Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EST Wed Nov 20 2019 Valid 12Z Sat Nov 23 2019 - 12Z Wed Nov 27 2019 ...Pattern Overview and Guidance/Predictability Assessment... Models have continued to struggle the timing of shortwaves in the northern stream during the medium range. Farther south, the multi-day trend toward faster progression of a southern stream trough eastward seems to have finally come to and end. Models now show good agreement in a low somewhere across the Tennessee Valley early on day 3 (Sat). The significant question now pertains more to the timing and amplitude of northern stream energy, which impacts the degree of phasing which occurs heading into day 4 (Sun), and the track of a low off the Northeast U.S. coastline. The 12Z CMC/UKMET showed less phasing, and a surface low track farther offshore, while the 12Z ECMWF and the past few runs of the GFS have shown a greater degree of phasing sooner, with a low track closer to the coast. With this system preferred a blend of the 12Z ECMWF/GFS, which shows a moderately phased solution, but not to the degree of the 18Z (or 00Z) GFS. Farther west, guidance shows consensus that a vigorous shortwave should reach the Pacific Northwest on Sun and then quickly amplify across the interior West on Mon. Much colder air should spread into the western states as the trough amplifies, with a cold front reaching the Plains by Mon. Models show an above average degree of consensus by days 6-7 (Tue-Wed) on developing a wave of low pressure along the front across the southern/central Plains strong shortwave energy rounds the base of the western trough, pushing strong height falls east into the Plains. While models vary on timing and track, the latest runs of the deterministic ECMWF/GFS/CMC all deepen this surface low as it moves northeastward into the Midwest by Tue night/Wed. Ensemble members have shown a cluster around this solution for at least the past few runs, indicating a surprising degree of consistency given the day 7 time range. Lastly, models also show consensus that another significant shortwave should reach the Pacific Northwest Tue night/Wed, bringing significant height falls and potentially a relatively strong area of low pressure. The WPC forecast was based on a blend of the deterministic 12Z ECMWF/GFS during days 3-4, with a gradual shift toward heavier weighting of ECENS/NAEFS ensemble mean through time during days 5-7. ...Weather Highlights/Threats... Low pressure crossing the eastern U.S. on Sat is expected to bring showers and potentially a few thunderstorms to areas from the Southeast and Southern Appalachians to the Mid-Atlantic, with locally heavy rain possible. Greater uncertainty exists farther north due to the variability surrounding the surface low track off the Northeast coast. At this point in time, it appears that at least some precipitation is possible across southern/eastern New England, with thickness profiles marginal for rain/snow across the region. Given the range of model solutions at time time, it is possible that precipitation could stay entirely offshore if the low tracks farther east, or that somewhat heavier precipitation reaches the coast if the low tracks farther west than forecast. Farther west, rain and mountain snow are expected across the Pacific Northwest and northern Great Basin/northern Rockies Sat-Sun as the first of the two significant shortwaves and cold fronts move inland. As low pressure begins to develop across the central U.S. early next week, models hit at the potential for rain and thunderstorms to affect a large area from the Gulf Coast to to the mid-Mississippi Valley, with heavy rain possible. Farther north, depending on the eventual intensity and track of the surface low across the northern Plains/Midwest/Great Lakes, winter weather (potentially significant) is a possibility. Based on the WPC forecast and current ensemble data, the great probability for significant snow during the Tue/Tue night time frame is across portions of the Upper Midwest, including southeastern Minnesota, western Wisconsin, and portions of northern Iowa. Please refer to WPC Day 4-7 Winter Weather Outlooks for specifics on the medium range winter weather threat. Finally, as the next low pressure system reaches the Pacific Northwest toward the middle of next week, expect a renewal of potentially heavy rain/snow for that region. Expect chilly temperatures to overspread the eastern U.S. Sat-Sun, with highs 5 to 10 deg F below average. A warming trend will bring high temperatures 5 to 10 deg above average to the northern Plains and Upper Midwest over the weekend. Meanwhile, as the upper trough digs into the West by early next week, expect temperatures to drop, with highs 5 to 15 deg below average across much of the interior western U.S. Below average temperatures should spread east into the central U.S. by the middle of next week behind the cold front and the strengthening low pressure system. 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