Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 521 AM EDT Fri Mar 13 2020 Valid 12Z Fri Mar 13 2020 - 12Z Mon Mar 16 2020 ...Northwest CONUS... A potent, positively tilted shortwave trough will dig south along the WA/OR coast today before closing off in place tonight before merely drifting south down the West Coast through Wednesday. ...Northern Rockies and Montana... Days 1-2... East of this developing low, mid-level divergence will intensify, combining with increasing diffluence within the right entrance region of the downstream jet streak to produce large scale ascent across the Pacific Northwest and Northern Rockies. In this pattern, 700-500 RH will increase quickly on moist SW flow. The near-surface cold air sinking southward interacting with warm advection aloft will lead to intensifying 850-700mb frontogenesis which is likely to persist and become intense across parts of Montana. The combination of synoptic ascent with this intense frontogenesis and low-level upslope flow will produce an extended period of moderate to heavy snow. The greatest snowfall is likely across north-central MT between 110W and the eastern slopes of the Rockies/Glacier NP. Day 1 WPC probabilities are moderate for 12 inches or more from the crest of the Lewis Range in Glacier NP onto the adjacent High Plains. As the upper low establishes itself along the OR Coast tonight into Saturday, the axis of snow shifts into northeastern MT and south into adjacent northwest WY. Day 2 probabilities for 6 or more inches are moderate to high in a swath from north-central MT to the Tetons and Wind River Range. A shortwave impulse coming off the closed low tracks east from MT Sunday, allowing generally light snow to develop in ND on low level convergence and ascent. Day 3 probabilities feature low probabilities for 4 or more inches centered over east-central ND. ...Pacific Northwest to central California... Days 1-3... A mid-upper level cyclone establishing off the OR coast tonight will stall off the far northern CA coast through Sunday night. This allows a focus of convergent, Pacific moisture laden air to shift south down the Cascades in Day 1 to the northern Sierra Nevada and CA Cascades (and Klamath/Siskiyous) for Days 2/3. Prolonged synoptic ascent through upper divergence, jet streak diffluence, low level convergence, with aid from upslope conditions in windward terrain will produce widespread heavy snow across western WA and northern OR today with snow levels around 1000ft. Day 1 probabilities are moderate to high for 8 or more inches for the WA/OR Cascades and Olympics. Snow levels drop to near sea level behind the upper low as precip generally cuts off. Any trailing precip would be light snow at all elevations including the valleys of western WA and as far south as northern Oregon on Day 2. Confidence continues to increase in prolonged heavy snow over the mountains of northern CA for Days 2/3. In particular, probabilities for 24 or more inches on Days 2/3 are high between Lassen Peak and the central Sierra Nevada a little south of Lake Tahoe with 3 to 5 feet likely through this time for the northern High Sierra. ...Four Corners through the Central Plains to the Midwest... Days 1/2... A closed low shifting inland over the southern CA coast this morning will open and eject northeast across the Four Corners today and the central Rockies tonight. The progression of the upper jet across AZ/NM places favorable upper divergence maxima across northern AZ and southern UT across northern NM and southern CO. The heaviest snow are expected where moderate to locally precipitation occurs where it remains coldest for the event in the mountains of southern UT and northern NM/southern CO with snow levels generally 6000 to 7000ft. Day 1 snow probabilities are high for 8 or more inches over the southern Wasatch and San Juans of CO/adjacent northern NM as well as central CO. Also today, precipitation will expand in coverage through warm/moist mid-level advection and height falls/PVA as the upper feature shifts northeast to the central Rockies on to the southern and central Plains. 700mb frontogenesis is particularly note worthy from eastern CO to northwestern KS/southwestern NE this afternoon with dynamically cooled air resulting in locally heavy snow across these areas despite being around midday in mid-March. This has expanded the moderate risk for 6 or more inches of snow into northwest KS from west-central NE. A secondary jet streak is expected to develop and move across NE, so pockets of upper divergence combine with the low-mid level warm advection to produce area with several inches of snow expected. This shortwave trough shifts east from the central Plains into the Midwest tonight through Saturday with wrap around moisture along a low level trough promoting diminishing snow across IA and IL into IN. Low Day 2 probabilities for 2 inches extend across this area. ...New Hampshire and Maine... Day 1... A trough swinging east across the northern Great Lakes early this morning will close over northeastern Ontario later this morning before shifting into Quebec this afternoon. Low-mid level warm/moisture advection ahead of the trough/developing low is producing a swath of precipitation progressing steadily across the northern Mid-Atlantic and southern New England with a wintry mix for only the highest elevations. After 12Z snow accumulations will be limited to the highest White Mtns of NH and Maine as well as far northern Maine which will be north of a developing surface low that tracks across central Maine this afternoon. A quick cut off of precipitation is expected late this afternoon as this low shifts northeast into the Canadian Maritimes. Day 1 snow probabilities for 4 or more inches are moderate to high for the aforementioned areas along with a light icing threat for the highest White Mountains such as Mt. Washington. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Jackson