Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 357 PM EST Thu Feb 21 2019 Valid 00Z Fri Feb 22 2019 - 00Z Mon Feb 25 2019 ...Southwest into the Central and Southern Rockies... Day 1... A closed low over the NV/CA border open as it shifts south to the mouth of the CO River tonight before shifting south of AZ Friday and ejecting east from NM Friday night. A low level southwest flow ahead of the system transports 0.50 inch precipitable water air over Baja CA and into AZ and the Mogollon Rim. The combination of elevated moisture and strong lift from upper divergence will continue to produce heavy snow at lower elevations than normal in the Desert SW. A couple more inches above 3000 feet in the mountains east of San Diego and mountains in the southern Great Basin can be expected tonight. The eastward shift of the moisture across AZ into NM allows one to two feet to fall along the higher portions of the Mogollon Rim with snow as low as 2000-3000ft across AZ (with notable snows east from Tucson) through Friday. Similar amounts are expected over favored upslope areas of the San Juan Mountains of southwest Co and adjacent northern NM through Friday. Day 2... The upper trough axis shifts east from AZ Friday evening, driving the upper jet east across the mountains/front range of CO/NM on to the southern High Plains Friday night. After an initial period of snow, 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. Day 2 WPC probabilities are moderate for an additional six inches in the high terrain of NM with a swath of moderate probabilities for six inches east along the CO/NM border north of the developing lee surface low. ...Great Plains across the Upper MS Valley/upper Great Lakes... Days 1-3... The 12Z GFS and FV3 were considered too fast/deep/north for inclusion in the QPF/winter precip blend for Day 2. The preference was for the 12Z NAM/ECMWF. 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 6 inches of snowfall, centered on the SD/NE border. Strong lift associated with a long wave trough crossing NM Friday night with lee surface low developing as it crosses the TX/northwest OK Saturday morning will induce a coupled jet region moving northeast out of CO from the eastern Plains and then into southwest NE by 12Z Saturday. The track of this low and its dynamic/strong TROWAL and narrow stripe of heavy snow left of the track remains uncertain with both timing and placement. The upper divergence maxima and mid-level frontogenesis maxima extends northeast from KS to the UP of MI Saturday through Saturday night. The track of the 700 mb low is the favored axis for heavy snow. 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. 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, front end 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 2/3 probabilities for 0.25+ from IA across northern WI to the eastern UP. ...Central Appalachians to New England... Day 2... A 1033mb surface high shifts east across the northern Mid-Atlantic Friday night with cold air damming setting up again east of the Appalachians to the central Mid-Atlantic. Low Day 2 WPC probabilities are west from the Shenandoah Valley into southwest PA as return flow overruns the low level cold air. Day 3... Powerful low tracking over the Great Lakes bring a warm front north from the Central Mid-Atlantic Saturday, reaching New England Sunday. Front end snow begins Sunday over interior New England with snow magnitude increasing north with moderate Day 3 WPC probabilities of six inches over northern NM and interior Maine. A warm nose allows northeast icing and a changeover to freezing rain farther north. Moderate Day 3 WPC probabilities for a tenth inch over a couple pockets of northern PA and over the Catskills/Adirondacks, and higher elevations of interior New England. ...Pacific Northwest into the Northern Rockies... Days 1-3... Yet another low shifting south down the BC coast brings onshore flow and low elevation and mountain snows to the Pac NW starting Friday morning over WA and Friday night for OR/ID/western MT. Snow levels only rise to around 1000ft through Friday night with high Day 1 and 2 probabilities for four inches across higher elevations of these areas with some accumulation possible in Seattle/Portland on Day 2. The trough amplifies off the coast through Monday as atmospheric river moisture shifts into OR/northern CA. Snow levels rise to 2000 to 3000ft over OR/northern CA indicating the moist air is not of true tropical origin. The increased moisture spreads east from OR on Day 3 with moderate probabilities for 12 inches over the OR Cascades, the Trinity Alps, Salmon/Sawtooth in ID, to the Tetons with snow levels 1000 to 2000ft over the northern Great Basin. Jackson