Extended Forecast Discussion NWS Weather Prediction Center College Park MD 202 PM EST Tue Dec 31 2019 Valid 12Z Fri Jan 03 2020 - 12Z Tue Jan 07 2020 ...Heavy rainfall threat continues across the Southeast/southern Appalachians on Friday... ...Overview/Guidance Assessment... The pattern across the CONUS during the medium range looks to remain progressive with an active stream of shortwave troughs from the North Pacific eastward across North America. An amplified upper trough initially in place across the central U.S. on day 3 (Fri) is forecast to move east by days 4-5 (Sat-Sun) resulting in a deepening low pressure system across the Great Lakes and the Northeast. Model consensus for this system was relatively good with relatively minor timing/intensity differences noted. Farther west, additional systems moving onshore along the West Coast and tracking eastward through time show increasing spread among the guidance, especially toward the end of the forecast period. A relatively intense frontal system is forecast to move quickly into the Pacific Northwest and across the Rockies days 3-4 before moving into the central U.S. on day 5. Deterministic solutions were reasonably well clustered with this feature, although some timing differences began to show by Sun, even among the ensemble means with the GEFS mean quite a bit slower than the ECENS mean. By days 6-7 (Mon-Tue) spread becomes much larger with the next shortwave approaching the West Coast. The ECMWF was quite a bit slower and deeper than the GFS with the shortwave, and the GFS has yet another shortwave (this one stronger) reaching the Pacific Northwest shortly after the stronger/slower previous wave shown by the ECMWF. These large differences by early next week lended to heavy use of the ensemble means in the forecast. The WPC forecast was initially based on a blend of the 00Z ECMWF/06Z GFS (and gradually increasing ECENS/GEFS ensemble means) during days 3-5. By days 6-7, the ECENS/GEFS means comprised a majority of the forecast blend given the high spread by that time frame. ...Weather Highlights/Threats... Gulf of Mexico moisture will get drawn ahead of the trough lifting through the mid-south continuing a threat for heavy rainfall from the end of the short range period into parts of the Southeast/southern Appalachians. Surface low across the Ohio Valley deepens, and reforms off the Northeast coast by Saturday, and depending on development of the upper system, could bring a period of snow north and west of the low from the Upper Midwest to northern/interior New England. Nearly daily bouts of precipitation are likely for the Northwest into the Northern Rockies during the period, focusing more over coastal locations and the Coastal Range/Cascades. Above to well above normal temperatures across the eastern third of the country Friday and Saturday should moderate to near normal following passage of the cold front, with above normal temperatures also moving across the Western U.S. in the Central states this weekend. Ryan/Santorelli 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Fri-Sun, Jan 3-Jan 5. - Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Fri-Sat, Jan 3-Jan 4. - Flooding possible across portions of the Middle Mississippi Valley, the Great Lakes, the Ohio Valley, and the Upper Mississippi Valley. - Flooding occurring or imminent across portions of the Northern Plains. - High winds across portions of the Southern Appalachians, the Mid-Atlantic, the Central Appalachians, and the Ohio Valley, Sun, Jan 5. 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