Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 345 AM EST Tue Feb 12 2019 Valid 12Z Tue Feb 12 2019 - 12Z Fri Feb 15 2019 ...Midwest... Days 1 and 3... A mid-level trough will deepen and close off as it advects toward the Great Lakes today. This will allow a surface low to intensify as it lifts into Michigan ahead of the upper feature this afternoon. Accompanying this feature will be significant height falls and robust mid/upper diffluence to provide ascent, which will be aided by 700mb deformation in the vicinity of the 700mb low and increasing frontogenesis with the cold conveyor belt. A band of moderate intensity snow over the IL/WI border and extending east into MI will continue to lift north across the Great Lakes today. The surface low center pushes across Lake Huron tonight with wrap around lake enhanced and lake effect snows for the UP and the western shore of MI through tonight. Day 1 WPC probabilities are moderately high for 8 inches across northern across the UP to northern lower MI with high probabilities for 4 inches extending south into central WI and central MI. An ongoing wintry mix east from northern IL will be over southeast MI and extreme northwest OH at day break. Day 1 WPC probabilities indicate moderate probabilities for an additional tenth inch of ice over this area after 12Z. Lake effect snow tapers off Wednesday morning as the low shift north of the St. Lawrence Valley. The next CO low crosses the Plains Wednesday night into Thursday with another northeast track into the Great Lakes Thursday and toward Lake Huron Thursday night. A surface ridge over the Gulf coast keeps Gulf moisture suppressed/out of the system until Thursday, so little precip is expected until Thursday night over the Great Lakes where there are moderate probabilities for two inches over the northern Great Lakes. ...Mid-Atlantic and Northeast... Days 1-2... A surface low moving through the Great Lakes today will continue low level southerly flow over the Mid-Atlantic today while surface cold air damming from a ridge extending from a sprawling high pressure centered over Quebec keeps the daytime erosion of cold air in place very slow until the cold front crosses the central Appalachians this evening. Low level frontogenesis on the leading edge of the WAA will cause a "front-end" thump of snow from western NY and northeast PA and northeast into upstate NY and New England through tonight. The warm nose quickly gets above freezing with a changeover to sleet then freezing rain across the northeast. Plain rain is expected to spread in from the coast, but be blocked by inland terrain. A new coastal low developing over the Delmarva tonight and becoming the dominant low by the time it reaches Maine late Wednesday. Snow/sleet accumulations increase farther north with high Day 1 probabilities for eight inches over the Adirondacks and Green and White Mountains and spreading into northern Maine into Day 2. Due to mixing, probabilities greatly decreased to the south, a narrow stripe of moderate probabilities for two inches of snow/sleet extends across northern PA and southern New England. As precipitation falls into the cold high pressure initially, the wedge will be reinforced and a period of heavy freezing rain is expected from western MD/southwestern PA into southern New England. There is good model consensus that the highest accretions will occur across central and northeastern PA with lesser amounts to the northeast where sleet will mix in more significantly. A period of heavy precip rates seems likely beneath robust isentropic lift/WAA, and without any notable dry advection to cool the web-bulb temperatures, the self-limiting process inherent to freezing rain will dominate, aided by runoff due to the aforementioned heavy precip rates. Still expect a large area of 0.25" or more across central and northeastern PA where Day 1 WPC probabilities are moderate for a quarter inch. Low probabilities for a half inch are over these areas as well. A small area of moderate probabilities for a quarter inch are also over the Berkshires/Litchfield Hills in MA/CT while high probabilities for a tenth inch of ice extends from Garrett Co MD to central MA. ...Western U.S.... Days 1-3... A very positively tilted trough off WA will slowly drift south through tonight with the focus of ample Pacific Moisture south of the trough shifting to OR. Snow levels of 1-3kft are expected through tonight over the Cascades of northern Oregon and Southern Washington. Day 1 WPC probabilities are moderate for 18 inches over high terrain of far northern CA and the OR Cascades to Mt. Rainier (and downstream in the high terrain of ID/MT). Snow levels are expected to remain high enough for less than an inch in the Willamette Valley of OR. Wednesday into Thursday /Day 2 and 3/ the trough over the Pac NW splits with northern stream energy shifting east over the northern tier while low pressure closes off the OR/CA border. The Pacific Jet will dig southward as another low descends the persistent ridge into AK and off the BC coast, reaching the Pac NW coast Thursday night. This southward shift will focus the heaviest snow into the mountains of California, including the Sierra Nevada where 48-hr snowfall may exceed 5 feet, with heavy snow also likely in the Shasta/Trinity and Siskiyou Ranges. Considerable lift within the exit region of the upper jet will tap anomalously high moisture as far east as Colorado and south as Arizona. However, with the source being more of tropical origin, snow levels rise into Day 3 remaining above 5000 feet south of 40N latitude and around 7000ft over the Sierra Nevada. This will confine the heaviest snow to the highest terrain outside of the California mountains, with WPC probabilities showing a moderate to high risk for 8 inches in the high mountains of UT/ID/WY/CO Thursday and Thursday night. Jackson