Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 337 PM EST Thu Feb 24 2022 Valid 00Z Fri Feb 25 2022 - 00Z Mon Feb 28 2022 ...Great Lakes, Ohio Valley, Mid-Atlantic, and New England... Day 1... A lifting shortwave trough over the Central Plains and strengthening subtropical jet (upwards of 170 kts) will help increase the span of a developing precipitation shield that currently stretches from the Midwest to the Mid-Atlantic. At the surface, organizing area of low pressure will lift northeastward through the Ohio Valley tonight and eventually weaken as it runs into the northern Appalachians on Friday morning. Meanwhile, arctic high pressure located over southern Quebec and a later developing area of low pressure off the southern New England coastline will allow for a wedge of cold air to linger at the surface as far south as the northern Mid-Atlantic. Light to moderate snow associated with the approaching upper-level shortwave tonight will spread across the Great Lakes region and parts of the Midwest. A deep DGZ will allow for high snow ratios (~20:1) and the potential for localized snowfall amounts to exceed four inches. Enhancement off Lake Michigan should aid in achieving local maxima around inches near Milwaukee and Chicago . Strong mid-level frontogenesis suggests a burgeoning area of moderate to heavy snow along the PA/NY border at precip onset as the secondary area of low pressure develops east of Delaware Bay along the oceanic boundary, eventually becoming the dominant center as the primary low pressure weakens over central NY Fri morning. The surge in southerly flow aloft will bring a mixed precip zone from southwest to northeast but disrupted by local topography, creating the usual headache for p-type transition/amounts as the models keep wavering on how far north to bring in milder air (and to what depth). At this point, most model differences are due to typical thermal biases. NAM would typically handle narrow warm noses better than the GFS but it has wavered on how far north in recent runs and trended slightly colder, but have taken a probabilistic approach toward a slightly milder solution within the confines of the National Blend of Models p-type guidance. For northern sections, in the all snow zone, strong FGEN will support a period of at least 1"/hr rates (supported by the CAM guidance snowband tool) over northern NY across central New England. Profiles suggest strong lift at times into the DGZ (perhaps not a large depth but quite sufficient) with higher SLRs ~20:1 closer to the Canadian border. There, snowfall should maximize just north of the QPF maximum where some mixing will be possible up toward the CT/MA border. Column will cool as the coastal low deepens and winds back to northerly as the precip starts to exit Friday afternoon/evening. WPC probabilities of at least eight inches of snow are moderate to high over much of eastern NY north of I-84 eastward into central New England and along I-90. Maximum snowfall amounts may exceed a foot in parts of eastern New York, Vermont, and southern New Hampshire, where terrain benefits from a period of upslope flow and a pivoting band lingers during the transfer from the primary to the secondary low pressure system. To the south, there is likely to be a zone of sleet before a transition to freezing rain from eastern Indiana eastward into PA then a more expansive southward extension east of the Appalachians into northern MD northeastward into the Poconos. Colder air in sheltered areas will support a longer duration of freezing rain in excess of a tenth of an inch to over a quarter of an inch. WPC probabilities for at least a quarter inch of ice are high (>50%) across the Laurel Highlands and Allegheny Mountains of SW PA and far W MD (a well-known prolonged icing area in cold air damming). Low probabilities of a quarter inch of freezing rain extend eastward along southern PA and into northern New Jersey, as well as far northern MD. ...Pacific Northwest... Day 3... Shortwave in the Pacific will move northeastward into WA/OR late Sat into early Sun, driving a cold front into western WA with a plume of moisture (+1 to +2.0 sigma) on southerly flow. Snow levels will rise in advance of the front but linger around 2000-3000ft along the Cascades with several inches likely at pass levels around that altitude. Snell Key Messages for Midwest through Northeast Winter Storm tonight/Friday: - A winter storm strengthens tonight into Friday, producing a swath of heavy freezing rain from Ohio through the northern Mid-Atlantic and a swath of heavy snow over New York State and much of New England. - Significant ice accumulations are expected from central Ohio through the northern Mid-Atlantic and into southern New England. Ice accumulations over 0.25†are most likely from west-central Pennsylvania through far western Maryland where particularly hazardous travel and localized power outages are likely. - Heavy snow with rates of 1†to 2†per hour will develop late tonight over New York State before expanding over much of New England on Friday. A swath over 8†is likely from the Adirondacks, across Vermont, New Hampshire, northern Massachusetts, and southern Maine where travel will become difficult. - A swath of wintry mix just south of the heavy snow will also make for impactful wintry weather late tonight into Friday.