Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 442 AM EDT Wed Apr 10 2019 Valid 12Z Wed Apr 10 2019 - 12Z Sat Apr 13 2019 ...Northern/Central Plains to the Great Lakes... Days 1-3... Rapid surface cyclogenesis will occur today in the lee of the CO Rockies as a deepening low shifts east over the CO Rockies. A strong southerly LLJ across the Great Plains will provide ample Gulf moisture into the system. Sprawling surface high pressure centered over Hudson Bay persists over the northern Plains, aiding frontogenesis on the north side of the low. 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. The TROWAL that has developed over SD persists today and merely pivots through Thursday as the low shifts east across KS this afternoon and night and then up the NE/IA border Thursday. Therefore a prolonged heavy snow threat is expected and two feet of snow are likely over east-central SD. The positive tilt of the mid-level trough allows Gulf moisture to spread up the Great Plains and into this system, but the typical convective thunderstorm complexes so commonly south of Plains systems (that are usually neutrally or negatively tilted) will be largely absent through Thursday. Heavy rain is expected near the front before it changes over, but a great percentage of the incoming moisture will be precipitated as snow which should maximize the snowfall, even with this being the second week of April. There are moderate to high 48hr probabilities for 12 inches from the Panhandle of Nebraska to Duluth for Days 1-2. There are a moderately high Day 1.5 probabilities for 18 inches in 24 hours in east-central SD and thirty percent probabilities for 30 inches there in 48 hours. Another area of concern is the ptype changeover line across southeast SD and across southern MN. The intense frontogenesis, isentropic lift and surface temperatures cooling under the southerly LLJ will make for a tight transition from rain to sleet and freezing rain to snow and the slow movement could allow for heavy sleet and thick icing. A stripe of low probabilities for quarter inch of ice are along the MN/IA border for Day 1 and over northern WI/near the Strait of Mackinac on Day 2. The low lifts over the upper Midwest for Day 2 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/GFS. 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 moderate to high probabilities for 8 inches from eastern SD to western Lake Superior on Day 2. ...Pacific Northwest... Days 1-2... A ridge into the Gulf of Alaska direct shortwave troughs down the BC coast to the Pac NW today through Thursday night before a break in precip as the ridge axis pushes ashore Friday night. Onshore flow and snow elevations hovering around 3000 to 4000ft make for moderate probabilities for 8 inches on the WA/OR cascades both Days 1 and 2. Jackson