Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 340 AM EST Sat Jan 4 2025 Valid 12Z Sat Jan 04 2025 - 12Z Tue Jan 07 2025 ...Western U.S.... Days 1-3... An amplified shortwave trough will continue to move progressively east from the Intermountain West into the Rockies today, bringing mountain snows to portions of the northern and central Rockies. The heaviest amounts are expected to fall along the northern to central Utah and the northwestern Colorado ranges. Snowfall rates increasing up to 2-3 inches an hour later this morning, are expected to result in snowfall totals of 6-12 inches, with locally heavier amounts across portions Wasatch and Uinta Mountains. Meanwhile, unsettled weather will continue in the Northwest, as a warm front associated with a shortwave trough moving over the top of an upstream ridge approaches the region today. This is expected to produce snowfall accumulations of 4-8 inches for many Cascade locations above 4000 ft, with heavier totals across the higher peaks. This system will slide southeast across the northern Rockies, bringing some additional locally heavy snows to the northern Idaho and western Montana ranges as another wave approaches the Pacific Northwest on Sunday. Followed by a sharp, amplifying ridge, moisture and the threat for additional heavy snow will be limited with this third system as it drops south from the Pacific Northwest into California and the Great late Monday into Tuesday. ...Great Lakes LES... Days 1-2... The strengthening -NAO block over Greenland and the North Atlantic will keep a large gyre-like 500mb upper low parked over southeast Canada and the northwest Atlantic over the next few days. On its western flank, it will work in tandem with strong 1040mb+ high pressure over central Canada to keep a tight pressure gradient over the Great Lakes, resulting in cold NWrly flow over the Lakes. This will prompt favorable LES on northwesterly flow along with a few upper level disturbances traversing the Great Lakes through Sunday. The Great Lakes will gradually cool, but lapse rates will remain steep enough over and downwind of the lakes to where lake- induced instability may exceed 500 J/kg on occasion. The snow belts of the Michigan U.P. and from the shores of eastern Lake Ontario to the Tug Hill will be most favored for heavy snow through this weekend. A prolonged WNW- NWrly fetch connecting a single band from lakes Superior, Huron, and Ontario will help produce the more significant amounts (1-2ft+) in Upstate NY between Syracuse and the Tug Hill. The WSSI shows Major to Extreme impacts for the Syracuse metro region and points northwest to Lake Ontario. Latest WPC probabilities are high (>70%) for at least 12 inches across this narrow LES band downwind of Lake Ontario. ...Northern High Plains... Day 1... The ongoing winter storm in the Northern Plains is a great example of mesoscale banding via strong frontogenetical forcing. Snow is underway from northwest MT to western SD as healthy mid-level divergence exists associated with an elongated sharp trough axis forecast to cross Alberta and the Intermountain West today. Closer to the surface, lower pressure over the western U.S. and strong 1040mb+ high pressure over Canada is leading to winds near the surface to be unusually strong out of the SE that is directing moisture, with origins out of the Gulf of Mexico, into the Northern High Plains. This is leading to exceptional 850mb WAA emanating out of the Southern High Plains at low level. In addition, this mean SErly flow at low- levels favors upslope enhancement through Saturday as the region becomes favorably placed on the northern flank of the 700mb low by 18Z today. Lastly, SWrly 700mb winds out of the southwestern U.S. are causing their own WAA and FGEN forcing as warm 700mb temps collide with much colder air in the Northern Plains and create a stripe of moderate snowfall. The end result is a setup for bands of heavy snowfall setting up from the heart of "Big Sky country" on south and east into western South Dakota. The WPC Snowband Probability Tracker still shows HREF guidance sporting 1-2"/hr rates early this morning across southwest MT. Areas as far south as northeast Wyoming also could see these heavy snow rates, primarily east of the Big Horns where upslope flow can enhance snowfall rates. As the best upper level dynamics shift southeast this evening, some lingering easterly flow may keep periods of snow in the forecast through the overnight hours, but snow should finally taper off by Sunday morning. WPC probabilities show a wide swatch of high chance probabilities (>70%) for at least 4" of snow from southwest Montana to northeast South Dakota. The WSSI shows widespread Minor Impacts through the Northern High Plains with some spotty Moderate Impact areas in north-central Montana (combined with snow occurring before 12z this morning). ...Central Plains through the Mid-Atlantic... Days 1-3... ...Widespread significant winter storm to develop tonight across the Central Plains today and then move through the Mid-Atlantic early next week... A closed low currently moving over the Northwest will fill as it moves towards the interior, and then weaken briefly during its dive near the Central Rockies. However, as it exits the Central Rockies tonight it will re-amplify, becoming a closed low once again into the Central High Plains. Over the past few days models have converged on a stronger and slightly north solution regarding the initial placement of this upper low, with still some differences regarding it's track and ability to maintain strength as it moves eastward to the Mid-Atlantic. Downstream confluence is quite strong and limits to what extent the precipitation shield can lift north. The GFS/GEFS remains alone in a stronger solution which leads to more potent WAA and a farther north heavy snow axis, as well as mixing issues spanning farther north than most other guidance. The answer likely lies somewhere in between, but may be closer to the ECMWF suite given latest trends. As the upper low shifts almost due east through Monday with continued deepening, it will force a strengthening jet streak aloft to help enhance ascent. This will cause the accompanying surface low to deepen slowly as it moves east, leading to a large swath of heavy snow, sleet, and freezing rain from eastern Kansas through Maryland/Virginia. While confidence is high in an impactful winter storm, some details are still in question. Downstream moisture advection will begin to intensify this afternoon as reflected by intense 290K isentropic ascent blossoming across Kansas Saturday afternoon and then continuing eastward in advance of the low. This will spread PWs that reach as high as +2 sigma across the Central Plains/Ozarks, with subtly lower anomalies into the Mid- Atlantic. The accompanying theta-e ridge is then progged to lift cyclonically into a TROWAL, especially Sunday when it becomes quite strong from KS into IN, before weakening in response to the filling of the upper low and shunting of the moisture to the east. The overlap of this TROWAL with impressive WAA will help deepen the DGZ (SREF probabilities for 100mb of depth now reach 70% in KS/MO) to increase the potential for heavy snow rates. Additionally, cross sections indicate a threat for CSI/CI where folded theta-e surface intersect with -EPV, resulting in the potential for thundersnow (through CI) and snowfall rates of 2+"/hr. The greatest risk for banded snow appears to be within the WAA from Missouri to Indianapolis as the setup matches the conceptual model for a laterally translating band, but then behind the low an extremely impressive deformation axis is likely which would have higher SLRs and the greater potential for excessive convective snow rates over northern KS. This deformation will likely weaken towards the east as the upper low begins to fill, but still strong WAA/fgen ahead of the low within the narrow translating band to the east will support impressive rates of 1-2"/hr at times. South of this band, the WAA will overrun the cold surface temps, but forecast soundings indicate an impressive E/NE dry advective flow beneath the warm nose to maintain sub-freezing wet bulb temperatures. This will support an axis of heavy sleet and freezing rain, generally from southern MO through northern KY, where sleet accumulations above 1", and freezing rain accretions of 0.25-0.5" are likely. As the system moves east and weakens a bit, there becomes more dichotomy between the northern GFS/NAM camp and other guidance, but still a heavy corridor of snow and mixed precipitation is likely from Ohio to Delaware. The wave is opening during this time, but the downstream WAA/fgen remains impressive especially in the 850-700mb layer, and a continuation of the upstream translating band is likely right through the Mid- Atlantic states. The intensity and accompanying snowfall is likely to be reduced from upstream, but could still feature intense rates and heavy snow from west to east, reflected by the most recent EFI reaching 0.8-0.9 with a co- located SoT of +2, highest in northern VA extending to the Delmarva Peninsula. The placement of this band remains in question, but where it sets up just north of the warm nose aloft, 1"/hr snows are likely at times. With the event not really ramping up until the second half of D1, WPC probabilities for snow on D1 are modest and confined to northern KS and southern NE, where probs for at least 4 inches are 40-70%. Freezing rain chances however are high (>80%) for at least 0.1" through Sunday morning across parts of central/southern KS and southern MO. By D2, snow and ice probabilities light up and stretch east across the Midwest and Ohio Valley. WPC probabilities for 6+ inches reach above 70% from central KS through just south of the Indianapolis metro area, and locally 12+ inches of snow is likely, especially in far NE KS or close to Kansas City, MO. Gusty winds and blowing snow may also lead to near blizzard conditions over parts of KS and MO. During the end of D2 and into D3 the 6+ inch probabilities race eastward, and Sunday night/Monday features a moderate risk (50-70% chance) of 6+ inches from southern OH through the Washington, D.C. metro area, central MD, and DE. South of the heavy snow, a swath of significant, even isolated damaging icing, is likely, with WPC probabilities for 0.25" reaching 50-70% from southern MO through western KY, pushing into eastern KY and the central Appalachians on D3. Some places, especially in eastern MO, southern IL, or western KY, could receive 0.5" of ice resulting in damage to trees and power lines. This system has prompted the issuance of key messages which are linked below. Snell/Weiss/Pereira ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png