Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 321 PM EST Fri Feb 22 2019 Valid 00Z Sat Feb 23 2019 - 00Z Tue Feb 26 2019 ...Southern Rockies across central Great Plains to the Upper MS Valley/ Upper Great Lakes... An amplified trough over AZ this afternoon ejects northeast across the Great Plains to the upper Great Lakes through Saturday night. A mid level low closes off over the central Plains Saturday aiding rapid development of the surface low as it crosses the Great Lakes. Day 1... Convergence across the highlands of NM and CO leads to moderate WPC Day 1 probabilities for eight inches across the southern Rockies. Surface low development begins late tonight in the lee of the NM Rockies over the TX Panhandle as the low shifts east before taking a northeast turn. Convergent northeasterly flow north of the low lasts for several hours allowing a stripe of 8 or more inches (moderate probabilities) to develop from southeast CO and into north-central KS. The 12Z GFS/FV3 remain too far north (and FV3 too high magnitude) for inclusion in WPC QPF with the Day 1 QPF preference for the 12Z HREF mean/12Z ARW (which is NAM based) and some 00Z UKMET/ECMWF. This results in a one inch QPF stripe from SW KS to southern IA in the strong trowal band. This will also be a very windy system with blizzard conditions possible. Ahead of the low, increasing deep layer southerly 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. Day 1 WPC probabilities are moderate to high confidence for an additional two inches, particularly across northern MN. Farther south ahead of the low, light icing develops in eastern NE to northern Iowa and adjacent southwest WI as warming aloft brings temps above freezing while surface temperatures are still below freezing for a multi-hour period. Most areas are forecast to receive measurable to a tenth of an icing. Low to moderate probabilities of a quarter inch reflects the higher 12Z NAM amounts in northeast IA to adjacent southwest WI. Day 2... The track of the 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 increasing amounts in the stripe of heavy snow from southeast NE across 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 with low Day 2 probabilities for 18 inches over the western UP. The Day 2 QPF preference was for the 12Z ECMWF/NAM with some UKMET/SREF and in house bias corrected blend. The 12Z NAM becomes too fast during day 2, but with most associated QPF in the CONUS in the first 12 hrs, it was included. 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 probabilities for 0.25+ from central WI the eastern UP/far northern lower MI. After the low passes northeast from northern Lake Michigan into Ontario, the synoptic snow decreases and what is normally strong LES flow over Lake Superior develops. However, significant ice coverage, particularly on the western arm of Lake Superior should reduce the overall LES risk. ...Pacific Northwest into the Northern Rockies... Days 1-3... Low pressure shifting south down the BC coast continues to spread onshore flow and low elevation and mountain snows to the Pac NW through tonight. An omega shape to the ridge into AK allows the trough to amplify west off BC Saturday into Monday. This drives Pacific moisture/atmospheric river into southern OR and far northern CA Saturday through at least Tuesday with considerable snows for terrain through this time. 72hr probabilities for 18 inches are high over the OR Cascades and Salmon/Sawtooth mountains of ID and Tetons of WY with three feet more likely. A baroclinic zone develops in this plume with snow elevations increasing south across OR. This may lead to low enough snow elevations to affect the Portland metro area by Day 3. There are moderate probabilities for two inches in Portland on Day 3. ...Central Appalachians to Northern Appalachians/New York/England... Days 1-2... A strong surface high shifts east across the northern Mid-Atlantic tonight with cold air damming setting up again east of the Appalachians in the interior central Mid-Atlantic. Low Day 2 WPC probabilities for a tenth inch of ice area west of the Shenandoah Valley as return flow overruns the low level cold air. The continuing 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. Day 2 WPC probabilities for a quarter inch of icing, across these higher areas. Days 2-3... 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 early Sunday over interior New England with snow magnitude increasing north across interior Maine where ptype remains snow. Moderate to high WPC probabilities of 8 inches of snow span Days 2/3 over northern 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 toward Monday morning, so the precip coverage/intensity over northern New York and New England should start to taper Sunday night. Jackson