to warm temperatures and rain, but higher elevations of the interior Northeast from Upstate NY through Maine could still see impactful accumulating snow. Confidence is low, and trends for this event need to be monitored over the next few model cycles, but WPC probabilities are modest at just 10-30% for 4+ inches of snow in the higher elevations. Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png