Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Mon Dec 10 2018 Valid 12Z Thu Dec 13 2018 - 12Z Mon Dec 17 2018 ...Heavy Precipitation Threat for the Northwest... ...Heavy Rain Threat for the Lower MS Valley through the East... ...Overview and Guidance Evaluation/Preferences... Models and ensembles this morning continue to indicate that a low pressure system should bring significant precipitation across the eastern half of the country late this week into the weekend. Meanwhile, weather systems will move rapidly across the Pacific Northwest. Considerable spread in the guidance is shown regarding the depth and intensity of the low forecast to move across the eastern U.S. The ECMWF and EC mean continue the tendency of slowing down the low followed by cyclogenesis just off the East Coast. In particular, the 00Z ECMWF showed the low center position lagging behind the 00Z EC mean position by a few hundred miles during the weekend! On the other hand, the discrepancy between the GFS and GEFS low center positions were more reasonable, placing the low center over the Mid-Atlantic on Saturday, which agrees very well with the 00Z EC mean position as well as WPC continuity. This lends more credibility to the GEFS/GFS solutions regarding this low. By Day 7 Monday, the ECMWF has been indicating the possibility of an upper-level vorticity breaking off from a longwave trough near the West Coast. The GFS did not indicate that but the 06Z GFS began indicating this feature. Therefore, the WPC medium-range morning grid package was derived mainly from a blend of 40% 00Z EC mean and 60% 06Z GEFS, mixing in with varied percentages of their deterministic solutions, with little or no 00Z ECMWF for Days 5 to 7. ...Weather Highlights and Hazards... Precipitation/wind will substantially increase into the Northwest into CA Fri/Sat and again by Mon/Tue as heights fall with a succession of approaching energetic Pacific shortwaves/frontal systems. Higher Pacific Northwest terrain will observe the heaviest rain/mountain snow (with varying snow levels as upper temperatures rise and fall between each system), with activity then working to the north-central Great Basin/Rockies. Lead shortwave energy digging from the Rockies to the southern Plains Thursday will reinforce upper troughing in an emerging southern stream and spawn surface cyclogenesis/frontogenesis. Ample Gulf moisture/warm advection increasing ahead of the deepening system will help to expand the precipitation shield to the east and also wrap to the north of the surface low. Relatively slow movement of the organized system and favorable dynamics support a heavy rain threat for the lower MS Valley/Mid-South to Southeast. Heavy rainfall spreads to the east side of the Appalachians and coastal plain with added infusion of Atlantic moisture. Runoff problems will be enhanced for areas with recent heavy snowfall in a lead-in pattern with moderating temperatures. Ice is possible across the central Appalachians Thursday night to Friday morning before the deepening low pulls mild air from the south. Some snow and winter precipitation can be expected to wrap back across the Midwest/Great Lakes into interior New England from Thursday and into the weekend. Kong/Schichtel 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