Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Tue Dec 08 2020 Valid 12Z Fri Dec 11 2020 - 12Z Tue Dec 15 2020 ...General Weather Pattern... A surface low will develop and deepen over the Central Plains before lifting north/east through the Great Lakes and Northeast regions. This system will be a focus of rain across the Plains and Ohio/Tennessee Valleys with snow possible across the Upper Midwest/Great Lakes/Northeast. Meanwhile, a well developed Pacific storm system is forecast to affect Washington and Oregon by Sunday and Monday with additional rain/mountain snow and gusty winds. The overall pattern is devoid of arctic air masses with most of the flow originating from the Pacific. ...Model Guidance Evaluation and Preferences... The confidence with the overall pattern decreases for the latter half of the period as timing/phasing of the trough across the central/eastern U.S. increases. The ECWMF is suggesting a deeper, further south track into the Gulf states; whereas, the 00/06Z GFS runs favor a faster and northern progression through the Tennessee Valley/Appalachians. The CMC continues to build a second upper trough over the Rockies/Plains by Tuesday, which remains an outlier solution and was not preferred for the majority of the extended periods. The model blend for this cycle utilized 00Z ECWMF/ 00/06Z GFS/ 00Z ECWMF ensemble means/ 00Z NAEFS means/ 12Z WPC forecast, with deterministic weighting decreasing with time. ...Weather/Threats Summary... Above average temperatures, around 5 to 15 degrees, are expected from the Ohio Valley to points East through this weekend ahead of an approaching Midwest storm system. Cooler air will filter in behind the system and replace the warmer air, lowering the daily values to near or below average for the remaining of the extended period. Over the southern and central part of the country, scattered to widespread rain will accompany this storm system- ahead of and along the advancing cold front. On the northern periphery of the moisture shield, there may be wintry precipitation near the Upper Mississippi Valley/Upper Midwest and Great Lakes region, possibly resulting in some lake enhanced snow in the favorable downwind locations. With limited Gulf moisture return and the overall progressive nature of the system, precipitation amounts are expected to become excessive. The Pacific Northwest will be in a fairly wet and unsettled pattern through the extended with a couple of systems moving onshore and through the Interior West toward the Plains. Coastal rain and mountain snow are expected this weekend with another round along with gusty winds early next week. Accumulations are anticipated to remain modest, however, locally heavy amounts will be possible over the multi-day period. Campbell Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: 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