Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 22 2023 08:28:33 FOUS11 KWBC 220826 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 326 AM EST Fri Dec 22 2023 Valid 12Z Fri Dec 22 2023 - 12Z Mon Dec 25 2023 ....Southern Rockies... Days 1-2... Upper low just southwest of San Diego this morning will weaken as it moves along the AZ/Mexico border tonight into early Saturday. However, the combination of upper diffluence and a hefty surge of moisture on southwest to southerly flow will promote widespread snow over the higher elevations of the Southwest/Four Corners this afternoon/evening over AZ and into the southern Rockies by late this evening and overnight. Orientation of the San Juans will maximize upslope over southwestern CO and significant snowfall is likely above 9000ft. WPC probabilities for at least 6 inches are high over the San Juans but also the higher peaks in eastern AZ for parts of the Sangre de Cristos in NM. ....Pacific Northwest to the Northern Rockies... Days 1-2... Sharp mid-level shortwave and surface cold front will push into the Pac NW today with a weak moisture surge but modest divergence aloft over the Cascades, along with upslope enhancement. Snow levels around 5000ft at precip onset will quickly fall as the cold front moves through, down to around 2000ft by the time the precipitation ends. Passes across the Cascades will likely see several inches of snow, with much higher totals above pass level in the WA Cascades. By Saturday morning, the shortwave will move into Idaho, with generally light snow for many of the ranges across central/northern portions and into northwestern WY (driven mainly by upslope and some lower level convergence). WPC probabilities of at least 6 inches are highest over the Cascades, with low values (10-40%) over much of ID into WY. ....Central Rockies to the Plains and Upper Midwest... Day 3... Combination/interaction/evolution of the aforementioned two systems, the northern stream shortwave and southern stream ex-upper low, will spread snowfall eastward out of the Rockies and onto the Plains by Sunday into Monday (Christmas) and likely beyond. Models/ensembles have shown a fair amount of inconsistency in their depiction of these two players, now favoring the northern stream shortwave to close off into an upper low over western CO by late Sunday and become the dominant feature into the Plains. This, in turn, suggests a weaker initial round of snow (rain to snow) as the lead southern shortwave takes a modest wave of low pressure out of southeastern CO toward the Upper Midwest beneath a ~100kt jet streak. Here, light to perhaps modest snows are likely from eastern CO/WY northeastward into the NE panhandle and into SD. There, WPC probabilities for at least 4 inches are low (10-40%) to moderate (40-70%), focused along the central SD/NE border. More appreciable snow is forecast for the CO Rockies (above 7000ft) as the shortwave closes off on Sunday and pushes through the southwestern corner of the state. To the north, cold front may move in a bit sooner than the precip spreads northward (again, fairly uncertain extent on the northern side) which may allow for sub-freezing boundary layer beneath a warm nose off the deck, promoting freezing rain over parts of SD across eastern ND into northwestern MN. There, WPC probabilities of at least 0.10" icing are low (10-40%) and are highly dependent on the amount of QPF and speed of cold air intrusion. Probabilities of at least 0.25" are quite low (5% or less) at this time but some ensemble members are quite aggressive. The second part to this system will unfold into day 4 as the upper low wobbles eastward toward the Corn Belt. Fracasso $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .