Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 410 PM EST Sun Jan 07 2024 Valid 00Z Mon Jan 08 2024 - 00Z Thu Jan 11 2024 ...Southern Rockies across the Plains and through the northern Great Lakes... Days 1-3... A digging shortwave trough at the base of what is now a full-latitude trough is over Arizona and will close into a low near the Four Corners this evening and cross the southern Rockies overnight. 500mb height anomalies with this low reach -4 sigma according to NAEFS ensemble tables from eastern NM across the southern Plains Monday and into the Midwest Tuesday. This extreme low will be accompanied by increasing upper diffluence as coupled jet streaks intensify downstream of the amplifying longwave trough axis, which will result in a surface cyclone that continues to intensify along its track from the lee of the Southern Rockies late tonight through the Midwest Tuesday. Increasing forcing from the developing will bring increasing SW flow between 700-500mb over the southern Rockies which will continue to advect modest Pacific moisture which will upslope into terrain, especially the San Juans and Sangre de Cristos where moderate to heavy snow can be expected above 3000-4000 ft and Day 1 snow probs are 30-60% for an additional >6" after 00Z. Snow levels crash under the low with light snow accumulations likely into many of the valley floors of NM/CO tonight. Intense confluent and southerly flow downstream of the closed low will drive strong moist advection northward into the Southern and Central Plains and around the lee cyclone that tracks into the TX Panhandle early Monday. A TROWAL band is expected over the south-central High Plains northeast from the eastern NM/CO border through western KS. Guidance still varies with the placement of this narrow/intense swath with 12Z CAMs generally north over eastern CO/western KS than globals which are more toward the OK Panhandle. Given the increasing winds, conditions will quickly reach blizzard levels, so extra caution is advised on travel through this area. As deep layer ascent maximizes both through synoptic forcing and the warm air advection, an expanding shield of heavy precipitation will quickly develop up the Plains from OK through KS/Neb and into SD Monday morning. While the column in the Southern Plains will be too warm for snow, areas into KS/MO and points north will experience a long duration of moderate to heavy WAA snow, with snowfall rates of 0.5-1"/hr likely as fgen maximizes beneath a deepening DGZ. This leading precip, then transitions to the back side of this system, with pivoting or laterally translating deformation bands surging eastward behind the low from the High Plains then lifting into lower Missouri Valley Monday night/Tuesday, and IL/WI into MI Tuesday afternoon/night. Day 1.5 WPC snow probs for >6" are 40-80% along the eastern CO/NM border up through northwest KS. Given the WAA to wrap around snow causing prolonged snow in eastern Neb/northeast KS into southwest IA/Northwest MO where there is an expansive area for 80-90% over 6". This prolonged snow then continues across the IA/MO border with Day 2 probs for >6" over 80% across southeast IA into south-central WI which is a bit of a bump north from the previous cycle. Over toward the western shore of Lake Michigan, the proximity to the surface low center and surface water temps in the low 40s per GLERL makes for a less certain snow accumulation forecast for Chicago up to the WI border which is evidenced by a tight gradient in probabilities for the southwestern Lake Michigan shore. The system becomes more progressive Tuesday night as it crosses the L.P. of MI with Day 2.5 snow probs for >6" generally 40-60% over the northern L.P. and eastern/central U.P. ..Northwest... Days 1-3... Back-to-back potent shortwave troughs will cross the Pacific Northwest Monday night and Tuesday night making for a particularly active stretch for an often stormy region. The head of an atmospheric river (AR) reach the WA/OR coast late tonight with precip spreading over the WA/OR Cascades to the northern Rockies Monday with snow levels slowly rising from 1500ft. The core of the AR arrive Monday evening with heavy precip and snow levels reaching 5500ft over the Cascades that persist there into the night before crashing to around 2000ft as a powerful cold front crosses the Cascades late Monday night and pushes into the northern Rockies on Tuesday. Powerful onshore flow behind the front maintains heavy precipitation over the Cascades Tuesday through the passage of the next potent shortwave trough Tuesday evening. Veering flow from an approaching digging trough behind this second wave makes for more NWly flow, snow levels to dip down to 1500ft in the Cascades, and an expanding foot print to precip down into CA and the Great Basin and northern Rockies on Wednesday. More moderate rates and the increasing snow levels Monday night makes for only 40-60% probs for >6" over the WA/OR Cascades for Day 1. However, the powerful cold front, continued strong onshore flow, and arrival of the second wave brings Day 1 snow probs for over one foot into the 60-90% range for the length of the WA/OR Cascades and 49080% in the northern ID Rockies. The expanding trough over the west focuses precip into the OR Cascades which have 80-90% probs for over a foot with more moderate, 40-70% for the WA Cascades, northern and central ID ranges, ranges around Glacier NP, and the Trinity Alps/Klamath in far northern CA for Day 3. This digging trough on Wednesday will spread impactful winter weather across the rest of The West, much like the current system is, trough Thursday with another likely Plains to Midwest track Thursday night through Friday night, so stay tuned for further details. ...Blue Ridge and New England... Days 2/3... First off, bands of snow wrapping around the low near the 40N/70W Benchmark will continue to shift east across eastern New England this afternoon as the low ejects east. Snow will taper off pretty quickly this evening with potential for an additional 1" after 00Z for eastern coastal Maine. The next strong system will further intensify as it tracks over the Southern Plains on Monday and then lift northeast over the Midwest Tuesday and into the St Lawrence valley Wednesday. This track is not favorable for cold/snow across New England, but there will be some resident cold air from the strong high pressure left in the wake of the current storm (1030mb high over the Northeast Monday night). This should lead to winter weather at the onset, both near the Blue Ridge in NC/VA where Day 2 ice probs for >0.1" are 10 to 40% through this corridor and snow over terrain in NY and New England. Powerful warm air advection will allow a changeover to rain (with a lot of heavy rain, please see the Excessive Rain Outlooks). Heavy/wet snow is expected in terrain until then with Day 2.5/3 snow probs for >6" 30-50% in the highest Catskills, Adirondacks, and Greens with 80-95% in the White Mtns of NH into Maine where more than 12 inches is likely in the highest terrain. Jackson ***Key Messages for Jan 7-11 Major Winter Storm Southern Rockies through Midwest and Eastern U.S.*** --Heavy Snow Will Create Hazardous Travel The winter storm will deliver heavy snow in the Plains and Midwest on Monday and Tuesday. A heavy, wet snow will develop over the interior Northeast terrain late Tuesday. The heaviest snow totals are most likely in parts of the Midwest, where local maxima of 12 inches are likely. --Extremely Dangerous Blizzard Possible Blizzard conditions are most likely in the central Plains where wind gusts in excess of 50 MPH will lead to near zero visibility at times and extremely dangerous travel. Blizzard conditions may extend into the Midwest. --Significant River, Flash, and Coastal Flooding Widespread and potentially significant river and flash flooding are likely for sections of the central Gulf Coast and the Eastern U.S. early this week. Powerful onshore winds will lead to widespread significant coastal flooding in portions of the Gulf Coast and much of the East Coast. --Damaging Winds May Lead to Power Outages Widespread wind gusts over 50 MPH are likely in the eastern Gulf Coast, central Appalachians, much of the East Coast, and New England. Prepare for power outages. ***Key Messages for Jan 8-11 Northwest Winter Storm*** --Major Winter Storm for the Cascades Back-to-back storms will cross the Pacific Northwest Monday night and Tuesday night, with several feet of heavy snow and gusty winds across the Cascades. Snow levels will rise to around 5000 feet Saturday night with an atmospheric river but quickly fall to between 1500 and 2500 feet behind a cold front on Tuesday, leading to considerable impacts for many mountain passes with the second storm. --Heavy Snow Expected Elsewhere in the West Over a foot of snow is likely (70-90%) in the higher elevations of the Northern Rockies, with the heaviest snow falling on Tuesday and Wednesday. Snow levels falling to below 1000 feet later on Wednesday and early Thursday may lead to some snow accumulations in the valleys also. --High Surf, Gusty Winds on the Pacific Northwest Coast Renewed periods of high surf and gusty onshore winds are expected early this week on the coastlines of Washington and Oregon. --Low Pressure System to Move over Central U.S. The storm system is forecast to dig across the Western U.S. through Thursday and reach the Plains late in the week. People in the Plains and Midwest should prepare for another winter storm and stay tuned for updates.