Extended Forecast Discussion NWS Weather Prediction Center College Park MD 114 PM EST Thu Dec 12 2019 Valid 12Z Sun Dec 15 2019 - 12Z Thu Dec 19 2019 ...Winter weather threat over the central Plains Sun/Mon may move to the East Coast by midweek, with heavy rainfall likely across the south... ...Pattern Overview and Guidance/Predictability Assessment... Broad troughing to start the period (Sunday) will shift focus to the shortwave ejecting out of the Four Corners region and through the southern/central Plains. This system will slide east-northeastward to the Mid-Atlantic by Tuesday and then out to sea. Another Pacific system will stumble into California Wed/Thu. A main surface low will track quickly to the east/northeast from the Southern Plains Sun/Mon into the Mid-Atlantic and Northeast by Tuesday. There remains some question on the exact track of this system with the latest run of the ECMWF (12/00z) farther east than the other deterministics and many of the ensemble members. The GFS/UKMET/CMC are all a bit slower/farther west with the low, but the ensemble means tend to cluster somewhere in the middle. Thus, a kind of middle ground solution (maybe a bit closer to the GFS camp though) was preferred for this system which also maintains good continuity with the previous forecast. In the West, the GFS/ECMWF were fairly close with the trough approaching the coast which is also in line with the ensembles. However, the GFS continued to be flatter with its downstream ridge into the Four Corners next Wednesday. With uncertainty in how the stretched western system evolves, coupled with the amount of cold air across the Upper Midwest mid to late next week, a trend towards a bit more ensemble weighting was preferred for the end of the period. ...Weather Highlights/Threats... An initial wrapped-up surface low over northern Maine on Sunday will move into the Canadian Maritimes on Monday with high pressure in its wake which should bring in cooler air (20s/30s) to the Northeast. Over the Plains, as low pressure exits the Foothills it will spread rain (south) and snow (north) over the area. There remains some north-south spread in amounts and precipitation type (depending on the track of the low in relation to the colder/warmer air mass interface) but a swath of at least plowable snow seems fairly likely over parts of Kansas. Winter weather outlook probabilities 30-50% encompassed most of the Sunflower State Sun-Mon. As the area of low pressure moves eastward, rain may expand and increase over the Southeast from the Lower MS Valley to the TN Valley and southern Appalachians. To the north, snow is possible on the northern side of the precipitation shield across the Ohio Valley and interior Mid-Atlantic, but this is highly dependent on the eventual track of the surface low. To the West, precipitation with the incoming Pacific front will focus along the coast and into the Sierra. Santorelli/Fracasso 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