Extended Forecast Discussion NWS Weather Prediction Center College Park MD 136 AM EST Sat Nov 16 2019 Valid 12Z Tue Nov 19 2019 - 12Z Sat Nov 23 2019 ...Heavy rain possible for portions of the Southwest Tue-Thu... ...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. days 3-4 (Tue-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 cold front across the Midwest/Great Lakes Wed night/Thu and the eastern U.S. by Fri. Farther west, northern stream energy is forecast to dig southward along the West Coast Te-Wed, and models/ensembles show consensus that the feature should eventually close off into an upper-level low, which should the slowly progress eastward across the Southwest. This feature becomes 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/CMC on the slow side of the spread, and ensemble means largely 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 at least somewhat toward the faster side of the spread with the upper low. The WPC forecast was initially based on a blend of the 12Z ECMWF/18Z GFS solutions during days 3-4 (Tue-Wed). After that time, given the above train of thought, leaned a bit more toward the GFS along with the ECENS/GEFS ensemble means. By day 7 (Sat), spread was quite high and opted to go entirely with ensemble means, but with a bit more weight on the slightly more progressive GEFS mean relative to the ECENS. ...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, shortwave energy and a passing front will bring valley rain/mountain snows to much of the northern Great Basin and northern Rockies on Tue. 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 Tue-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. Temperatures will initially be well above average across much of the West and the High Plains on Tue (highs 5 to 15 deg F above average), but these will quickly cool by Wed as a cold front crosses the central U.S. and heights fall across the West in association with the digging trough. Expect highs to near average to as much as 10 deg below average across much of the western and central U.S. from Wed onward, with cooler temperatures returning to portions of the Ohio Valley, Great Lakes, and eastern U.S. by Fri-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