Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 504 PM EDT Tue Apr 09 2019 Valid 00Z Wed Apr 10 2019 - 00Z Sat Apr 13 2019 ...The West... Days 1-3... A trough currently pushing onshore over the Pac NW digs and amplifies as the southern end reaches the Desert SW tonight with the mid level closed low center over UT before the system shifts east across the northern/central Rockies Wednesday. Snow elevations decrease with the trough as does the influx of Pacific moisture resulting in heavy snow across much of the high terrain from the Cascades to the northern Rockies and mountains of the Great Basin on Day 1. Rapid cyclogenesis in the lee of the CO Rockies Wednesday focuses heavy snow across WY and CO as well as the mountains of UT on Day 2. A blizzard develops on the Great Plains which is discussed below. As the trough axis becomes quasi-stationary across the Rockies by mid-week, this will set the stage for continual moisture feed and jet energy across the Pacific Northwest ahead of the next storm system which is posed to cross Thursday into Friday. Anticipate renewed snow to the elevations above 3000 to 4000 ft from the WA/OR Cascades eastward across the Northern Rockies. ...Northern/Central Plains to the Great Lakes... Days 1-3... Rapid cyclogenesis will begin tonight in the lee of the CO Rockies as an amplifying trough pushes across the Great Basin. This low will stall over eastern CO Wednesday as the parent upper low begins to close and deepens as it moves over the CO Rockies. A strong southerly LLJ will provide ample moisture into an already saturated environment characterized by Pacific moisture and PWAT anomalies of +1.5 standard deviations. While the airmass east of the surface low will be quite warm, north and west of the low cold air will be in place, and will intensify as high pressure centered over Hudson Bay noses southward into the Plains aided by to the deepening low, eventually shifting over the Great Lakes on day 3. As the surface low deepens, the parent upper trough will close off helping to stall the progression of the low across east KS/ east NE and west IA. A stripe of potent deformation and intensifying 850-600 mb frontogenesis will drive mesoscale ascent, and heavy snow is likely north and west of the low track. Precipitation breaks out over SD Wednesday afternoon and dynamic cooling is expected to cause a rapid changeover to snow Wednesday night as the TROWAL sets up north of the developing low. This TROWAL axis merely pivots as the low shifts east across KS Wednesday afternoon and night and then into western IA Thursday. Therefore a prolonged heavy snow threat is expected and two feet of snow are possible over eastern SD. Another note about this system is the positive tilt of the mid-level trough that allows Gulf moisture to spread up the Great Plains, but not the typical convective thunderstorm complexes so commonly south of the system that reduces the Gulf moisture before it wraps around the system as snow. Heavy rain is expected before it changes over, but a great percentage of the incoming moisture will be precipitated as snow which could maximize the snowfall, even with this being the second week of April. There are a moderate probabilities for 18 inches in 24 hours in the Day 2 time frame. In the high plains/Black Hills WPC probabilities are moderate to high for 12 inches for Days 1 and 2 from eastern WY into central NE and southern SD. It is across this area where blizzard conditions are possible as this intense lift occurs atop a deep isothermal layer in conjunction with -EPV near the DGZ to produce the potential for thunder-snow and 1-2"/hr snowfall rates, along with increasing winds. North of the low track, theta-e advection wrapping cyclonically around the low within a robust TROWAL will produce a prolonged period of heavy snowfall, and WPC probabilities are high for 12 inches, and even moderate for 18 inches, from northeast NE across southeast SD and into southwest MN. Despite relatively lowered SLRs due to high liquid-water content snow, tremendous QPF and heavy snowfall rates suggest the potential for more than 20 inches in some locations. There will likely be a very sharp gradient between heavy wet snow and rain, and some uncertainty remains in where this axis will settle over the northern Plains into the upper Mid-West. Another area of concern is the ptype changeover line across SD across southern MN along the IA border on the onset of this system. It is quite a challenge with respect to the precipitation type, intensity and the resultant ice/snowfall amounts due to the intense frontogenesis, isentropic lift and surface temperatures cooling under the southerly LLJ. Models are starting to suggest ptype issues with this precursor precipitation band that may lead to a narrow corridor of higher ice accretion/snow. Moderate probabilities remain for 0.1 inches on Day 2 with the potential to approach 0.25 inches, though localized. Some limiting factors for more widespread icing will be the temperatures around freezing, lack of drier air for wet bulb potential and the precipitation rates (with some instability noted). By the time the system lifts over the upper Midwest a cold front will have set up to the south allowing more intense moisture advection ahead of the front. Overrunning, ahead of the cold front will result in freezing rain/sleet becoming a bit more widespread over both northern WI and northern MI, though amounts should remain below a quarter of an inch. The low lifts over the upper Midwest for Day 3 with the heavy snow axis in the MN/WI area with the aforementioned ptype issues occurring later on in the event. Models have remained rather consistent with the low tracking into south-central MN by early Friday with the model preference toward the EC/CMC. The low will be occluded/filling at this point which can lead to broad dry slots. Also, with convection ongoing across IA this may aid in reducing the available moisture thus diminishing snowfall rates/amounts as the low tracks northeast. For now there remains moderately to high probabilities for 8 inches from eastern SD to western Lake Superior on Day 3. ...Northeast... Day 1... A surface low will cross central New England through the overnight resulting in continual snow across much of ME and the higher terrain across northern NY/VT/NH. Low probabilities for six inches into western Maine. A light icing is also possible near the low track in sheltered valleys of northern VT/NH. Jackson/Pagano