Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 254 PM EST Mon Feb 3 2025 Valid 00Z Tue Feb 04 2025 - 00Z Fri Feb 07 2025 ...Western U.S. across the Northern Rockies... Days 1-3... ...Atmospheric River (AR) and upper low to sustain prolonged heavy snow from Oregon/California border and Sierra Nevada to the Northern Plains through Wednesday... A positively-tilted upper-low off the Pacific NW coast trapped beneath a strong upper ridge over Alaska is responsible for an active jet stream pattern that is resulting in a highly active storm track to open the month of February. The 200mb jet stream pattern today sets the diffluent right-entrance region over interior Northwest with a strong 1040mb+ high located over southwest Canada (NAEFS shows the dome of high pressure tops the 97.5 climatological percentile over North Dakota by Tuesday AM). This expansive area of high pressure will work to lock a cold Canadian air-mass in place along and east of the Continental Divide, while also causing easterly upslope flow to enhance precipitation rates along the leeward-facing slopes of the Northern Rockies. In tandem with the busy Pacific jet stream pattern, which will deliver wave after wave of IVT surges into the western U.S., periods of heavy mountain snow will common from the West Coast ranges (Sierra Nevada, OR/CA coastal ranges) on east to the Sawtooth, Bear River, Bitterroot, Teton, Absaroka, Wind River, and Lewis Ranges. In terms of the time line, a stream of precipitation associated with low pressure in the Great Basin is tied to period of mountain snow this afternoon from the northern Sierra Nevada on north and east to the Northern Plains. By Tuesday morning, the next wave of low pressure approaches the CA coast with IVT values topping the 99.5 climatological percentile over the Sierra Nevada by Tuesday afternoon. Strong WAA will keep the heaviest snowfall in the Trinity/Shasta and northern Sierra NEvada to elevations >6,000ft, while in the central and southern Sierra Nevada, snowfall will be heaviest at elevations >7,000ft Tuesday afternoon and through Tuesday evening. This same stream of moisture will keep heavy snow in the forecast in parts of southeast OR, the Blue Mountains, and both the Sawtooth and Bitterroots through Tuesday night. By Wednesday morning, heavy snow will be common in the Teton and Wind River Ranges. A brief reprieve in the heavy snowfall (sans some ongoing upslope flow into the Cascades, Olympics, and coastal ranges along the Pacific NW coast) will arrive Wednesday afternoon and linger into Wednesday night, but the next Pacific storm system arrives on Thursday with colder 700mb temps over northern CA this time around per NAEFS. This should allow for heavier snowfall to occur at lower elevations throughout the northern California (excluding elevations <3,000ft) with some light snow accumulations as low as 1,000ft western OR. Through Thursday afternoon, WPC probabilities paint a very wintry picture for much of the northwestern U.S.. Snow will be measured in feet from the Sierra Nevada >7,000ft and the Trinity/Shasta/Salmon mountains of northern CA >5,000ft through the Sawtooth, Bitterroots, Absaroka, Tetons, and Wind River Ranges. The Sierra Nevada, Sawtooth, and Tetons in particular could are forecast to see as much as 3-5 feet of snowfall through Thursday. For western OR/WA, parts of the Cascades, Olympics, and coastal ranges could see up to a foot of snow with even light accumulations possible along the I-5 corridor from the Willamette Valley on north through the Seattle metro area. In fact, WPC probabilities through Tuesday afternoon show low chances (10-30%) for snowfall >2" from the Olympia on south to just north of the Columbia River. In the Northern Plains, WPC probabilities show high chances (70-90%) for >4" of snowfall for areas east of the Lewis Range and surrounding the Big Snowy/Little Belt mountains on east through northwest North Dakota. In fact, these area event show low-to-moderate chances (30-50%) for snowfall amounts >8". The WSSI shows the worst impacts (Extreme Impacts mostly due to Snow Amount and Snow Load)) for the Sierra Nevada above 7,000ft. These areas are likely to contend with dangerous to even impossible travel conditions, as well as extensive closures and disruptions. Major Impacts (considerable disruptions and dangerous travel conditions) are highlighted in the Sawtooth, the Trinity/Shasta/Salmon of northern CA and southern OR, and even some isolated areas of the OR coastal ranges. Lastly, Moderate Impacts (hazardous driving conditions) are most common in these areas lower elevated areas (still generally above 2,000ft) from northern CA and the WA/OR coastal ranges through southeast OR and into the northern Rockies. Mullinax ...Northeast and Upper Great Lakes... Day 1... Persistent zonal flow regime will continue through D1 with an analyzed speed max over the Great Lakes this afternoon pushing eastward toward New England within the west to east aligned flow. Over Upstate New York and New England, this jet presence upstream over the Great Lakes will shift focus into the interior Northeast with increasing ascent generally aligned over areas downwind of Lake Ontario within the Tug Hill and Adirondacks, eventually encompassing Northern New England with the greatest impacts in-of the Green and White Mountains of VT/NH. The strongest signal for heavy snow is within the Adirondacks and adjacent North Country with probs for >4" upwards of 70-90% within those zones. Snow levels across New England will be sufficient for snowfall above 1000', however greater impacts will be mainly relegated to zones above 2000' with a strong gradient in the snow probability fields for >4" with <20% in those lower elevations with a solid 50-80% within the highest elevations of the Green and White Mountains. ...Upper Midwest... Days 2-3... Disturbance originating upstream across the Northern and Central Rockies will eject eastward by midweek within a very progressive upper flow evolution. Northern wave over WY/MT will progress east- northeast across the Northern Plains and Upper Midwest with snowfall breaking out downstream within a increasingly diffluent mid-level pattern. Light to moderate snow will transpire over north-central MT, far northern ND, and into northern MN with the greatest impactful snowfall likely to occur over the Arrowhead of MN. Modest probabilities (30-50%) of >4" exist from north- central MT on D2 and passes just north of the ND-Canada border before expanding into the Arrowhead of MN. ...Midwest to Mid-Atlantic... Days 2-3... System referenced in the section above will motion more west to east across the Plains into the Ohio Valley and Mid Atlantic by the late-D2 into much of D3. High pressure over the Great Lakes will shift eastward into the Northeast U.S by Wednesday night with cold air funneling south into the Mid Atlantic and guidance signaling a classic CAD wedge pattern initiating east of the Appalachian front. Limited blocking downstream over the North Atlantic will preclude any chance for the high pressure to hang on across the region with the continued eastward progression of the surface ridge eventually shifting off the Atlantic seaboard. Despite its positioning, time frame of interest for precipitation across the Ohio Valley and Northeast will be occurring after peak diurnal heating with wet bulb temperatures into the upper 20s and low 30s by the time precipitation breaks out across areas of IN/OH over into the Central Mid-Atlantic. Primary concern with the pattern is the low to mid-level WAA regime as the surface reflection encroaches from the west leading to a mixed ptype likely correlating to more icy (IP/ZR) hydrometeors compared to plain snow that we will see for points well to the north. Recent trends within deterministic have been for colder wedge to hang on more thoroughly for places east of the Apps. This colder trend is partly in response to greater confluence downstream across the Gulf of Maine. The highest ice probs located in- of the Laurel Highlands of PA, western MD, and the WV Panhandle due to elevation factors leaning colder at precip onset, as well as a secondary but weaker ice maxima over the northern Ohio Valley where cold air will be able to hang on through much of the event given stronger WAA pattern displaced further downstream into the Mid- Atlantic comparatively. Ice probs of >0.1" are very high (80+%) across west-central PA with the focus in-of the Laurel Highlands where even some modest 30-60% probs for >0.25" of ice accretion are forecast as of the latest WWD progs. Northern Ohio Valley doesn't carry the probabilistic maxima for >0.25" like the mountains to the east, however >0.1" ice accretion is sufficiently within the 50-80% range, very much a non-trivial depiction that will have to be monitored closely. Ice probabilities also exist within the Central Mid Atlantic east of the Blue Ridge, however a similar prob of 50-70% for >0.1" of ice exists for locations northwest of the fall line across MD extending into southern PA with lower, non-zero probs further east into the I-95 corridor and adjacent lowlands. Setup is delicate with multiple variables involved including depth of cold air, strength of LLJ providing WAA pattern, as well as timing of precip onset. These factors will determine the magnitude of ice potential in-of the Mid-Atlantic and Ohio Valley with guidance starting to hone in on specifics as we move closer to the event. While there may be some snow at onset across the Mid- Atlantic on Wednesday and throughout the Interior Northeast and New England as WAA increases on D3, probabilities for >4" remains low and confined to parts of VT and ME. Given the relatively large geographic coverage of at least 0.10" ice potential and the increasing chances for over 0.25" of ice accretion in parts of WV, MD, and PA... Key Messages have been initiated for this event. Kleebauer/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