Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1101 AM EST Fri Dec 07 2018 Valid 12Z Mon Dec 10 2018 - 12Z Fri Dec 14 2018 ...Overview... The progressive yet amplified flow is forecast to slow down and increase in amplitude by the end of next week, potentially setting up a heavy rain and winter weather threat in the east. Until then, the pattern will remain quite active through the Pacific Northwest downstream of a swift 200kt jet. With upper ridging developing over Hudson Bay, a deep trough or closed low is likely to its south through the lower/mid-Mississippi Valley by the end of the week. ...Guidance Evaluation/Preferences... As the complex eastern system finally exits stage right early next week (lead and secondary shortwave followed by the final kicker on Tuesday) ridging will move into the east as a western trough digs through the interior. To start, the latest deterministic models (00Z/06Z cycle) offer reasonable clustering with the evolution and a blend of the GFS/ECMWF/UKMET/Canadian served as a starting point. By Wednesday, the models diverged with the speed of the trough out of the west into the southern Plains (GFS quicker than the ECMWF/UKMET but Canadian slowest and not preferred) as well as the track/amplitude of an upstream trough into the Pac NW Wednesday. By Thursday, the 06Z GFS and 00Z ECMWF were close to the agreeable ensemble means (06Z GEFS/00Z ECMWF ens mean) as the lead trough lifts to the Lakes and weakens while the secondary trough digs/amplifies toward Arkansas next Friday. For several cycles the models have showed a rather wound-up system developing west of the Appalachians by the end of the week and likely tracking toward the Great Lakes with downstream ridging along the east coast into the northwestern Atlantic. Opted to keep a majority deterministic weighting due to good synoptic consistency in this late-week system though the details will likely shift around as ensemble spread expands quickly next Friday. Back to the west, third system in five days may impact the region. ...Weather Highlights and Hazards... In the east, exiting system will slide across the Atlantic in several pieces, the first of which as a maritime gale hazard being followed by OPC. In the wake of the low and upper trough, cool temperatures and high pressure will dominate the east early/mid-week. Temperatures will moderate by next Thu/Fri ahead of the developing system out of the southern Plains. The Pacific Northwest will be quite active with roughly a system every other day as the upper flow remains pointed right along 50N. Higher terrain of western Oregon/Washington have the potential for heaviest precipitation amounts (rain and mountain snow) with lesser amounts farther east into the north-central Great Basin/Rockies. By Thu/Fri, increase in Gulf moisture and developing surface low will produce and expanding area of precipitation along/east of the front and to the north/northwest of the low where some wintry weather is possible. The signal exists for a potentially modest and locally heavy rain threat late Friday into next Saturday (just beyond the medium range period) as Atlantic moisture flows into the Southeast and Mid-Atlantic. Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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