Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 355 AM EDT Mon Mar 17 2025 Valid 12Z Mon Mar 17 2025 - 12Z Thu Mar 20 2025 ...The West... Days 1-3... Shortwave trough positioned just west of northern CA at 12Z Monday will deepen into a closed mid-level low as it advects onshore and drops subtly southeast Monday morning. The core of this low is progged to track across the northern Great Basin this evening, but a strong lobe of vorticity immediately in its wake will dig farther south, amplifying the trough axis into southern CA by the end of D1. Downstream of this trough, squeezed mid-level flow will produce moisture confluence, lying directly beneath a strengthening and meridionally advancing subtropical jet streak. This setup will support impressive moist advection, reflected by PWs surging to above the 90th percentile across CA and the Great Basin according to NAEFS ensemble tables. Within this moistening column, and beneath the deep layer ascent, a wave of low pressure will move southeast along a baroclinic gradient/cold front that will be driven eastward by the parent trough evolution. This will maintain the focus of greatest moisture just south of the front, but will drive some enhanced mid-level isentropic ascent above and behind the boundary. This additional lift, combined with periods of upslope flow into terrain features, will result in widespread precipitation D1 from the Cascades southward through the northern CA terrain and into the Sierra, as well as points well east across the Great Basin and into the Wasatch/Tetons/Wind Rivers. Snow levels will vary considerably across the West during this period due to the passage of the cold front. However, within the region of greatest moisture/ascent and heaviest resultant snowfall, snow levels should be generally 4000-6000 ft from the Sierra through the Great Basin and into the Central Rockies. Snow levels crash behind the cold front, allowing for at least moderate snowfall but with more substantial pass level impacts in its wake. WPC probabilities for more than 6 inches of snow are moderate (30-50%) D1 across the Sierra and WA Cascades, but high (>70%) across the Wasatch and into portions of western WY. Additionally, along this front, impressive 0-2km fgen and increased low-level RH will move into a corridor of elevated instability (NAM SBCAPE 100-400 J/kg). This will occur within a region of high SnSq parameter above 2 from eastern NV Monday evening through UT, WY, and CO by Tuesday evening. This setup could produce convective snow showers or even snow squalls, which is additionally supported by simulated reflectivity in the high-res guidance. While any snow squalls won't produce tremendous snowfall amounts, dangerous travel is likely through snow-covered roads and rapidly changing visibility. This first system moves into the Central Rockies during D2, becoming a significant winter storm in the Plains, and leaving a brief respite much of D2 across the West within this otherwise continued active pattern. This break will be short lived, however, as yet another strengthening mid-level trough moves across the Pacific and approaches the Pacific Northwest D3, likely coming onshore Wednesday evening. Although the guidance varies considerably with timing of the associated mid-level waves within this next trough, they all agree that it will be potent, and potentially negatively tilted, as it moves onshore during D3. The downstream affects of this will be increased synoptic lift through mid-level divergence, height falls, and intensifying upper diffluence, all to pivot renewed moisture and forcing back onshore. The antecedent airmass is cool, so snow levels will are only expected to rise to around 3500 ft within the core of the strongest WAA, and otherwise be generally 2000-3000 ft, suggesting travel across area passes will again become problematic. This feature will likely be transient, but have sufficient moisture to produce heavy snow once again from the Olympics and WA Cascades southward into the Sierra, with some spill over occurring before the end of the period as far east as the Salmon River/Sawtooth/Northern Rockies. WPC probabilities for 6+ inches of snow on D3 feature a high risk (>70%) across the Olympics, Cascades, and Shasta/Trinity region of northern CA< with lower probabilities below 50% continuing into the Sierra and east across parts of the higher elevations of Idaho. ...Central Rockies, Central Plains, Upper Midwest... Days 1-3... ...Another significant winter system likely to bring blizzard conditions to portions of the Plains and Upper Midwest... This system manifests as two waves of low pressure, a modest lead wave, followed immediately by the more impressive cyclone. These will generate from an anomalous trough which will amplify across the Four Corners Tuesday /D2/ leading to impressive height anomalies that are progged to fall below the 1st climatological percentile between 700-500mb across the Central Plains by Wednesday. This deepening and intense amplitude will yield strong downstream ascent through height falls and mid-level divergence, and as the subtropical jet streak amplifies it will begin to shift almost due poleward, placing intense LFQ diffluence atop the region of greatest height falls. Together this will produce an environment extremely favorable for a rapidly deepening cyclone in the lee of the Rockies. However, before this occurs, a weaker wave is progged to develop near SE WY Tuesday morning in response to more modest height falls, but in the presence of increased baroclinicity as a cold front drops southward from the Northern Rockies/High Plains, aided by the ageostrophic response (fgen) of the RRQ of a departing jet streak into the Great Lakes. This first low will eject rapidly east and weaken, but the strong ascent and plentiful moisture will create heavy snow across the Central Rockies, especially in WY where low-level upslope ascent into the best frontogenesis will produce heavy snow rates. WPC probabilities for more than 4 inches of snow are high (>90%) across much of NW WY and into the Big Horns, with some elevated probabilities for 1-2" extending into the lower terrain of eastern WY as well. During D2, however, is when the significant low develops. As the synoptic pattern evolves as described above, lee cyclogenesis will occur, and this low will be located beneath extremely favorable synoptic ascent for rapid surface height falls, and nearly all the ensemble groupings suggest SLP falling to the 980s Wednesday morning over IA. There is generally good clustering of the ensemble member low-pressures, but D3 clusters still suggest that the GEFS is a bit deeper and faster with the progression, although the spread appears to have decreased in the past 24-hrs. This leads to high confidence in a rapidly strengthening low pressure, with moisture steadily increasing as 300K isentropic ascent surges theta-e northward and into the system, eventually lifting into a TROWAL to provide moisture and elevated instability. As has been the case with the past few of these March systems, the antecedent airmass is warm, and the moist/warm advection funneling in the higher PW air will cause the precipitation to begin as rain in many areas, especially outside of terrain and into the Plains. However, this should have minimal affect on snowfall accumulations, as extreme dynamic cooling combined with ageostrophic flow into low and cold advection as the low moves east will cause a rapid p-type transition from rain to snow, with snow likely becoming quite heavy within the deformation axis NW of the low Wednesday. Snowfall rates that may reach 1-2"/hr (or more? some signals for CI in a deep and saturated DGZ over IA indicating possible thunder- snow) will overcome the warm ground quickly. Thus, the limiting factor to snowfall amounts appears to be more the speed of this system as it remains progressive, and CIPS analogs indicate the 75th% is only generally 4-8", higher into WI/MI where leading jet- streak snowfall may add on to amounts before the main system. This is additionally reflected by WPC probabilities for 4+ inches that are below 30% D2, but increase to greater than 70% from western IA northeast into the U.P. of MI D3. These probabilities have trended upward, and locally 6+ inches of snow is likely within this band. Finally, despite snowfall that may be modest in many areas outside of the Upper Midwest, the intense winds driven by 850mb northerly flow of 60 kts will produce strong gusts for which DESI indicates has a 90% chance of exceeding 35 mph, and even a 50% chance for 50 mph across the Plains. This has led to high (>70%) probabilities for moderate blowing snow impacts from WSSI-P, and despite snow that may start as the heavier/wet variety, will transition to higher SLR and broken aggregates leading to likely blizzard conditions in some areas, especially from central Kansas northeast into southern Minnesota. By the end of the forecast period, this low will be well into Ontario, but lingering snow and gusty winds are expected to still be ongoing across MI, with some lake-enhanced snow across the U.P., western L.P. and maybe even into northern Indiana into early D4. 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