Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1259 AM 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 will move to the East Coast by midweek... ...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. Models and ensembles diverge into mainly two unequally-weighted camps with respect to the central/eastern system Sun-Tue. The larger (and preferred) cluster of solutions revolved around the 12Z ECMWF/UKMET (and the ECMWF and Canadian ensemble means) with the other smaller cluster around the 12-18Z GFS and 18Z GEFS mean. The 12Z Canadian was closer to the ECMWF-led cluster but slower. Ensemble trend supports a flatter solution though there are members from all three ensemble systems that were more developed and tracked farther to the northwest (west of the Appalachians) like the GFS. Thereafter, GFS/ECMWF were fairly close with the western trough moving into the West in line with the ensembles, though the GFS was much flatter with its downstream ridge into the Four Corners next Wed. With uncertainty in how the stretched western system evolves coupled with the amount of cold air in the Upper Midwest next Wed/Thu, trended toward a majority ensemble weighting for the end of the period. ...Weather Highlights/Threats... Wrapped-up system over northern Maine on Sunday will move into the Canadian Maritimes on Monday with high pressure in its wake. bringing in cooler air (20s/30s) to the Northeast after a brief mild intrusion. 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 over 30% 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 but this will depend on how far north the surface low may track. To the West, precipitation with the incoming Pacific front will focus along the coast and into the Sierra. 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