220 FOUS11 KWBC 021904 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 304 PM EDT Wed Apr 2 2025 Valid 00Z Thu Apr 03 2025 - 00Z Sun Apr 06 2025 ...Northern Plains & Upper Midwest... Day 1... ...Major winter storm continues to bring heavy snow and gusty winds through Thursday afternoon. Key Messages have been issued for this system and are linked below... The system which is expected to bring a late season major winter storm to the Dakotas and Minnesota is organizing this morning as reflected by an increase in reflectivity across the region in the vicinity of increasing 925-700mb PVU. This system will deepen today in response to a mid-level closed low deepening as it moves across Minnesota and into Ontario by Thursday morning. This low will become quite intense, reflected by NAEFS 850-700mb height anomalies falling below the 1st percentile in the CFSR database, with similar 500mb anomalies dropping below 2.5 percentile. This is reflective of a strong system, with surface low deepening encouraged by the robust height falls and concurrent/overlapping upper diffluence as a strengthening jet streak pivots poleward downstream of the mid-level low. The guidance has come into better agreement today with the track of this surface low, and it is expected to track from near Minneapolis this aftn through the western U.P. of MI by Thursday morning. This evolution will have a two-pronged impact on the winter weather impacts. First, this slightly more northern track will help push a dry slot and a warm nose northward leading to an axis of light sleet and freezing rain, but accumulations are expected to be minimal. More significantly, NW of the surface low, a potent deformation axis is progged to develop and pivot from eastern ND/SD through north-central MN. This deformation axis will occur concurrently with an intensifying TROWAL as theta-e advection on moist isentropic ascent increases and wraps cyclonically around the system. The TROWAL will be most impressive from generally 18Z today through 06Z Thursday across N-Central MN, and cross-sections within this region suggest a threat for CSI to enhance snowfall rates. This is additionally supported by both the WPC prototype snowband tool and HREF snowfall rate probabilities which indicate a moderate to high risk (50-80%) for 1+"/hr rates, with locally 2"/hr rates possible. This will overwhelm antecedent warmth and the April sun, leading to rapid snowfall accumulation beneath this deformation, and WPC probabilities are moderate (30-50%) for an additional 4+ inches after 00Z, with storm total snowfall of 6-10" in some areas. While there is still some uncertainty into the exact placement of this deformation, and the gradient on the NW and SE side should be pretty significant, this band of heavy snow will be impactful before the entire system exits to the northeast by late Thursday morning/early Thursday aftn. ...Northeast... Days 1 & 3... The same system that will be producing heavy snow across the Upper Midwest D1 will occlude to a secondary low and attendant warm front farther east. This warm front will lift northeast into Upstate NY and New England tonight into Thursday, bringing a period of heavy precipitation through Thursday aftn. The front will lift rapidly northward, so the duration of precipitation will be limited, and p-types will vary widely as the warm nose causes a change from snow, to sleet, to freezing rain, and eventually rain before ending (likely ending as freezing rain the highest terrain of NH and northern ME). Despite this, the multitude of precipitation types will create hazardous conditions, especially in the higher terrain from the Adirondacks through Vermont, New Hampshire, and Maine where WSSI-P probabilities indicate a 10-30% chance for moderate level impacts. The impressive fgen along this front should result in periods of heavy precipitation rates as well, regardless of the p-type. This could result in a few inches of snow and sleet, before changing to freezing rain. Total accumulations will be generally modest, but WPC probabilities indicate a high risk (>70%) for at least 4 inches of snow in the highest terrain of NH and ME, and a 50-70% chance for at least 0.1" of ice in the Adirondacks and portions of NH/VT/ME as well, ending by 00Z Friday. Another round of mixed precipitation is likely as moisture funnels northward on return flow through the Mississippi Valley and into New England Saturday. At this time any significant wintry precipitation accumulations are expected to be confined to the highest terrain of northern New England, with a mix of snow and freezing rain likely. Total accumulations are expected to be modest however, as reflected by WPC probabilities for 0.01" of ice that are generally just 10-30%. ...Intermountain West... Days 1-3... Expansive mid-level trough will remain entrenched across much of the West through the weekend, but evolution of embedded shortwaves will lead to amplification and wavelength shortening by Friday. Before this occurs, generally modest ascent and modest moisture will lead to widespread but light snow across much of the terrain from the Northern Rockies southward through the Four Corners states. Within this broad ascent, there is likely to be two areas of more consolidated ascent and heavier snowfall D1: the Northern Rockies and from the Wasatch to the San Juans. In the Northern Rockies, a cold front sagging southward will cause some enhanced fgen and post-frontal upslope on E/NE winds. This will more efficiently wring out available moisture leading to heavy snow in the vicinity of Glacier NP where WPC probabilities are high (>70%) for at least 6 inches of snow. Additionally on D1, a subtle shortwave ejecting from the southern stream will work together with downstream mid- level divergence and some upslope ascent to increase snowfall from the Wasatch into the San Juans where WPC probabilities are 70-90% for 4+ inches of snow. More widespread, generally light, snow occurs across the Intermountain West terrain on D2, but some focused heavier snowfall is likely in the vicinity of the MT/WY border, including the Absarokas, Wind Rivers, and in the vicinity of Yellowstone NP where the sinking cold front helps to enhance snowfall. WPC probabilities in this region reach as high as 30-50% for 6+ inches of snow. Then late D2 into D3, the amplifying trough and concurrent closing off of an upper low across the Four Corners will yield much more significant snow across the Southern Rockies and into Colorado. While there is still uncertainty as to how this system will evolve, and trends have been for a slightly farther south track, confidence is increasing that heavy snow will become widespread across northern NM and into CO. This will be in response to increasing synoptic ascent driven by both downstream divergence and increasing fgen/upslope flow as the cold front continues to sag southward. This will additionally cause isentropic lift to surge moisture northward, and as this pivots back to the west it will yield an expanding area of heavy snowfall, especially in the terrain from the Front Range, to the Palmer Divide, the Raton Mesa, and the Sangre de Cristos. Additionally, as snow levels crash behind the front, impactful snow may spread into the High Plains including the urban I-25 corridor, with at least moderate snow potentially pivoting into the TX/OK Panhandles late in the forecast period and into D4. At this time, WPC probabilities begin to rise late D2, reaching 30-50% for 4+ inches along the Front Range, before expanding and increasing D3 to 70-90% from the San Juans to the Raton Mesa and along the Sangre de Cristos. Additionally, with snowfall expanding and snow levels falling, WPC probabilities indicate a low risk (10-30%) for at least 4 inches as far east as the western TX/OK Panhandles and across the northeast NM High Plains. 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 $$