Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EST Fri Dec 27 2019 Valid 12Z Mon Dec 30 2019 - 12Z Fri Jan 03 2020 ...Significant winter storm expected to impact parts of the Upper Great Lakes into Upper Great Lakes/Northeast early next week... ...Heavy rainfall potential from the western-central Gulf Coast region northeastward mid-late next week... ...Overview and Guidance/Predictability Assessment... The guidance continues to show fairly good agreement at least with respect to the large scale mean flow. The upper low over the Upper Midwest early day 3/Monday will track gradually northeast while another vigorous upper system drops southward along or just inland from the West Coast. As the West Coast feature reaches over or near northwest Mexico, strong North Pacific flow should reach into western North America. Models show upper ridging over the east Pacific to strengthen by the middle of next week, which eventually should lead to amplified troughing over the central U.S./northern Mexico by next Thursday and Friday. Though large scale flow evolution shows good agreement through the period, there remains some fairly notable differences with respect to timing of the various systems. For the initial Midwest upper low, some timing differences remain, but there seems to be a trend towards the slower ECMWF/CMC. This would allow for more high pressure to build into eastern Canada for a time, leading to a bit slower northward progress of triple point wave development tracking from off the northern Mid-Atlantic coast into the Canadian Maritimes by Wednesday. Meanwhile, fairly significant differences continue for the energy dropping southward along the West Coast. The GFS/GEFS mean remain slower than the ECMWF/ECENS mean but have trended slightly faster with the past couple of runs. The latest CMC runs are slower than the ECMWF, but still faster than the GFS, while the latest UKMET sits right in between the ECMWF and CMC. This difference has significant influence downstream involving how quickly moisture spreads from near the Gulf Coast and northeastward Tuesday night onward with the GFS scenario the slowest. There remains some differences in the evolution of reinforced amplified troughing across the Central U.S. at the end of the period with respect to the North American energy dropping into the West. The CMC is much faster to push this trough eastward next Friday/day 7 than the remainder of the guidance and the means (even its own). Based on the above evaluation, the WPC blend for days 3-5 used a majority deterministic guidance blend between the 12z ECMWF/CMC/UKMET with smaller contributions from the 18z GEFS/12z ECENS means. After day 5, increased the weighting of the ensemble means and removed the CMC from the blend (and the UKMET since it is not available past 12z day 5). Was able to incorporate some of the 18z GFS past day 5 though as it became more similar to the ECMWF and the ensemble means with the larger scale pattern. This blend resulted in good continuity from the previous WPC shift. ...Weather Highlights/Threats... By the start of the medium range period (Monday), the storm system initially over the Upper Great Lakes should weaken in favor of a developing coastal wave off the Northeast coast. Precipitation with this system will be ongoing across the Upper Lakes into the Northeast Monday into Tuesday with snow likely across northern areas. The latest WPC medium range winter weather outlook shows the best chance for accumulating significant snowfall across far northeast Minnesota/northern Wisconsin, as well as across interior New England. Precipitation (lower elevation rain and mountain snows) across the Pacific Northwest will be ongoing through much of the period as subsequent waves of energy move onshore. Some moisture should reach into the Intermountain West, though with lesser intensity. Ahead of the upper low in northern Mexico, ample moisture from the Gulf of Mexico is likely to surge northward across the eastern half of the country by the middle to later part of next week. Expect a period of widespread showers and thunderstorms across portions of the western/central Gulf Coast and lower Mississippi Valley, which should eventually spread northeastward into the Tennessee Valley/central Appalachians as well. While confidence remains low for precise timing and duration, the models continue to show a decent signal for a meaningful rainfall event over this general area. The warm sector ahead of the initial Upper Great Lakes storm will contain much above normal temperatures to start next week, with some readings as much as 20F above normal over the Mid-Atlantic. Frontal passage will bring a cooling trend but only to near/moderately above normal levels. Central U.S. trough amplification aloft late in the period should encourage a warmer trend again over the East by next Thu. Areas from the central/southern Rockies into the Southwest are likely to see the most persistent below average temperatures with some areas averaging 5-10F below normal for highs through the Mon-Fri period. Santorelli 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