Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 358 PM EST Wed Nov 29 2023 Valid 00Z Thu Nov 30 2023 - 00Z Sun Dec 03 2023 ...Southwest and Southern and Central Rockies... Days 1-3... A series of systems are expected to impact the region, producing high elevation snow with locally heavy amounts possible. The initial wave is forecast to slide east along the U.S.-Mexico border into the Southwest. Favorable upper forcing generated by the wave along with a low-to-mid level front will support expanding precipitation from northeastern and east-central Arizona into northwestern New Mexico on Thursday, with some potential for heavy snow developing for areas mainly above 6000 ft. This initial wave will be a progressive system, moving into the southern Plains by late Thursday. However, a series of shortwaves moving quickly on its heels will maintain a trough across the region and produce additional high elevation snow across the Southwest into the southern Rockies, as well as across areas farther to the north into the central Rockies. While not expected to be a widespread heavy snowfall event, WPC probabilities continue to highlight some areas where locally heavy amounts are likely. This includes Arizona's White Mountains, the Jemez, San Juan, and Elk mountains. Probabilities for storm total amounts of 6 inches or more are high (greater than 70 percent) for much of these regions, with high probabilities for 8 inches or more across some of the higher peaks. ...Cascades through the Northern Rockies... Days 1-3... A prolonged period of wet and unsettled weather, that is likely to continue to next week, is expected to begin tomorrow with the first in a series of shortwaves. The initial shortwave is forecast to dig southeast from the Gulf of Alaska and the northeastern Pacific into the region on Thursday. Moisture with this initial wave will be limited and the system will be progressive -- limiting amounts. However, snow levels will be low, impacting the Cascade passes at the onset. While the leading wave moves into the northern Rockies on Friday, an upstream wave will quickly follow with an uptick in moisture and better forcing to produce heavier amounts across western Washington and Oregon, while snow levels remain relatively low. This second wave will be progressive as well, however a long fetch of onshore flow with embedded energy aloft will follow, supporting additional periods of precipitation through the end of the period. While there are some latitudinal differences, models overall agree that a plume of subtropical moisture will begin to take aim at the Oregon coast by late Saturday. Snow levels are expected to rise as a flat ridge begins to build along the coast. By late Saturday, snow totals of a foot or more are likely to cover much of the Washington and Oregon Cascades, including the major passes. Heavy accumulations are expected farther east as well across the Blue Mountains and portions of the northern Rockies, including the northern to central Idaho ranges, and the western Wyoming to northern Utah ranges. Some locally heavy amounts are possible in the northern Nevada mountains as well. WPC guidance shows at least moderate probabilities (greater than 40 percent) for accumulations of a foot or more across these areas by late Saturday. ...Central Plains into the Midwest... Day 2... Shortwave exiting NM Thursday afternoon will lift into OK later that evening as the upper jet strengthens over TX. Surface low pressure will develop and deepen into the TX Panhandle and cross over the Red River into OK overnight. With a surge of moisture northward out of the NW Gulf will wrap up and into the low amid a marginally cold air column on the northwest side of the precipitation shield. Model spread remains regarding thermal profiles but potential exists for a narrow stripe of snow 1-2" from southwest to northeast across KS though WPC probabilities remain low. Farther northeast, from northeast KS to southeast IA, a stripe of freezing rain and/or a mix is probable where WAA aloft will be stronger just to the north of the low track. A second shortwave may be a bit colder, and bring another bout of light snow to portions of southern IA eastward. WPC probabilities of at least 0.1" ice near the IA/MO border remain low (<30%). Pereira/Fracasso ***Key Messages for Pacific Northwest Winter Storm*** --A long-duration winter storm will start on Thursday along and west of the Cascades with rain for the valleys and snow at many mountain passes. Heavier precipitation is expected to push into the region on Friday and continue into the weekend. --Through early Sunday, there is a high chance (>70%) of at least 12" of snow across the Cascade passes, including I-90. Travel will be difficult due to snow and blowing snow. --Over eastern Washington and Oregon into northern Idaho, there is a moderate chance (>40%) of at least 8" of snow at many passes. Strong winds this weekend could bring down tree branches. --Rising snow levels Sunday will change snow to rain over all mountain passes through the Cascades. Several inches of rain will bring the threat of river flooding late this weekend into next week.