Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 407 AM EST Mon Feb 27 2023 Valid 12Z Mon Feb 27 2023 - 12Z Thu Mar 02 2023 ...Upper Midwest/Great Lakes to the Northeast... Days 1-2... A potent and negatively tilted upper low making its way northeast though the Midwest this morning is responsible for a power surface and low level circulation center directing a strong southerly LLJ and anomalous moisture content into the Upper Midwest and northern Great Lakes. 850mb winds as of 12Z this morning will be as intense as 70-80 knots over southern IL and cause a burgeoning warm nose with temps >0C in the 850-800mb layer over central WI. Surface temperatures look to remain sub-freezing long enough in eastern MN, central WI, and into central MI with the help of both a departing Canadian high over Quebec and linger snow pack. The snow pack will aid in keeping sub-freezing air long enough to allow for freezing rain persist through the morning hours. As the strongest 850mb flow moves east Monday afternoon and central WI becomes located on the northwest flank of the storm system, freezing rain will transition back to snow while central MI continues to contend with an icy wintry mix. The WSSI does show mostly Minor impacts in these areas for Monday with some embedded Moderate impacts in northwest WI. The WPC PWPF continues to show a large swath of 50-70% probabilities for >0.10" of ice accumulation with west-central MI featuring the low chances (10-30% chance) for >0.25" of ice accumulation. The surface low will march east through the Lower Great Lakes and direct its strong layer of WAA in the 925-850-700mb layers into the northern Mid-Atlantic and Northeast Monday evening. The Canadian high is in a slightly better position to provide sufficiently cold enough temperatures to support snow from northern PA and upstate NY into New England. The 00Z HREF did show a band of snowfall capable of producing >1"/hr snowfall rates (max probabilities between 60-80%) from the Finger Lakes and just south of the Tug Hill Plateau to the Catskills and Berkshires Monday evening. As a frontal wave forms off the NJ coast, a wave of low pressure will deepen along it and track east, positioning itself by early Tuesday morning south of Long Island. It is during this window (Monday night into early Tuesday) that Atlantic moisture funneling around the north side of the surface low can lead to a more prolonged period of banded snowfall from the Tri-State area to southern New England early Tuesday morning. Given the timing of when the periods of snow occur, this will likely lead to reduced visibility and slick travel conditions on roadways for the Tuesday AM commute. The PWSSI does show a large footprint of >90% probabilities for Minor impacts Monday night into Tuesday AM over the Lower Hudson Valley and across much of CT. Meanwhile, there are 60% probabilities for Moderate impacts in the Catskills, portions of the Poconos, and near the Hartford metro area. This is largely due to the Snow Rate algorithm, which suggests the potential for quick snowfall accumulations and poor visibility. Latest WPC PWPF does show 60-70% probabilities for >6" of snowfall in the mountain ranges (Poconos, Catskills, Adirondacks, Tug Hill, Berkshires, Green, White Mountains) for the event, suggesting some localized areas could see snow totals up to a foot. Probabilities are 60-80% for >4" from the Tri-State area to CT and RI, with localized areas >6" possible. ...Pacific Northwest and California to the central and southern Rockies... Days 1-3... A broad longwave trough over the West Coast will contain a pair of storm systems that usher widespread heavy snow for most mountain ranges along the West Coast and as far east as the Colorado Rockies. This morning, an upper low level low off the Pacific Northwest coast that is falling below the 10th climatological percentile according to NAEFS will funnel deep Pacific moisture into the West Coast and as far inland as the Intermountain West. Heavy snow will blanket the Cascade Range, the Coastal Range of southwest OR and northwest CA, and as far inland as the Wasatch and Tetons where snowfall accumulations over a foot are expected. The heaviest snowfall totals will be found in the Sierra Nevada where snowfall totals as high as 30" are possible in the highest elevations of the mountain range. The Day 1 WSSI shows Major impacts in all the mountain ranges previously mentioned with some Extreme impacts in the higher elevations of the Sierra Nevada. Snow amount is the primary driver in the WSSI for these areas, but there are also Major to Extreme impacts depicted from the Blowing Snow and Ground Blizzard algorithms thanks to wind gusts >40 mph expected in those highest elevations. California's mountain ranges will be primed for another busy day of heavy mountain snow on Tuesday as the next vigorous upper level disturbance races south from being off the Pacific Northwest coast Tuesday morning, to causing an amplification of the longwave trough over the Great Basin and into southern California Tuesday night. Heights and temperatures at the 500-700mb levels are likely to be <1st climatological percentile in the Northwest and the northern Great Basin Tuesday night, further allowing for abnormally cold temperatures and high SLRs throughout the region. This next upper level disturbance will continue amplifying into the Southwest on Wednesday where it will create a strong ~150kt jet streak at 250mb over the AZ and NM. As height falls and temperatures continue to cool in wake of a cold frontal passage, sufficient 700mb moisture flux will be funneled over the Rockies, setting the stage for round of heavy snow throughout the Four Corners region's mountains ranges on Wednesday. Latest WPC PWPF shows high probabilities (80% chance) for >8" of snow in the Mogollon Rim, the southern Wasatch, and into the San Juans. The Mogollon Rim of northern AZ features the best chances for >12" totals on Wednesday with probabilities as high as 70-80%. The Day 3 WSSI shows Major impacts are in the forecast presently in these ranges due to a combination of the Snow Amount and Blowing Snow algorithms. This shows that not only is there the heavy snow aspect of this forecast, but also a strong wind gust aspect that will help to reduce visibilities to near whiteout conditions at times, as well as blowing/drifting snow on roads and passes. Key Messages continue to be generated for this event. The latest bullet points are listed below this discussion. ...Northern Plains to the Upper Midwest... Days 2-3... The initial upper level disturbance helping to produce heavy snow in the Sierra Nevada today will race east across the Rockies Monday night and enter the Northern Plains by Tuesday. As cyclogenesis transpires in western SD Tuesday afternoon, a conveyor belt of 850mb moisture will be directed north towards the Dakotas and western MN. There is a weak Canadian high pressure area to the north helping to lock in sufficiently cold air while 850mb frontogenesis aids in developing precipitation falling in the form of snow. The deformation axis is likely to setup on the northern flank of the 850mb front with heavy snowfall rates up to 1"/hr possible. Latest WPC PWPF shows 20-40% probabilities for >6" of snowfall along the ND/SD border and into far western MN. These areas also contain a 50-60% chance for Minor impacts according to the experimental PWSSI. Snow Amount and Snow Rate are the primary drivers in this setup, but there is a chance for elevated wind gusts as well, which would favor both drifting snow on roadways and reduced visibilities Tuesday night. The front begins to undergo frontolysis by the time it reaches the Upper MS River Valley and will lead to diminishing snowfall rates. Still, cold enough SLRs and moisture aloft could lead to some areas across central MN and northwest WI to see over 4" of snow through Tuesday night. Latest WPC PWPF currently put those probabilities for >4" at 20-40% through Tuesday night and into Wednesday morning. Mullinax ...Key Messages for West Coast Winter Storms... --A series of storm systems will continue moving across the Pacific Northwest and into California through Wednesday with unusually cold temperatures making for lower than normal snow levels. --Mountains from southern Oregon to central California will see the greatest impacts with heavy snow rates in excess of 2 inches per hour at times and additional snowfall of 4 to 7 feet along the Sierra Nevada. Combined with high winds, blizzard conditions and dangerous to impossible travel are expected. --If you plan to travel by road at elevations above 1,000 feet through West Coast states, be prepared for rapidly changing conditions and have winter driving supplies. --Heavy snow will make it as far east as the Colorado Rockies and as far south as northern Arizona where hazardous travel conditions are expected Tuesday and Wednesday.