Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 340 AM EST Thu Jan 2 2025 Valid 12Z Thu Jan 02 2025 - 12Z Sun Jan 05 2025 ...The West... Days 1-3... Persistent troughing across the Pacific will maintain broad SW flow into a modest ridge along the coast until Saturday when a pronounced closed low digs eastward towards OR forcing a trough to move inland to the Great Basin. Periodic impulses shedding through the flow will move into a region of greater mid-level divergence D1, leading to broad ascent and increasing precipitation coverage. For today, this will be most pronounced along a low-level warm front/baroclinic gradient lifting east/northeast from OR into the Central and Northern Rockies. This will result in a swath of precipitation, primarily as snow, but with snow levels rising to as high as 6000 ft in southern OR/northern Great Basin, with a sharp gradient to less than 1000 ft into the Northern Rockies and Columbia Basin. Heavier snowfall is likely at the very end of D1 into much of D2 as more substantial moisture and ascent spread onshore Friday into Saturday. This more enhanced forcing/moisture is associated with the closed low moving onshore and causing the entire trough axis to pivot eastward. Impressive lift will be aided by the LFQ of a modest, but well placed, upper jet streak, and there is likely to be strong deep layer lift expanding across much of the Pacific and Interior Northwest Friday night, shifting as far as the Northern High Plains by Saturday night. The most impressive ascent will again be collocated with a secondary warm front lifting northward from OR to Canada on Friday, leading to enhanced WAA/isentropic ascent within the broad synoptic lift regime. While overall omega is modest, a swath of moderate to heavy snow is likely from the mountainous areas of northern CA through OR, WA, and into the Northern Rockies, with snow levels generally rising to 4000-6000 ft. This is reflected by WPC probabilities which suggest a high risk (70-90%) for more than 6 inches of snow in the Cascades and Northern Rockies, with more general onshore flow leading to upslope snow across the typical terrain features D3 from the Sierra, Cascades, and into the Northern Rockies again. Additionally, the warm advection overrunning cold high pressure centered over Saskatchewan is progged to leave low-level cold easterly flow from the Columbia basin into the Columbia Gorge. Although eventually this will be scoured out, the setup appears favorable for a prolonged period of moderate freezing rain in the eastern foothills of the Cascades and into the Columbia Gorge/Basin. The heaviest icing is expected along the Hood River Valley and into the Gorge which will take a longer time to scour out the cold air, and this is reflected by WPC probabilities that reach as high as 30% for 0.1" of ice accretion. ...Great Lakes and Northeast... Days 1-3... Surface low pressure moving across eastern Maine early this morning will usher in a long duration lake effect snow (LES) event into the weekend. This surface low will be pushed northeast in response to a potent vorticity maxima rotating through the negatively tilted trough, which eventually swings northwestward towards the Hudson Bay as a larger upper level low and stalls. This allows for strong and persistent cold northwesterly flow over the currently ice-free Great Lakes and substantial snowfall likely associated with the LES, especially D1 and through D2 as lake-induced instability climbs above 500 J/kg. The pattern will support heavy LES in the favored W/NW snow belts from the U.P., across the L.P., and then downstream to east of Lakes Erie and Ontario, with a favorable upstream moisture connection from Lakes Superior and Huron enhancing LES off of Lake Ontario. LES is expected to be most widespread and heavy through Friday, when 1-2"/hr snows, possibly greater at times, cause WPC probabilities to be high (>70%) for 6+ inches downstream of most Lakes, and reach above 90% for 8+ inches east of Lake Ontario D2. Total snowfall of multiple feet is likely, with the highest along the Tug Hill Plateau. LES will continue through D3 with just slightly lesser intensity and coverage as reflected by WPC probabilities for an additional 6+ inches being above 70% in just narrow channels near the Chautauqua Ridge and the Tug Hill Plateau. ...Midwest through Central Appalachians and Mid-Atlantic... Days 1-3... An inverted surface trough sandwiched between a retreating high pressure over the Southeast and an incoming strong Canadian high pressure from Alberta will become a focus for a developing wave of low pressure and an associated stripe of snowfall from west to east. The sharpening of this trough will initially be in response to a subtle vorticity lobe swinging through the broad trough across the east, ejecting from the High Plains into the Upper Midwest today. Immediately following this impulse, a potent jet streak will drop southeast into the Northern/Central Plains while amplifying to 150 kts, providing favorable overlap for deep layer ascent. This will stem cyclogenesis along the inverted trough, with increasing downstream warm/moist advection surging moisture along the 280-285K isentropic surfaces into the Upper Midwest. This WAA will help deepen the DGZ to the east, while 925-850mb fgen increases, although only modestly. This will result in a stripe of moderate snow from Iowa this morning through the Ohio Valley, where at least brief snowfall rates to 1"/hr are possible, leading to a general 1-3" of snowfall accumulation, with locally up to 4" possible. Farther downstream, as the low coalesces to be more pronounced and has stronger frontal features accompanying it, a period of strong upslope flow will occur in its wake across the Central Appalachians. The DGZ is shallow and thin, but pronounced lift into it will result in a period of heavy snow, especially late Friday into Saturday, from the Laurel Highlands through southern WV. During this time, moisture will be enhanced as well from upstream Great Lakes connection, enhancing the potential for heavy accumulations, and WPC probabilities D2 are high (>70%) for 6+ inches of snow, leading to local maxima around 12 inches by the end of the forecast period. Finally, although total accumulations are expected to be minimal, some snow showers and snow squalls may crest the Appalachians and move across the Mid- Atlantic states Friday afternoon as the vorticity max dives southeastward. The late afternoon timing of this vort max allows for steep lapse rates from the sfc all the way up to 500 mb and the potential for snow squalls to develop and potentially contain 1-2"/hr snowfall rates. Surface temperatures in the Washington D.C. to Baltimore region will start out above freezing, but areas just north and west may be right around the freezing mark. Either way 850 mb temps of -5 to -6C will allow for the heavier precip rates to quickly fall as snow and surface temperatures to quickly crash as well. This may lead to potentially hazardous travel and rapidly deteriorating road conditions for the Friday evening commute depending on the strength of the snow showers/squalls and where exactly they impact in the Mid-Atlantic. ...Northern High Plains... Day 3... A stripe of heavy snowfall is becoming more likely Saturday from central MT through western SD. This snowfall will be associated with a slowly advancing warm front topped by increasing divergence downstream of a potent shortwave digging into the Four Corners region. While jet dynamics are weak during this time, the downstream moist advection on impressive 280-290K isentropic ascent should expand precipitation in a NW to SE fashion late Friday night and through Saturday. Robust 850-700mb fgen into this moistening column will help intensify ascent resulting in heavier snow rates, but at this time the intensity of this snow is still uncertain due to modest DGZ depth probabilities and a lack of ideal overlap between the best ascent and the DGZ. Still, a band of moderate to heavy snow is becoming more likely. WPC probabilities have increased to 50-80% for 4+ inches across north-central MT, with 40-50% chances for at least 6 inches by Saturday night. ...Central Plains through the Mid-Atlantic... A significant winter storm is becoming more likely late Saturday through Monday, with the onset of light freezing or frozen precipitation beginning Saturday evening across Kansas. At this, time, WPC probabilities for significant snow are minimal through the end of D3 (Sun morning), but heavy snow is expected to be developing at that point and continuing to into D4 and beyond. The upper low swinging through the Great Basin on Saturday is expected to eject into the central Plains Saturday night while closing off and strengthening within a developing favorable duel jet structure. Strong WAA will develop and blossom the precipitation shield across the central Plains and Mid-Mississippi Valley Saturday night while also promoting a warm nose to lift northward. This creates a favorable setup for mixed ptypes and wide swath of sleet/freezing rain. Guidance has trended slightly north with QPF and the mix zone in recent runs as is common for WAA regimes with a surface High remaining displaced to the NW. It's possible the warm nose trends farther north as hires CAMs become available, also impacting areas downstream by D4. WPC ice probabilities have increased by the end of D3 and are 20-40% for at least 0.1" of freezing rain accretion from central KS to southern Missouri. For this system, WPC has initiated Key Messages which are linked below, and more information can be found in the WPC extended range discussion as well. Snell/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