Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 421 PM EST Tue Feb 11 2025 Valid 00Z Wed Feb 12 2025 - 00Z Sat Feb 15 2025 ...Mid-Atlantic... Day 1... ...Winter Storm Ongoing Across the Mid-Atlantic with Impacts from Significant Icing and Heavy Snow. Key Messages linked below... A winter storm ongoing across the Virginias has already produced multiple inches of snow from southern West Virginia through central Virginia, with icing reports along the Blue Ridge in North Carolina. Local storm reports show the highest snowfall amounts as high as 12 inches in the Allegheny Highlands of WV/VA. Meanwhile radar shows the heaviest snowfall rates in a swath across north-central Virginia into southern Maryland where snowfall rates over an inch per hour are also being reported. This storm is being caused by intensifying warm advection moving over a stubbornly cold air mass caused by a 1030 mb area of high pressure centered off of Nantucket. Thus, the predominant flow is easterly. This flow off the Atlantic will become increasingly warm with time as the flow turns more southeasterly. Any warming is being countered by the aforementioned heavy snow rates, leading to dynamic cooling by the sheer volume of the heavy falling snow. Meanwhile in the mid-levels (850 mb) the flow will be south to southwesterly throughout the event, though increasing in intensity through this evening. In the upper levels, all of the lift will be driven by the right entrance region (RER) of a 150+ kt jet over New England. It's important to note that other than the jet, there will be very little if any additional upper level support from any shortwaves. Therefore, most of the support will be from warm advection of a warm air mass over the colder air mass that remains persistent near the surface. Weak 700 mb waves will track along the plume of moisture which extends from east Texas to the Delmarva. These will lead to localized increases in intensity of the precipitation through tonight. The rain-snow line is unlikely to move much from its current position across much of southern Virginia through tonight. A weak surface low/trough will from late tonight (around 06Z) off the Virginia coast which will pull the precipitation plume east into the Atlantic and largely end the wintry side of the event from west to east. There is some disagreement in the high resolution models as to when this will happen however, as a secondary weaker wave may keep light frozen precipitation ongoing over northern VA and the Delmarva as late as sunrise Wednesday morning. Ultimately the snow event will end once drier air and a lack of mid-level disturbances can overcome an otherwise saturated air mass that stands ready to produce additional light precipitation at a moment's notice. While dynamically this snowfall event will be the weaker of the two storms (the other described below over the central Plains to the Great Lakes), it may be the longer-duration of the two for portions of the Virginias, so the highest snowfall accumulations between the two storms could very well be similar. To the south along the rain-snow line, there is expected to be a long-duration ice event due to freezing rain for portions of the Appalachians from central West Virginia through the Blue Ridge and Shenandoah Valley from Roanoke, VA south into far northwestern North Carolina. As mentioned above, strong warm advection in the mid- levels will override colder air near the surface, unlike further north where the warm air will fail to warm temperatures above freezing in the midlevels, further south the warmer air will succeed at both eroding the cold air enough to warm temperatures in the mid- levels above freezing, the cold air will remain stout at the surface, even as a shallow layer. This will set up the potential for prolonged icing. Most of the precipitation will occur with this first storm that is ongoing now. Some areas around Roanoke have started the storm as snow but will transition to freezing rain as the warm air aloft in this area has some success at impinging northward tonight. Icing for many may exceed a quarter inch, and could approach a half inch through Wednesday morning in the areas where both the surface cold air and precipitation persist the longest. Late tonight into Wednesday, most of the central Mid-Atlantic will be dry or light drizzle with pockets of freezing rain lingering over the VA Piedmont and on ridges which would further add to ice impacts. WPC probabilities are moderate (50-60%) for 1/4 inch of ice along I- 81 south of Roanoke south to extreme northwestern North Carolina, though amounts drop to very low (5-10%) for 1/2 inch of ice. On the snow side, probabilities are moderate to high (60-70%) for at least 6 inches of snow north and west of Roanoke, some of those have already been realized). Farther east, those probabilities are low to moderate (30-40%) from DC south along I-95 through Fredericksburg, VA. ...CO Rockies/Central Plains/Midwest/Great Lakes... Days 1/2... ...Winter Storm expected starting over Kansas/Nebraska spreads northeast across Lower Michigan through Wednesday Night... Troughing over the northern Great Basin will dip into the central Rockies tonight with broad WSW flow across CO. This will favor an expansion of snowfall over the Medicine Bow mountains and into the San Juans/Sangre de Cristos where upslope enhancement will wring out several inches of snow area-wide. Snow probs after 00Z for >6" are 40-60% above about 8000ft in portions of the Sawatch and San Juan Mtns. Downstream, a winter storm is developing over the Central Plains while the one over the Mid-Atlantic one persists into tonight. For forcing, there will be some notable differences between the two storms. While there will certainly be some warm advection bringing Gulf moisture north into the central Plains and Midwest, this storm will be much more dynamic in the upper levels. A broad longwave trough over the northern Plains will begin to shift eastward as a potent shortwave trough rounds the southern periphery of the trough. This will enhance the downstream jet, which will intensify the forcing needed for surface cyclogenesis from the southern Plains and eventually into the Ohio Valley. Thus, the expected swath of snowfall associated with this low will be associated with a well- developed surface low in much more of a "classic" form. With the intensifying and fast-moving low providing the primary forcing for the snowfall, expect heavy snow to develop across central Kansas tonight, and track northeastward across far northwestern Missouri, central and eastern Iowa, far northern Illinois, southern Wisconsin, and across much of the Lower Peninsula of Michigan through Wednesday night. The low will be plenty strong enough to produce bands of heavy snow embedded within the comma- head region as it picks up Gulf moisture to its south...but due to the fast moving nature of the low, the duration of any heavy snow, especially outside of any banding will be shorter than the current Mid-Atlantic storm. On Wednesday, as the storm moves into the left exit region of a southwest-to-northeast oriented jet, the snow will spread into the Midwest and upper Lakes. With lake-enhancement on the northeasterly winds, there may be locally heavier snow and higher accumulations southwest of Lake Michigan, which may include the Chicago and Milwaukee metros. Of course, this will be highly dependent on where any embedded bands associated with the storm can line up with the lake, resulting in the localized enhancement. Blowing and drifting of snow in the strong winds is likely right along the lake shore. Additional enhancements are possible along the Lake Huron shoreline into Wednesday night for the same reason...a rather unusual direction for Lake Huron enhanced snowfall. On the southern side of the comma-head region of the low, enough warm air is expected to move in around the center of the low to result in an area of mixing/freezing rain just south of the swath of snow from central Oklahoma tonight northeast across much of southern Missouri, including the St. Louis area, then extending northeast across central Indiana, including the Indianapolis area, and into far northwestern Ohio, including the Toledo area. Each of these areas, and of course those in between, have the potential to see some light to moderate icing, up to 1/4 inch. Once again the timing of any icing should be shorter than areas further east, which should limit any icing accumulations and thus, the magnitude of any impacts. WPC probabilities for ice are moderate (40-60%) for 1/10 inch of accumulation from northeast of Indianapolis northeast through Toledo, OH. Probabilities are low (5-30%) for 1/10 inch of ice from south of St. Louis northeast through Indianapolis. For snowfall, probabilities for 6 inches or more are moderate (40-60%) for much of northern Kansas through south-central Iowa, lower (20-40%) through eastern Iowa and northwestern IL, then increase back to the moderate category (40-70%) along the lakeshore in Wisconsin due to lake-enhancement from Milwaukee south to northern Chicago, as well as for much of eastern Michigan north of Detroit. ...Sierra Nevada and much of the West... Days 1-3... Day 1... An expansive, positively-tilted trough lingers over the west into Wednesday with northwest flow persisting over CA. A shortwave impulse rides along the central CA coast tonight into Wednesday producing areas of light to locally moderate precip with snow levels around 3000ft in the southern Sierra Nevada and around 4000ft in the SoCal transverse and peninsular ranges. Day 1 Snow probs for >4" are 30-50% in the southern Sierra Nevada and the highest SoCal ranges such as near Big Bear. Days 2-3... A stronger low shifts toward the OR/CA border Wednesday night, opening into a trough as it drifts east over the Great Basin through Friday. A strengthening and increasingly northerly jet shifts toward the West Coast Thursday night. An atmospheric river with IVT around 500 kg/m/s shifts over the CA coast Wednesday night through Thursday. A wave of moderate to topographically enhanced heavy precip shifts east with the accompanying warm front Wednesday night through Thursday. Snow levels over the Sierra Nevada are initially 3000ft Wednesday evening, but quickly rise to 6000ft by Thursday morning. Both Days 2 and 3 snow probs for >18" are high along the length of the Sierra Nevada with 4-6ft forecast for the High Sierra through Friday. Farther north into the Shasta/Trinity region and Oregon, pronounced cold air will be slow to retreat, with snow levels generally rising only from 2000ft to 4000ft through Thursday with lower precip rates than in the core of the AR over CA. Day 2.5 snow probs for >12" are 50-80% for the Shasta/Siskiyou/Trinity Mtns and Day 3 snow probs for >8" are 40-70% for the OR Cascades. The lingering cold air brings light lowland snow potential for much of the PacNW with an appreciable threat for freezing rain mainly Thursday over northwest OR. Day 2.5 ice probs for >0.1" are 30-60% for mainly the Coast Ranges west of the Willamette Valley. Moisture quickly spreads east throughout the Intermountain West Thursday through Friday ahead of the slow moving trough axis. Day 3 snow probs for >8" are above 70% for the Wasatch and Tushar Mtns in Utah as well as for western slopes of the CO Rockies and around 50% for the western Sawtooths and Tetons into southeast ID. ...Northeast... Day 2... The system coming from Michigan Thursday morning will bring widespread precipitation from SW to NE over the Northeast. Downstream of this system, moist advection will maximize in response to low-level WAA on S/SE flow overrunning a retreating high pressure that will try to wedge back to the west into New England. This will result in an expansion of wintry precipitation Wednesday night through Thursday with a wintry mix expected across the Northeast with the far northern NY border east of Lake Ontario and northern Maine getting the most snow. Day 2 snow probs for >4" are limited to the Tug Hill, Adirondacks, Greens, Whites, and interior Maine with 40% for >8" over the Thousand Islands area and 60% over northern Maine. Day 2 ice probs for > 0.1" are 20-30% in the Poconos, Catskills, and Berkshires into the Litchfield Hills. Jackson/Wegman ...Winter Storm Key Messages are in effect... Central Plains through Midwest Winter Storm tonight into Thursday: https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png Mid-Atlantic Winter Storm continuing tonight for snow/Wednesday night for ice: https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png Key messages are also in effect for the Atmospheric River into California Wednesday night into Friday and can be found on Weather Prediction Center social media.