Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 412 AM EST Sat Feb 23 2019 Valid 12Z Sat Feb 23 2019 - 12Z Tue Feb 26 2019 ...Central Great Plains to the Upper MS Valley/ Upper Great Lakes... Day 1... Blizzard warnings are in effect from western to northern KS, southeast NE, and much of western IA and southern MN as a developing storm brings snow and high winds to these areas. A surface low is forecast to move northeast from the TX/OK panhandles northeast to near the MO/IA around 00z and then northern Lake MI by 12z Sun. Low-mid level convergent northeasterly flow north of the low lasts for several hours underneath upper divergence maxima and mid level frontogenesis, supporting a stripe of 8-10 inches (moderate probabilities) from central to northeast KS across southeast NE, southwest to northern IA, northern WI, and the western UP of MI. This axis is close to what the 12z ECMWF and 00z UKMET and Canadian regional and global runs. The 00z ECMWF shifted the axis south in northern KS and eastern NE, so confidence is still not high yet. The NAM/UKMET/Canadian global all forecast an inch to inch and a half liquid equivalent. The steady forward motion suggest this is overdone, but signals these is potential for a foot of snow in mesoscale bands. 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, northern 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. Day 2... After the low passes northeast from northern Lake Michigan into Ontario, the synoptic snow decreases and northwest flow across western Lake Superior results in strong lee shore convergence underneath a band of mid level deformation. With high relative humidity and lift, several inches of snow should occur until starting to taper Sunday nigh once drier air aloft moves in from the northwest and the lee shore convergence weakens. A secondary max is expected over northwest lower MI in the lee of Lake MI. ...Pacific Northwest into the Northern Rockies... Days 1-3... The models show a well defined upper jet core streaming from the northeast Pacific across OR today and tonight, with front front entrance region upper divergence maxima coming onshore into the OR Cascades. Windward orographic lift combines with the long period of moisture fluxes and lift to produce sustained snow in the OR Cascades. The favored jet aspect with left exit region lift crosses the ranges of southern ID and northwest WY, so locally heavy snow is expected further inland as well on Day 1. One day 2, a wave develops on the front in OR and progresses inland. More heavy snow is expected along the low track and north of the wave, so northern OR will be most heavily impacted, particularly the western OR Cascades, where another foot to 20 inches of snow is expected due to the sustained jet core and enhanced upper divergence maxima. The sustained jet core starts drifting north, allowing additional heavy snows to occur over the Boise/Salmon River Mountains and WY Tetons. 72hr probabilities for 24 inches are high over the OR Cascades and Salmon/Sawtooth mountains of ID and Tetons of WY with event totals three feet or more likely. On day 3 the next wave of low pressure develops in the coastal waters and moves towards northwest OR. The wave is preceded by another surge of low level convergence underneath upper divergence maxima in the right entrance region of an upper jet crossing northern OR/southern WA to northern ID and MT. The north drift tot he jet allows moisture and ascent to drift north on day 3, expanding the area of heavy snow into the ID Bitterroots and adjacent ranges of western MT. The heaviest Mon snow are forecast to be inland in the ranges of southern ID following the longer duration ascent/upper divergence maxima. In OR, low elevation snow are expected in valley/Gorge locations in northern OR, including the Portland metro area. There are moderate probabilities for two inches in Portland on Day 2 and again on Day 3. Locally heavy snow are possible in the coastal ranges of OR. ...Central Appalachians to Northern Appalachians/New York/New England... Day 1... Cold air damming is expected to set up again east of the Appalachians in the interior central Mid-Atlantic. Low Day 1 WPC probabilities for measurable to a tenth inch of ice area west of the Shenandoah Valley into southwest PA is indicated as 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 minor snow accumulations as a result, continuing downstream into northeast PA, interior eastern NY, and adjacent western New England. Light icing in the Catskills/Adirondacks, and Berkshires of MA to the adjacent Green Mountains of southern VT. Day 2 WPC probabilities for a tenth of an inch of icing (locally as much as a quarter) are focused across these higher elevations. Day 2... 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. Snow driven by frontal lift and mid level warm and moist advection occurs early Sunday over northern New England with the snow magnitude increasing north across interior Maine where ptype remains snow. Moderate to high WPC probabilities of 8 inches exist in Day 2 over northern Maine. The other area of snow is in the lake effect region of the Tug Hill and western Adirondacks, where several inches of snow is expected on day 2. The mid level warm air turns the snow to sleet and freezing rain over VT and NH to adjacent southern Maine. The 00z SREF Mean shows potential for a quarter inch of icing in the Monadnocks and White Mountains of NH, where a low probability of a quarter inch of icing is shown. 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. The probability of significant icing on day 3 is less than 10 percent. Petersen