Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Dec 09 2019 Valid 12Z Thu Dec 12 2019 - 12Z Mon Dec 16 2019 ...Developing low pressure system could bring areas of heavy rain up the Appalachians Friday into the Saturday with wintry precipitation at first across the interior Northeast and central Appalachians... ...Moist and fast flow across the West could lead to next low pressure system developing over the Deep South early next week... ...Pattern Overview and Guidance/Predictability Assessment... Model guidance this morning continues to indicate that a cold core upper vortex will meander in the vicinity of Hudson Bay through this weekend. Across the U.S. mainland, the pattern will become more amplified this weekend as a shortwave exiting the southern Rockies/northern Mexico is forecast lift toward the northeast and interact with the northern stream flow over the eastern U.S. The GFS continues to show a faster motion of this shortwave across the northern Gulf of Mexico Thursday into Friday compared with the ECMWF, resulting in a much faster progression of the wave through the Eastern Seaboard during the weekend. A 30% blend of the 06Z GEFS with 70% the 00Z EC mean (together with some of their deterministic solution) was used to handle this system. This yielded a slower forward motion of this system with the associated moisture pushing further into the interior sections of the entire East Coast this weekend. A similar blend of the 06Z GEFS with 70% the 00Z EC mean was used for Days 6 and 7 to handle the uncertainty on the back side of the low over the Northeast. ...Weather Highlights/Threats... Models still offer varied low scenarios from the Gulf of Mexico to the Southeast then up the Eastern Seaboard into next weekend. Forecast spread and run-to-run variability remain substantial. But there appears to be a trend for the associated moisture pushing further into the interior sections of the entire East Coast this weekend. This could bring areas of heavy rain up the Appalachians Friday into the Saturday Subsequent system lift up through the East Coast and interaction with a slow to recede lower atmospheric cold air damming offers an interior snow/ice threat to monitor. As is often the case, the degree of phasing/interaction between the two initially separate streams and baroclinic zone/coastal setup will be determining factors of eventual evolution and any potential for any deeper coastal storm developments and threats. Nevertheless, wintry precipitation can be expected at first across the interior Northeast and central Appalachians Friday night into early on Saturday. Out West, the pattern will become wet across the Pacific Northwest into later this week as Pacific shortwave energies/height falls punch onshore. Moderate to heavier snows will develop with system progressions inland, especially for favored terrain of the north-central Great Basin/Rockies later week through the weekend. Kong/Schichtel 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