Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 243 PM EST Thu Jan 30 2025 Valid 00Z Fri Jan 31 2025 - 00Z Mon Feb 03 2025 ...Northeast & Northern Mid-Atlantic... Days 1-3... An anomalous closed upper-low over the Mississippi Valley Friday morning will direct a broad and strong IVT at the Northeast U.S.. Rich moisture from the south and increasing low-mid level WAA aloft will overrun boundary layer temperatures that start out below freezing in the interior northern Mid-Atlantic and Northeast on Friday. Pockets of freezing rain are most likely to occur from northern PA on east through the Poconos, Upstate NY, and into central New England beginning Friday morning and lasting for the more northern locations through Friday evening. WPC probabilities show low chances (10-30%, pockets of 40% in some areas) in north- central PA and the Catskills for ice accumulations >0.1" through Friday evening. Minor ice accumulations <0.1" are possible as far south as the Lehigh Valley in PA, but with a quickly modifying air- mass, a changeover to plain rain should limit the impact- potential in these more southern areas. Meanwhile, the mountain ranges of northern New York and northern New England will have better chances of staying primarily snow for the duration of this event (Friday afternoon through Saturday night). WPC probabilities show low-to- moderate chances (30-50%) for snowfall totals >4" in these areas with localized amounts within the peaks of the Adirondacks, Green, and White Mountains topping 6" by early Saturday morning. As the primary low weakens Friday night across NY, a new coastal low will form east of the MA Capes Friday night. Some guidance shows an increasingly low-level easterly fetch to the north of the low may deliver some Atlantic moisture over southern and central Maine Friday night and into Saturday morning. Snow should taper off by midday as the low races into the North Atlantic Saturday afternoon. WPC probabilities show low chances (10-30%) for snowfall totals >4" across southern and central Maine, which may result minor impacts for travelers Saturday morning. ...Upper Great Lakes... Days 2-3... A fast moving upper level shortwave trough tracking across the Northern Plains will coincide with the diffluent left-exit region of a 250mb jet streak to provide modest ascent over the Upper Midwest. In addition, as a warm front lifts north Saturday evening, increasing 850-700mb WAA will provide additional low-level ascent that results in periods of snow from northern MN on east across the Upper Great Lakes. The combination of best lift and elevation is currently being highlighted over northern WI and the southern MI Upper Peninsula. By 06Z Sunday, a consolidating area of low pressure will allow for southeast-to-easterly flow to allow for added lake-enhancement along the MN Arrowhead. Light snow may ensue over northern MI through the day on Sunday while the heavier snowfall over northern MN/northern WI tapers off Sunday afternoon. WPC probabilities show moderate chances (40-60%) for snowfall amounts >6" along the MN Arrowhead and low-to-moderate chances for snowfall totals >6" in parts of northeast Wisconsin through Sunday morning. ...Pacific and Interior Northwest/Great Basin... Days 1-3... The start of an active and increasingly wintrier pattern for much of the Pacific Northwest/Interior West/Great Basin is now in sight as the lead atmospheric river event starts tonight. Initially mild temperatures will limit heavier snow to the higher mountain locations, but colder air will become entrenched as light to modest precipitation continues to spread across the region. This will support snow into the lowlands by the end of the period. For D1, strong IVT (>90th percentile over much of the region tonight into Fri) and WAA will drive snow levels up to 4000-5000ft along the Cascades and northern Rockies/Great Basin (and over 6000-7000ft over NorCal into the Sierra). With over an inch QPF in the terrain of the Cascades, well over a foot to two feet of snow are likely well above pass level. Light to modest snow is likely farther east into eastern WA/northern ID/western MT along with some light icing due to very cold surface/ground temperatures even as 2m temps warm up to just over 32F. Into D2-3, colder air will filter in (in typical fashion) then rush in with a vengeance by Sunday as the upper low over coastal BC sinks closer to the region. Though QPF will be lighter overall over WA/OR as the moisture plume focuses into NorCal/northern Sierra, SLRs will rise and snow levels will fall, bringing accumulating and impactful snow to the passes D2 then into the lowlands D3 and beyond. For areas north of CA/NV, continued onshore flow ahead of the upper low will lead to less QPF each day but more snow overall. For the D2-3 period, WPC probabilities of at least 8 inches of snow are high (>70%) above 1000-2000ft (north) to around 4000ft (south) along the Cascades and across much of the Bitterroots, Sawtooth, Tetons, Absarokas where 1-3ft of snow is likely. By D3 with colder air mostly in place (at least aloft and mostly to the sfc), snow levels will drop to near sea level which will favor some accumulation depending on 2m temps and time of day. WPC probabilities for at least an inch of snow D3 is non-zero through the I-5 corridor from WA into OR though higher chances may be into D4. To the south, with the strong AR settling into NorCal, snow levels will remain on the higher side (6000-8000ft) into the northern and central Sierra with WPC probabilities for at least 12 inches of snow >50% above these elevations. Low SLRs over much of CA will promote a heavy and wet snow ("Sierra cement") esp Sat/Sat night. Impacts per the WSSI may be at least moderate to major (considerable disruptions). Mullinax/Fracasso