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   FOUS11 KWBC 250703   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   203 AM EST Thu Dec 25 2025   
      
   Valid 12Z Thu Dec 25 2025 - 12Z Sun Dec 28 2025   
      
   *** Continued Heavy Snow on the Sierra Nevada through Friday ***   
      
      
   ...Great Lakes to Northeast...   
   Days 1-2... Maximum impact level from WSSI: Moderate   
      
   Quasi-zonal flow will direct a plume of moisture originating from=20   
   the West along the US/Canadian border today/tonight with WAA-driven   
   snow over the MN Arrowhead into the U.P. of Michigan. With cold=20   
   sub-32F sfc temps over parts of MN/WI/Lower MI at precip onset=20   
   tonight, some freezing rain is likely with minor accumulations over   
   MN/WI. WPC probabilities for at least 4 inches of snow are low to=20   
   moderate (20-60%) over the far northeastern MN Arrowhead into the=20   
   Keweenaw Peninsula.=20   
      
   Into D2, shortwave will push through northern Lower MI with its=20   
   sfc reflection over IN/OH and the 850mb low moving through BUF. The   
   sfc low will weaken as it approaches the Appalachians, getting too   
   far behind the mid-level shortwave, and ultimately jumping into=20   
   the Atlantic by 12Z Sat. From Friday to Saturday morning, a variety   
   of ptypes are expected over the region (central Appalachians/Mid-=20   
   Atlantic to the Northeast). The stubborn cold surface air will hold   
   tight over Lower MI and especially into the Laurel Highlands in=20   
   western PA where significant freezing rain is possible. WPC=20   
   probabilities of at least 0.10" icing are high (>70%) from around=20   
   Detroit and into western PA/western MD, with 40-60% probabilities=20   
   of at least 0.25" icing in these areas as well. Light icing is=20   
   likely on either side of this zone of maximum probabilities -- on=20   
   the south side along the I-66 corridor (DC) and into eastern WV. To   
   the north/northeast, deeper cold air at the surface will allow for   
   sleet to be a dominant ptype over central/eastern PA with up to an   
   inch of so possible. To the north, from central NYS through=20   
   northeast PA into the Catskills/Poconos and to the NYC Metro area,=20   
   an axis of moderate snow is possible, driven by the stronger height   
   falls at the nose of the incoming 130kt jet. The whole scenario is   
   still uncertain in where the transition zones align as some of the   
   00Z guidance (e.g., the NAM) were much farther northeast with the=20   
   WAA than the other global guidance. The recent trend has been=20   
   toward this milder scenario but only nudged the forecast somewhat=20   
   given the lead time still.=20   
      
   WPC probabilities for at least 4 inches of snow are >50% in a zone   
   bounded between I-890/Rt 17/I-476 to the west, the southern=20   
   Adirondacks on the north side, Berkshires/I-91 to the east, and=20   
   into the NYC Metro/Long Island to the south. Though the system will   
   be progressive, combination of higher SLRs and upslope could yield   
   more than 8 inches of snow into the southern Catskills.=20   
      
      
   ...California/Sierra Nevada...=20   
   Days 1-3... Maximum impact level from WSSI: Extreme   
      
   The next moisture plume with the long-lived atmospheric river=20   
   event will arrive this morning into CA amid lower snow levels=20   
   around 55000-6500ft and heavy snow rates of 2-4"/hr through midday=20   
   before rates again decrease in showery conditions through the=20   
   afternoon. The last larger punch of moisture arrives this=20   
   evening/overnight with persistent snow levels of 5500/6500ft=20   
   through the night before dropping to 5000ft for Friday with=20   
   continued moderately heavy rates (1-2"/hr). Precipitation is=20   
   expected to finally wane by Friday night as the upper trough moves=20   
   ashore with ridging to follow. The lower snow levels and rounds of=20   
   heavy snow through Friday will lead to increasing travel and=20   
   infrastructure impacts. Additional snowfall of 2-5ft can be=20   
   expected above 6000ft along the extent of the Sierra Nevada.   
      
   Far northern CA ranges (Shasta Siskiyou/Klamath/etc) can expect=20   
   moderately heavy snow above 4000ft or so as snow levels dip to=20   
   start the period. Some snow impacts to I-5 near Mt Shasta are=20   
   possible mainly early Friday. Most snow will end by 12Z Saturday.=20   
   Over SoCal, snow levels drop to around 7000ft tonight where they=20   
   linger through Friday. Impactful snow looks to remain higher than=20   
   Big Bear Lake.=20   
      
   Key Messages for the CA atmospheric river and subsequent low that=20   
   maintains impacts through Friday are linked below.   
      
      
   ...Pacific Northwest, Great Basin and Rockies...=20   
   Days 1-3... Maximum impact level from WSSI: Moderate to Major   
      
   Though the AR affecting CA will infuse some moisture into the=20   
   Rockies, an incoming trough out of the Gulf of Alaska will drop=20   
   into the PacNW on Friday with a renewed increase of moisture to the   
   area as snow levels remain on the lower side -- generally=20   
   2500-3500ft in the Cascades. That trough will exit WA early=20   
   Saturday and bring widespread light to moderate snows to the=20   
   northern Rockies the CA AR (via the separate upper trough) moves=20   
   inland as well. This will act to expand the area of snow across=20   
   most of the Great Basin to central Rockies Saturday into early=20   
   Sunday before the entire full- latitude trough then exits onto the=20   
   Plains just beyond this forecast period. Snow levels will be much=20   
   lower over MT/ID and western WY compared to UT/CO (closer to the=20   
   warmer AR) -- generally 4000ft to the north (and falling to the=20   
   valley floors) ramping up to around 7000-8000ft over CO, falling to   
   below 6000ft by early Sunday morning.=20   
      
   For days 1-2, WPC probabilities for at least 12 inches are >50%=20   
   above about 2500ft in the WA Cascades (still affecting the passes),   
   5000ft in western MT, 6000ft in central ID, 7000-8000ft in the=20   
   western WY ranges, and about 9000ft in the Uintas. For days 2-3,=20   
   the focus will be on the CO Rockies where WPC probabilities for at=20   
   least 8 inches of snow are >50% above about 9000ft.=20   
      
      
   Fracasso   
      
      
   ...Winter Storm Key Messages are in effect. Please see current=20   
    Key Messages below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!9DtysQKcTuXQhxMcKfHeYjCpIeqFJmrWgTAvy3gKqa5WQ=   
   6_sMtVxVGdijBYGfI2ZZ8pAjxJzPTa_xFdkiKV4mKY6LGw$=20   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_2.png__;!!DZ3fjg!9DtysQKcTuXQhxMcKfHeYjCpIeqFJmrWgTAvy3gKqa5WQ=   
   6_sMtVxVGdijBYGfI2ZZ8pAjxJzPTa_xFdkiKV4LAyvh_8$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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