Extended Forecast Discussion NWS Weather Prediction Center College Park MD 126 PM EST Sun Jan 12 2020 Valid 12Z Wed Jan 15 2020 - 12Z Sun Jan 19 2020 ...Potential for significant late-week winter storm from the Upper Midwest to the Northeast... 1830 UTC Update... The 00Z ECMWF was favored over the GFS for this update through much of the extended forecast period, with a gradual shift to more ECENS/NAEFS ensemble mean weighting by days 6-7. The GFS has struggled with northern stream flow and the embedded shortwaves across southern Canada and the northern CONUS Wed-Fri (days 3-5), and the ECMWF was well-situated in the center of an ensemble-based consensus. Thus, the ECMWF Was heavily used during the day 3-5 time period, along with a small amount of the GFS and ECENS/NAEFS ensemble means. Farther west, by Fri, models/ensembles continue to suggest that a potentially significant low pressure system will begin to develop across the central U.S. as a vigorous mid/upper-level shortwave crosses the Rockies. Ensembles have shown a remarkable reduction in spread for this system over the past couple cycles, especially considering the time frame (day 5-7). Models/ensembles show general consensus that a surface low should deepen as it crosses the mid-Mississippi Valley and moves toward the Great Lakes Fri night/Sat morning, with a swath of winter weather expected north and west of the low track. By Sat night/Sun, guidance shows rapid development of a new triple point/coastal low across the Northeast U.S., with some solutions quite deep by 12Z Sun as the system passes along the New England coastline (the ECMWF/GFS/CMC were all below 980 hPa at that time). Ensemble members are quite well-clustered around this consensus, suggesting some degree of above average forecast confidence. One note of caution, however - there is some possibility that the apparent excellent consensus could be due, at least in part, to underdispersion among the ensemble guidance. Thus, while confidence is certainly above average relative to what is typical for days 5-7, it may not be quite as high as the guidance consensus would suggest. Would like to see this solution persist for another model/ensemble cycle or two to further bolster confidence in this event. Nonetheless, Winter Weather Outlook probabilities associated with this system were raised in this update over what was issued overnight. Otherwise, no major changes were made to the ongoing extended forecast across the remainder of the CONUS. Ryan Previous Discussion (issued at 0613 UTC)... ...Weather Pattern/Hazard Highlights and Guidance/Predictability Assessment... Mid-upper level troughing over the West will periodically be reinforced with dynamic eastern Pacific energy, but with a trend toward less amplified flow through later week. Guidance supports a deep low off the Pacific Northwest Wed/Thu that would enhance maritime hazards and inland flow/weather, with energies then working into the West. Details are less certain for upstream storm potential, but a favored guidance composite signal is growing for low development next weekend more off the Pacific Northwest/western Canada. WPC QPFs and winter weather outlook probabilities show a widespread snow threat for the Northwest that reach lower elevations given the extent of cold air filtration. Activity spreads into CA and the north-central Great Basin/Rockies mid-late week. Snows over the northern High Plains/Rockies will be enhanced by post-frontal upslope flow. Activity works back in earnest into the Northwest next weekend. Upper trough energies and surface systems will eject northeastward from the West over the central and eastern U.S. well overtop a lingering and warming/stabilizing Southeast ridge and interacting with a lower atmospheric Arctic airmass. A preferred composite blend of varying GFS/ECMWF and GEFS/ECMWF ensemble guidance highlights a moderately deepening low threat from the central Plains to the Northeast Wed/Thu. The blend is now better clustered with deep cyclogenesis from the Plains to the Midwest/Great lakes to the Northeast Fri-next Sunday, bolstering forecast confidence in this potentially significant system. This would favor heavier rainfall potential later week/next weekend from the south-central U.S. and mid-lower MS/OH Valleys to southern New England. The pattern offers several heavy snow/ice threats in the cooled northern tier from the Midwest/Great Lakes to the northern Mid-Atlantic/Northeast as the deepening frontal lows wrap in deeper moisture as shown by WPC progs, grids, QPFs and winter weather outlook probabilities. Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml