Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 334 PM EST Tue Jan 16 2024 Valid 00Z Wed Jan 17 2024 - 00Z Sat Jan 20 2024 ...Maine... Day 1... A strengthening coastal low will be near Downeast Maine this evening but then lift rapidly northeast into the Canadian Maritimes by Wednesday morning. Significant ascent will persist across Maine as this low departs through PVA/height falls ahead of the trailing shortwave and modest LFQ diffluence as the upper jet streak pivots to the northeast. Moisture will continue to wrap onshore within cyclonic flow around the surface low, with PW anomalies progged to remain above +1 sigma early in the period before waning rapidly. This will result in moderate to heavy snow, especially in far northern Maine, with additional accumulations exceeding 4 inches likely as reflected by WPC probabilities reaching 50-80%. ...Prolonged Great Lake Effect Snow... Days 1-3... Expansive upper low centered southwest of Hudson Bay this morning will gradually elongate eastward through the end of the week, but maintain a large gyre over southern Canada which will keep cyclonic flow in place over the Great Lakes. Around this closed low, spokes of vorticity are likely to shed south and then east, enhancing ascent at times across the Great Lakes, but also re-energizing W/NW flow through the region as a cold front crosses the lakes from NW to SE on Wednesday night, with a secondary reinforcing trough progged to drop southward by early Friday. The Great Lakes are almost entirely ice-free according to GLERL, with water temperatures ranging from around +3C over Lake Superior to as warm as +8C within Lake Ontario. The persistent CAA across these still warm lake temperatures will result in steepening lapse rates to drive inversion depths to potentially above 10,000 ft, highest east of Lake Ontario, which will support lake-induced instability of 500-750 J/kg. Additionally, forecast soundings indicate a favorable cross section of ascent into the DGZ, further evidence of potential for heavy snowfall. With 850mb temperatures wavering at times between -10C and -25C, it is likely that any bands of lake effect snow will produce heavy rates of 1-2+"/hr, and although the multiple boundaries swinging through the region during the forecast period will result in wavering bands due to differing wind directions, any time a band persists it will result in heavy snow accumulations. For D1-D2, WPC probabilities for heavy snow maximize in the favored N/NW LES belts across the U.P. of MI, and east of both Lakes Erie and Ontario. WPC probabilities for more than 6 inches reach 50-80+% both days in these areas, and 2-day snowfall could add up to several feet in some areas, especially in the Tug Hill Plateau. By D3, the flow shifts to become more northerly, focusing the heaviest LES and greatest probabilities for more than 6 inches of snow near Cleveland, OH south of Lake Erie, and on the S/SE side of the long fetch of Lake Michigan. ...Pacific Northwest through the Central Rockies... Days 1-3... An active pattern will continue to bring rounds of heavy precipitation to the northwest CONUS, with heavy icing and heavy snow both expected through the end of the week. The period begins with a closed 500mb low moving onshore near the WA/OR border. This feature is fast moving and will weaken into an open wave over the northern Great Basin by Wednesday afternoon, but will provide considerable forcing for ascent as it races southeast in conjunction with the LFQ of a potent 150+kt Pacific jet streak. Downstream of this low, confluent mid-level flow combined with the aforementioned jet streak will surge IVT onshore as an atmospheric river with moderate (50-70%) probabilities of IVT exceeding 500 kg/ms tonight into Wednesday. Behind this shortwave, 700-500mb flow remains zonally oriented into the Pacific Northwest which will maintain efficient moisture advection onshore, and a prolonged period of IVT exceeding 250 kg/ms is forecast through Saturday (D4) along the coast by both the GEFS and ECENS ensemble systems. This persistent IVT will surge PWs to +1-+2 sigma across much of the western CONUS, manifesting as ares of heavy precipitation within the regions of most significant ascent. While the most intense forcing is likely D1 before shortwave ridging blossoms over the region on D2, another round of precipitation is likely D3 as another, potentially more potent and negatively tilted trough, approaches the coast of CA/OR by the end of the forecast period. Although the precipitation will be driven by accompanying WAA off the Pacific Ocean, it will encounter an extremely cold airmass anchored across the northwest, characterized by ECMWF EFI of -0.8 to -0.9 both Wednesday and Thursday. This cold air will remain entrenched at the surface as high pressure reinforces down from Canada through the period, with low-level easterly flow maintaining this cold air nearly to the coast, especially along and within the Columbia Gorge. This setup appears extremely favorable for heavy freezing rain from the Willamette Valley through the Columbia Basin, including the Portland, OR metro area and the Columbia River Gorge. Ice accumulations are likely to be significant to damaging, and WPC probabilities for more than 0.25" are as high as 70% in this area, with locally more than 0.75" of ice possible within the Columbia Gorge. More modest but still significant ice accretions are likely across much of the western Lowlands of WA/OR where WPC probabilities for more than 0.1" reach 10-30%. After a brief respite from icing on D2 (although some light icing may persist), another round of freezing rain is forecast for Thursday night into Friday as the next wave of precipitation spreads onshore. WPC probabilities D3 for more than 0.1" of ice reach 50-70% in the Columbia River Gorge and southern Columbia Basin. Snow levels west of the Cascades and into the Great Basin/Central Rockies will climb to 3000-4000 ft D1-D2, and then rise more steadily to around 6000 ft D3, with these higher snow levels expanding well eastward on Friday as well. Above these levels, however, heavy snow accumulations are likely. On D1, WPC probabilities for more than 6 inches of snow are above 80% from the Cascades through the Northern Rockies and as far south as the Wasatch of UT and Park Range of CO. Locally 1-2 feet is possible in the higher terrain of most of these ranges, with 3+ feet likely in the Cascades. On D2, the southeast motion of the best ascent pushes the highest WPC probabilities for more than 6 inches of snow from the Northern Rockies through the NW WY ranges and into the CO Rockies, where again locally 2+ feet is possible in the higher terrain. By D3 most of the activity winds down, but ramps back up in the WA Cascades where WPC probabilities for more than 6 inches reach 30-50% once again. 3-day total snowfall could reach 4-6 feet in the higher terrain of the Cascades. ...Northern/Central Plains... Days 2-3... Broad cyclonic flow expanding across the eastern half of the CONUS will result in upstream ridging across the Pacific Northwest by Thursday evening, leaving NW to SE mid-level flow from the Intermountain West into the Central Plains. A lobe of vorticity is progged to advect southeast within this fast flow, and the accompanying shortwave will likely weaken due to increased shear as it approaches the Central Plains after 00Z/Friday. Despite the modest amplitude of this feature, forcing for ascent will be substantial due to the PVA/weak height falls overlapping with the diffluent LFQ of a potent 150kt upper jet streak diving through the flow. This may yield a shallow wave of low pressure moving across WY, with modest downstream WAA along a leading warm front surging moist isentropic ascent into NE/SD late D2 into D3. While total moisture and ascent are likely to be modest, there may be some enhancement due to the fgen response to the favorably placed upper jet streak above the low-level fgen along the sloped warm front. Regional soundings are quite cold and feature a deep DGZ characterized by SREF probabilities for 100mb of depth reaching above 50%. This could result in a fast moving band of very high SLR snow which could accumulate to several inches as it advects east, and WPC probabilities for more than 4 inches of snow are as high as 10-30% near the SD/NE border, with lower probabilities continuing into western MO D2.5. ...Mid-South... Days 2-3... A weak shortwave and accompanying PVA will slide southeast from the Missouri Valley late Thursday into Friday /late D2 into D3/ bringing weak height falls and PVA as the shortwave fills with time. Modest moist advection on increasing isentropic ascent downstream of this feature will surge PWs favorable to support at least light precipitation, but PW anomalies are progged to remain less than +1 sigma according to NAEFS, indicating generally near normal moisture for the time of year and location. The near-normal moisture being acted upon by modest deep layer ascent will result in a stripe of light to moderate precipitation from the lower MS VLY through the western Carolinas. Regional soundings indicate a warm nose slightly above 0C may exist within the warm/moist advection, but more notably is a lack of sufficient RH within the elevated DGZ. This suggests that a swath of light freezing rain is possible as this wave traverses eastward, but WPC probabilities for 0.1" of ice D2 and D3 are less than 5%, highest in western TN. ...Ohio Valley, Mid-Atlantic, Northeast... Day 3... A fast moving shortwave dropping southeast upstream of the broad trough across the eastern CONUS will amplify as it shifts towards the Ohio Valley Friday morning, and then maintain its amplitude during its trajectory eastward across the Mid-Atlantic. The flow is fast, and the shortwave is of modest intensity, but it will be efficiently topped by a potent 170+kt jet streak diving through the trough to enhance deep layer ascent. At the same time, a cold front will be making its way eastward from the Great Lakes to the Mid-Atlantic states, and it is possible the associated fgen along this front could be enhanced by the upper jet streak pivoting overhead. This will likely enhance ascent and will result in a surface low pressure developing near the Carolinas and then racing east-northeast into Saturday /D4/. PWs downstream of the best ascent are expected to be modest within the generally zonal and fast flow, although some enhancement towards +1 sigma may surge northward into VA as reflected by the NAEFS ensemble tables, but this will likely be dependent on the intensity of the surface low to drive more robust theta-e advection northward. The GFS/ECMWF and their associated ensemble clusters have trended weaker/farther southeast today, but the CMC and its ensemble remain more intense and closer to the coast. The threat for heavy snow appears to be a little lower than from previous model cycles, but uncertainty is still high, and regional soundings indicate support for high SLR snow due to a cold column and SREF DGZ depth probabilities for 100mb reaching 10-30%. WPC probabilities for more than 4 inches of snow on D3 are modest but expansive, reaching 10-30% in a west-to-east oriented swath from eastern Indiana through Long Island, although probabilities have decreased today. The exception is along the WV Appalachians where colder temperatures and some upslope flow should result in efficient snowfall accumulations, and WPC probabilities for 4+ inches are above 50% in this area. Weiss ***Key Messages for Northwest Winter Storm*** --A "1-2 Punch" of Pacific Storm Systems An upper low ushers Pacific moisture through the Pacific Northwest today and into the Northern Rockies and Plains mid-week, while a second storm system directs another round of Pacific moisture towards the Pacific Northwest for both Thursday and Friday. --Hazardous Ice Accumulations into Wednesday A prolonged freezing rain event will unfold today through Wednesday with a 40-80% probability for more than 0.25" of ice in the Willamette Valley, Portland Metro, and Lower Columbia Basin. Maximum ice amounts around 1" are possible for the western Columbia Gorge. This will likely result in impacts including power outages and tree damage. --Heavy Snow Wednesday into Thursday Snow overspreads the interior Northwest tonight and into the Northern Rockies & Plains for both Wednesday and Thursday. Snowfall rates exceeding 2" per hour are expected in parts of the Cascades and northern Rockies Wednesday into Thursday.