Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EST Sun Nov 17 2019 Valid 12Z Wed Nov 20 2019 - 12Z Sun Nov 24 2019 ...Heavy rain possible for portions of the Southwest through midweek... ...Pattern Overview and Guidance/Predictability Assessment... Amplified and moderately progressive northern stream flow is expected to persist across the CONUS through the medium range, with a tendency for periods of split flow as energy separates and cuts off south of the strongest westerlies. An upper-level trough is forecast to exit the eastern U.S. on Wed as a deepening coastal low moves quickly into the Canadian Maritimes. A second northern stream wave should quickly follow on its heels, moving a low pressure system and trailing cold front across the Midwest/Great Wed night/Thu and the eastern U.S. by Fri. Farther west, northern stream energy is forecast to dig southward along the West Coast Wed, and models/ensembles show consensus that the feature should eventually close off into an upper-level low, which should then slowly progress eastward across the Southwest. This feature continues to be the biggest forecast problem of the medium range, as solutions differ on exactly how far south/west the feature will dig before moving east, and also on the speed with which it moves eastward (which in turn introduces further complexities and potential interactions with a northern stream wave). The GFS has consistently been quicker to move the upper low eastward, with the ECMWF on the slow side of the spread, and the CMC along with ensemble means in the middle. Taking a look at the hemispheric pattern, the broader pattern shows very little in the way of significant flow blocking, with things remaining largely progressive from Asia across the North Pacific to North America. Additionally, teleconnections associated with persistent positive height anomalies becoming established across western Canada favor colder conditions/lower heights across the eastern half of the CONUS, suggesting that the flow should have a tendency to remain somewhat progressive. Given these considerations, decided to lean away from the slowest ECMWF solution and follow more closely with the 12Z CMC, which seemed to represent a compromise. Given the favored timing, this idea would favor the development of a wave of low pressure along the trailing frontal boundary across the lower Mississippi Valley by Fri night-Sat which would then move northeastward into the Ohio Valley/central Appalachians by Sat night-Sun. The WPC forecast was initially based on a blend of the 18Z GFS and 12Z ECMWF/CMC during days 3-5 (Wed-Fri). After that time, given the above train of thought, a blend of the 12Z CMC and 12Z ECENS/NAEFS ensemble means was used, with increasing weight place on ensemble means through time. ...Weather Highlights/Threats... Showers and perhaps a few thunderstorms are possible across the Southern/Central Plains and mid-Mississippi Valley Wed-Thu in association with the cold front crossing the region. Rain and areas of snow associated with this system are expected to spread into the Great Lakes/Northeast Thu-Fri. Farther west, as the trough digs along the West Coast and the upper low develops, moisture will be drawn northward into the southwestern U.S., potentially including moisture associated with the future remnants of Tropical Storm Raymond. Widespread and potentially heavy rains will be possible across much of the Southwest Wed-Thu, with some areas expected to receive at least a couple inches of rainfall. Snow will be possible at the higher elevations from the southern Great Basin to the central/southern Rockies. The wave of low pressure expected to develop across the lower Mississippi Valley Fri-Sat will bring the potential for a large area of overrunning rain from the Gulf Coast/lower Mississippi Valley, eventually reaching the Southeast, Appalachians, and Mid-Atlantic Fri-Sun. Model spread remains fairly high, but in the scenario described above, rainfall totals of a couple inches would be possible from the lower Mississippi Valley to portions of the Southeast. Below average temperatures are expected to overspread much of the western and central U.S. Wed-Fri with the upper low digging into the Southeast and the cold front crossing the central/eastern U.S. High temperatures across the West are expected to be 5 to 15 deg F below normal through this time period. While the central U.S. should initially be warm (highs 5 to 10 deg above average) on Wed, cooler temperatures will arrive in the wake of the front, with highs 5 to 15 deg below average Thu-Sat. 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