Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Wed Dec 25 2019 Valid 12Z Sat Dec 28 2019 - 12Z Wed Jan 01 2020 ...Significant winter weather event possible for portions of the Midwest and Upper Great Lakes Saturday and Sunday... ...Overview and Guidance/Predictability Assessment... Models have rapidly come into much better consensus over the past 24-36 hours on the evolution/progression of an upper low which is forecast to have already begun the process of phasing with a northern stream shortwave across the central Rockies/High Plains on Sat (day 3). At least during the Sat-Sun time period the majority of guidance has now come around to the idea which the GFS has shown for multiple days of a phased and highly amplified trough/upper low with a relatively deep surface low expected to move from the Central Plains Sat morning to the Upper Midwest/Great Lakes Sun morning. Models begin to diverge during the day on Sun, with the ECMWF again making an attempt to slow the system down substantially with a forecast upper low lingering across the Great Lakes into the middle of next week, while the GFS keeps things much more progressive. The 12Z UKMET was somewhat of a middle ground, although a closer to the more progressive GFS idea. Given the superior performance of the GFS in recent days in terms of forecast consistency, opted to once again lean toward the GFS in this forecast cycle. In this solution, a triple point surface low should develop across the Northeast by Sun night/Mon, with the cold front quickly sweeping off the Eastern Seaboard by that time. Farther west, a similar scenario as seen in at least a couple instances recently looks to occur once again, with nearly all guidance forecasting a shortwave to separate from an active North Pacific jet and dive southward along the U.S. West Coast as it amplifies, with development of a closed upper low expected on Mon. Most guidance show the system continuing a slow drift southeastward through Tue before eventually falling under the influence of an amplifying trough farther north and moving east more quickly by the end of the forecast period/middle of next week. The ECMWF was a bit less amplified initially and slower to close the feature off, although it has been rather inconsistent from run-to-run. The GFS has again shown a higher degree of consistency with this system, and a blend of the GFS along with the ensemble means was the preferred solution by late in the forecast period. The WPC forecast during days 3-5 was based heavily on the 18Z GFS/12Z UKMET solutions along with some influence from the GEFS/ECENS ensemble means. Starting on day 6 (Tue), increased weight was place on the GEFS/ECENS ensemble means, along with some continued use of the GFS through day 7 (Wed). ...Weather Highlights/Threats... As low pressure develops and begins to deepen across the Central Plains on Sat and moves northeastward into Sun, an area of precipitation is expected to develop and expand northward and eastward across the Plains and eventually the Midwest/Mississippi Valley. Sufficient cold air may exist for precipitation to fall as a wintry mix/and or snow in the northern portion of the precipitation area. The greatest potential for significant winter weather Sat/Sat night appears to be in a band across portions of the Upper Midwest and Upper Great Lakes. Given improved confidence in this scenario, boosted winter weather outlook probabilities for these areas. By Sun/Sun night, some potential winter weather may continue for portions of the Upper Midwest/Great Lakes on the west side of the low pressure system, while accumulating winter weather will also be possible for portions of the Northeast and especially northern New England. Please see the medium range winter weather outlook products for more details on the winter weather threat. Farther south along the cold front, rain is expected from the lower Mississippi Valley to the Appalachians and Mid-Atlantic regions Sat through Sun night before the cold front quickly moves offshore by Mon morning. Across the Pacific Northwest, as the shortwave moves onshore early next week, rain and mountain snow are forecast to develop across the coastal ranges and Cascades in Oregon and Washington. Temperatures are forecast to be well above average across the central and eastern U.S. Sat-Sun ahead of the low pressure system. High temperatures of 10 to 20 deg F above average are forecast across a large area of the central/eastern U.S., with highs forecast to exceed 20 deg above average across portions of the Mid-Mississippi and Ohio Valleys as well as the lower Great Lakes. The cold front forecast to arrive by Sun-Mon will bring an end to the warm temperatures from west to east, with temperatures returning to near seasonal norms. Meanwhile, given persistent lower heights across the interior western U.S., highs are forecast to be 5 to 10 deg below average from the Great Basin to much of the Southwest over the weekend, with temperatures moderating somewhat by early next week. Ryan 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