Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 353 AM EST Thu Mar 02 2023 Valid 12Z Thu Mar 02 2023 - 12Z Sun Mar 05 2023 ...New Mexico and Southern High Plains... Day 1... Deep low pressure currently over southern AZ will shift east across southern NM into west TX today. Tropical Pacific moisture wrapping in ahead of the low is largely wrung out over Mexican Highlands, but there is some making it over along with western Gulf moisture which will make for moderate snow over northern NM terrain to the Raton Pass in CO (where there are moderate probabilities for 4 or more inches after 12Z) with an fgen band over the southeast CO High Plains into northwest KS where generally 2-4" are forecast). ...Maine... Day 1... An amplifying shortwave trough currently over the northern Great Lakes will cross northern New England this evening as it rides the strong SWly jet extending northeast from southern NM ahead of the deep low currently down there. The developing coastal low will lock in the cold air across northern New England and throw moisture back into Maine while also dragging cold air down as cold air advection commences in its wake. This will result in a swath of moderate to heavy snow today over northern/down east Maine. Day 1 WPC probabilities remain around 50% for >6 inches across northern Maine. ...Pacific Northwest through Central California... Days 1-3... A deep low over the Gulf of Alaska will continue to shear apart as it moves into the short-lived ridge currently extending northeast from the Pac NW. The leading edge of this upper trough pushes east over WA today with a plume of Pacific air shifting inland ahead of the axis/cold front. Moderate precip and snow levels around 2000ft shift over the Pac NW to the northern Rockies today with high Day 1 snow probabilities for >8" over the WA into northern OR Cascades and moderate over the Clearwater Mtns of ID. The next wave dives southeast from the Gulf of Alaska Friday, closing into a deep low that wobbles to the WA/OR border Saturday night. Persistent onshore flow and moisture advection combined with continued ascent, especially including upslope enhancement where mid-level flow is orthogonal to the terrain, will provide a long duration moderate to heavy snow event for the northern Cascades. Snow levels will be quite low tonight into Saturday, generally around 500ft. Day 2 snow probabilities for >8" are moderate to high over the northern Cascades and Olympics (with some values on the OR Coastal Ranges). The focus for precip shifts south to southern OR and through central CA for Sat/Sat night. Snow levels will generally be to 1000-3000 ft across this region on Day 3 where snow probabilities for >12" are high (Klamath and the northern Sierra Nevada) with moderate probs for >4" over northern OR into WA. ...Central Plains, Upper Midwest into the Great Lakes and Northeast... Days 1-3... The potent closed low swinging across TX/the southern Plains tonight turns northeastward across the lower MS VLY Friday morning before crossing the eastern Midwest Friday afternoon and over the Northeast Friday night. Mild air across the southern Plains keeps the snow risk generally northeast from the Central Plains. In addition to the deep layer ascent which will spread ahead of the low, moisture will become quite anomalous downstream of the parent trough as meridional flow surges out of the Gulf of Mexico. Isentropic ascent becomes impressive along the 285-290K surfaces, with PW anomalies surging to +2 to +3 standard deviations above the climo mean in a narrow ribbon collocated with strongest low-level winds out of the south. This will concurrently push a strong theta-e ridge axis northward which will rotate cyclonically through the warm conveyor belt into a TROWAL by Friday. An expansive shield of precipitation will spread from the mid-MS VLY through the Mid-Atlantic Friday, crossing the Northeast Friday night into Saturday. Where the primary low tracks into the Great Lakes, a region of enhanced deformation will develop NW of the 850mb low, and as this occurs, especially during surface low deepening, a p-type transition from rain to snow is likely as CAA surges southward, aided by cold ageostrophic drainage into the low. Despite an unfavorable sfc high position well northeast of the surface low, there should still be a rapid changeover to heavy snow and at least moderate accumulations from the dynamics of the powerful and often negatively-tilted trough. Day 2 WPC snow probabilities for >6" are moderate over Chicagoland and much of the central L.P. of MI with low probabilities in southeast WI and toward northwest IN as well as across southern MI. Lake enhancement is expected in the ENE flow over Lakes Michigan and Huron. Similar to the system earlier this week, the primary vorticity structure associated with this low will be sheared out into the northern stream westerlies as it crosses the Northeast. However, sufficient energy is translated to a coastal low the tracks over/near Long Island - farther north than the Monday night system. So this system starts off stronger than the early week system and there is more moisture with a wide open Gulf of Mexico. Impressive upper diffluence as the downstream zonally oriented jet streak intensifies to 190 kts over New England in conjunction with a jet streak rotating through the base of the primary trough and arcing poleward to produce an intensely coupled jet structure. This overlap of robust synoptic ascent will result in a rapidly deepening surface low moving from OK to the Great Lakes Thursday night and Friday, before energy begins to transfer off southern New England Day 2.5 snow probabilities are high for >8" over much of the Adirondacks, southern Greens, and southern NH over the Monadnock and Merrimack regions of southern NM into the southern Maine coast. greatest risk for heavy snow exceeding 6 inches is stretched from near the Tug Hill Plateau eastward through Portland, ME and including much of the higher terrain from the Catskills through the Greens, Worcester Hills, and Monadnock region of NH where WPC probabilities for more than 4 inches are above 80% D3. South of here, at least modest ice accumulations are possible for the Laurel Highlands where Day 2 WPC probabilities for more than 0.1" peak around 20%. Jackson ...Key Messages for Plains to Northeast Winter Storm... --A potent winter storm will produce possibly narrow bands of heavy snow and mixed precipitation, including sleet and freezing rain to the south of the snow, from the north-central Mississippi Valley through the Great Lakes and central New England Friday through Saturday. --The combination of heavy snow rates exceeding 1"/hr and strong winds would result in hazardous travel as well as impacts to infrastructure. --Uncertainty remains with the location and strength of the snow swaths over the Midwest/Great Lakes as well as the areas of greatest impact over the Northeast. Keep up to date with the latest forecasts as this storm evolves.