Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 443 AM EST Tue Jan 17 2023 Valid 12Z Tue Jan 17 2023 - 12Z Fri Jan 20 2023 ...West Coast... Days 1-3... Despite weak ridging aloft, an approaching surface warm front will help guide some moisture off the Pacific into the WA Cascades today into early Wednesday, with only a few inches at high elevations. The cold front will progress through the region D2, as elongated mid-level troughing splits southward. PW values will be near climo, but IVT may briefly surge to 250 kg/ms/s on the upslope side of the Cascades early Wednesday before weakening. Snow levels around 3000ft will drop behind the front below 2000ft, bringing more appreciable snow to the passes where several inches are possible. WPC probabilities for at least 4 inches of snow are around 50% near 3000ft. Into D3, the southern portion of the sharp upper trough will dig through NV and likely close off by late Thu/early Fri. Given the trajectory, it will lose a lot of its moisture but will still capitalize on enough IVT into NorCal to yield some modest snows to the Shasta/Trinity/Siskiyou region as snow levels lower from 4000ft to below 3000ft as precipitation winds down into D3. A bit to the east, the northern Sierra will see modest snows before the moisture source from the Pacific diminishes, leaving the central portions of the mountain range with little snow. WPC probabilities for at least 6 inches in the northern Sierra are high (above 70%) generally above 4500-5000ft. ...Great Basin/Four Corners... Days 1 and 3... Troughing over the Great Basin/western Four Corners early D1 will become more negatively-tilted as it pushes past the Rockies, with trailing vort maxes lagging behind. This will continue the snow over the mountains D1 and finally end from west to east by early D2. 150kt jet streak will provide divergence aloft over the region especially early, before progressing eastward. PW anomalies +1 to +1.5 sigma early D1 coupled with upslope into the Mogollon Rim, White Mountains, and especially the San Juans will drive heavy snowfall this morning that winds down during the late afternoon into the early evening. WPC probabilities for an additional 6 inches of snow D1 are high (>70%) in the San Juans (where over a foot is likely) and many southwest CO peaks, and moderate over portions of AZ above 6000-7000ft. After a break on D2, the system moving through the Great Basin on D3 will bring mostly light snow to Nevada, Utah, and northern Arizona. Due to limited moisture, despite closing off at 500mb, WPC probabilities of at least 4 inches of snow are low (<40%) over southwestern UT. ...Eastern Colorado/Central Plains through the Upper Midwest/Western Great Lakes... Days 1-3... Mid-level trough moving out of the Four Corners D1 will will close off over southeastern CO by the start of D2. Upper divergence in the LFQ of a 130kt jet east of the Rockies will promote a broad area of light to moderate snow overnight tonight into early Wednesday from CO into WY and then onto the central Plains. Gulf moisture will stream northward and wrap into the developing surface low, moving east-northeastward out of southeast CO D2 as increasing frontogenesis on the NW side of the low increases snowfall rates to 1-2"/hr. This will manifest as at least a modest TROWAL wrapping cyclonically around the low into the Central Plains, with a weak TROWAL signature continuing into the Upper Midwest as the system begins to move more rapidly eastward. While overall this low should remain relatively transient, there is increasing potential for pivoting bands of heavy snow to develop where the deformation axis overlaps with this TROWAL. Some omega is enhanced through easterly/upslope flow, and modeled mid-level theta-e lapse rates fall to < 0C/km suggesting the greatest risk for CSI. Although regional soundings do not look ideal for excessive SLR, increased values to near and over climo where the best banding potential exists. ECMWF EFI reaches as high as 0.95 with a shift of tails of 2 on Wednesday. Outside of the heaviest snow axis, a longer duration of light to moderate snow is also likely within a region of broad, but slightly less intense, ascent stretching from CO through MI. Despite a relatively lower-than-average spread vs mean snowfall forecast per the WPC Super Ensemble and NBM, there remains details that are only slowly being refined; namely, strength of the warm nose and dry slot and amount of freezing rain that may occur over southeastern NE into Iowa (trend is up). In addition, deviations in the track of the surface low may result in significant shifts in the snowfall gradient which is fairly tight from SE to NW (spread of a few inches within a county). The surface low will lift into Michigan late Thursday as the jet relaxes to the east, allowing the triple point to take over and scoot the system more eastward than northeastward. Northeast flow around the low may aide in increasing some amounts on the west side of Lake Superior as well as prolonging the longevity of light snow through D3. WPC probabilities of event totals of at least 8 inches are high over northeastern CO into central NE, where amounts over 10-12" are quite possible. Farther northeast, WPC probabilities of at least 4 inches are high from northwestern IA across central WI into the southern U.P. and northern Lower Michigan. Additionally, south of the heaviest snow, an axis of moderate to heavy mixed precipitation is likely as WAA spreads a warm nose >0C northward atop still cold surface temps, followed by the impressive dry slot which will dry out the DGZ resulting in at least periods of light freezing rain/freezing drizzle. The uncertainty into the placement and timing of both the warm nose and dry slow makes this part of the forecast more uncertain than that of the heavy snow, but it is likely a swath of impactful freezing rain will occur from southeastern NE through southern IA where WPC probabilities for 0.10+" of freezing rain have increased to 40-70%. Probabilities of at least 0.25" of accretion are still low (<20%) but it bears watching. ...New England... Day 3... Triple point low from the Great Lakes system will deepen over the Mid-Atlantic and eventually take over as it slips eastward just south of Long Island overnight Thursday into early Friday. Surface high pressure will move eastward in tandem well north of the area in central northern Quebec, but a lingering wedge will help to keep the column sub-freezing over much of northern New England. Enough moisture from the south will wrap around the new surface low as it remains progressive, yielding several inches of snow across the North Country in NY into VT/NH and western Maine. WPC probabilities of at least 4 inches of snow are generally less than 50% through early Friday. Closer to the occlusion, enough warm air will override the sub-freezing boundary layer to produce a broad area of light icing over interior New York. WPC probabilities of at least 0.10" ice are below 40%. Fracasso/Weiss Key Messages for the High Plains through the Great Lakes Winter Storm: --A winter storm will develop over eastern Colorado tonight and track into the Great Lakes by Thursday with a mixture of heavy snow, sleet, and freezing rain. --Intense snowfall rates of 1-2"/hour are possible within the heaviest snow bands from northeast Colorado and southern Nebraska to northwestern Iowa. This could result in snowfall totals above 10 inches. --The combination of heavy snowfall rates and gusty winds will likely result in hazardous travel conditions due to blowing and drifting of snow creating reduced visibility. --An icy wintry mix of freezing rain and some sleet is likely from northern Kansas through southeastern Nebraska and into southern Iowa. This could result in slippery roads/sidewalks and isolated power outages.