Extended Forecast Discussion NWS Weather Prediction Center College Park MD 232 AM EDT Sun Oct 13 2019 Valid 12Z Wed Oct 16 2019 - 12Z Sun Oct 20 2019 ...Pattern Overview and Guidance/Uncertainty Assessment... A relatively amplified but progressive pattern will be in place across the CONUS during the extended period. A trough is forecast to amplify across the Great Lakes/Northeast Wed/Thu, with a closed upper-level low expected to develop. At the surface, a low pressure system and surface frontal system are forecast to cross the eastern U.S. ahead of the trough. With the initial surface low crossing the Great Lakes on Wed, formation of a new surface low is expected across the Mid-Atlantic region, which is forecast to quickly intensify as it moves northeastward along the New England coast Wed night/Thu. Farther west, an intense Pacific upper-level jet is forecast to reach the Pacific Northwest, along with a train of shortwave energy, starting Wed night. This should quickly lower heights across the Northwest and eventually the northern Rockies as the systems move inland. By Fri, the first significant shortwave in this series should exit from the Rockies into the central U.S., pushing a leading cold front into the Plains. Models show this perhaps being followed by amplification of a more significant shortwave across the Four Corners region or Rockies by Sun, poised to move east into the Plains beyond the end of the forecast period. The WPC medium range forecast was initially based on a blend of the 12Z ECMWF/GFS/CMC on days 3-4 (Wed-Thu). The 12Z GFS was preferred over the 18Z run due to a track closer to the model/ensemble consensus with the northeastern U.S. coastal low during this frame. Additionally, a blend of these solutions should smooth out timing differences with the arrival of the West Coast trough on Thu. During days 5-7 (Fri-Sun) a gradual trend was shown toward heavier weighting of ensemble means (ECENS/GEFS). Model differences were largest with the potential amplification of shortwave energy across the Four Corners region by next weekend. In general, the trend in the guidance has been toward a slower and more amplified system, although the 00Z GFS seemed to back off that trend (while the CMC continued to trend slower/deeper). Thus, model spread remains relatively high by next weekend, supporting the heavier ensemble weighting. Consensus appears much better across the eastern U.S., where most guidance agrees that subtropical ridging will build off the Southeast U.S. coast, lending to greater overall amplification of the pattern and setting up a broad region of southerly/southwesterly flow across the Mississippi Valley ahead of the trough. ...Weather Highlights/Threats... Widespread and potentially heavy precipitation is expected across the Pacific Northwest by mid to late week as the series of shortwaves and surface fronts move inland. Snow-levels are forecast to fall as lower height begin to press inland. The heaviest precipitation is expected to fall across the coastal ranges as well as the Cascades, with multi-inch liquid equivalents possible. By next weekend, moisture return across the central U.S. ahead of the western trough could generate scattered showers and thunderstorms across portions of the Mississippi Valley. Meanwhile, a lingering surface front in the Gulf of Mexico may focus areas of showers and storms along the western/central Gulf Coast. Relatively cool conditions are forecast across much of the central and eastern U.S. Wed into Thu, with highs for many areas 5 to 15 deg F below average. These temperatures should warm by Fri and into next weekend. Meanwhile, initially above average temperatures across much of the West (highs 5 to 10 deg above normal) will be replaced by much cooler conditions as upper troughing moves inland. Highs are forecast to be 5 to 10 deg below normal across the Northwest by Thu. By next weekend, colder conditions should overspread much of the western U.S. as the main upper trough axis presses inland and amplified, with highs forecast to be as 15 deg below normal for some areas. 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