Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 424 AM EST Fri Feb 22 2019 Valid 12Z Fri Feb 22 2019 - 12Z Mon Feb 25 2019 ...Great Plains across the Upper MS Valley/ Upper Great Lakes... Day 1... An increasing deep layer southwest flow transports moisture up and over colder air near the surface from NE through SD and into ND/across northern MN. The column remains cold enough to support snow, with the confluent flow leading to multiple jet maxima and differences where lift occurs within the broad confluent zone. There is a multi model signal of potential for a band of 3 to 5 inches of snowfall, centered over southeast SD, with a separate area in western NE. The 00Z GFS has higher QPF and heavier snow amounts in western NE, with significantly lighter amounts in the nam/ecmwf/cmc/ukmet. The model majority was preferred as the GFS suffers from a high QPF bias, with a cross check with the GEFS mean showing lighter amounts than the 00z GFS. On the other side of the circulation to the east, light icing develops in eastern NE to northern Iowa and adjacent southwest WI as warming aloft brings temps above freezing while surface termperatures are still below freezing for a multi-hour period. Most areas are forecast to receive measurable to a tenth of an icing. A low probability of a quarter inch reflects higher forecast NAM amounts in northeast IA to adjacent southwest WI. Day 2... A lee surface low and corresponding 700 mb low developing as it crosses northwest OK Saturday morning will correspond with a coupled jet region moving northeast out of CO from the eastern Plains and then into southwest NE early Saturday. The track of this low and band of convergence along the track of the 700 mb low combine with upper divergence maxima and mid-level frontogenesis maxima to support a narrow stripe of heavy snow from southwest KS to southeast NE, central IA, southeast MN, and then northern WI and the western UP of MI. Given the strength of the low a narrow band of 8 to 12 inches of snow is expected. This will also be a very windy system with blizzard conditions possible. The 00z NAM was an eastern outlier on the low track so given no weighting on the axis/orientation of the snow and freezing rain. While the GFS is further north in NE and joins the majority cluster in the low track across the upper MS Valley and upper lakes. East of the low-mid level circulation, strong warm air advection rides up and over retreating low level cold air from southeast NE across central to northeast Iowa, northwest Illinois, southern to northeast WI, and finally into northern lower MI and the eastern UP of MI. In the strong mid-level warming, light snow transitions to freezing rain. There was a signal for measurable to 0.10+ inch ice amounts in the aforementioned locations, with low Day 2 probabilities for 0.25+ across eastern WI to the eastern UP/far northern lower MI. Day 3... On day 3 (Sun), the low causing the mixed winter precip event treks from over northern Lake Michigan northeast into Ontario. The synoptic snow comes to an end and then transitions to lake effect in the UP of MI and northwest lower MI, where several additional inches of snow are expected. The icing aspect of the event should end. ...Pacific Northwest into the Northern Rockies... Days 1-3... A low and associated low level front shifting south down the BC coast brings onshore flow and low elevation and mountain snows to the Pac NW starting this morning over the WA Cascades and tonight for the rest of the WA Cascades down to the OR Cascades and inland across the Blue Mountains into the ranges of ID/western MT. The higher Day 1 probabilities are for the WA Cascades due to the longer duration of snow. On day 2, the low off the coast continues a slow south drift. The persistence of a low level frontal boundary and 700 mb west-east jet focuses moisture across the OR Cascades and the further east into the Boise/Salmon River Mntns and the continuing east into the Tetons of WY. The highest probabilities for 8 inches are in the OR Cascades due to the closer proximity to the Pacific moisture source plus longer event duration. Light snow accumulations are possible in Portland OR on Day 2. On day 3, the trough amplifies off the coast through Sunday night/early Monday as atmospheric river moisture shifts into southwest OR/northern CA. The low level front drifts south across OR and ID, with 700 mb theta-e advection and convergence maxima allowing prolonged snows. The increased moisture spreads east across OR on Day 3 across southern ID and northwest WY with moderate probabilities for 12 inches over the OR Cascades, the Trinity Alps, Salmon River/Boise Mntns in ID, to the Tetons. ...Southwest into the Southern Rockies... Day 1... The models show the mid to upper level front/trough over UT to AZ this morning. Confluent flow at 700 mb leads to an axis of convergence across southeast AZ, NM and CO that drifts east across eastern CO and northeast NM tonight. Snow occurs within the convergence band and is enhanced within upslope areas. The heaviest amounts on day 1 are in the San Juan mountains of CO/NM and then the Sangre de Cristo mountains, where areas of an additional foot of snow are possible. After an initial period of snow in AZ, the departure of the upper divergence in conjunction with the eastward moving jet and drying aloft leads to the snow tapering and the event winding down. ...Central Appalachians to Northern Appalachians/New York/England... Day 2... A strong surface high shifts east across the northern Mid-Atlantic Friday night with cold air damming setting up again east of the Appalachians in the interior central Mid-Atlantic. Low Day 2 icing amounts/WPC probabilities are focused on southwest to central PA as return flow overruns the low level cold air. The continuing mid level warm advection over surface cold air favors a rapid transition from snow to sleet and freezing rain, with light icing in the Catskills/Adirondacks, and Berkshires of MA to the adjacent Green Mountains of southern VT. The SREF Mean shows potential for up to a quarter inch of icing, which is more than the ecmwf or gfs. Day 3... The powerful low tracking over the Great Lakes drives a warm front north across New England Sunday. Low pressure is expected to develop along the front, enhancing low level convergence and lift. Front end snow begins Sunday over interior New England with snow magnitude increasing north across interior Maine, with moderate to high Day 3 WPC probabilities of 8 inches of snow over interior Maine. The mid level warm air turns the snow to sleet and freezing rain over VT and NH to adjacent southern Maine. The continued forward progression of the 700 mb wave leads to drying aloft the latter half of day 3(towards Mon morning), so the precip coverage/intensity over northern New York and New England should start to taper. Petersen