Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Wed Dec 23 2020 Valid 12Z Sat Dec 26 2020 - 12Z Wed Dec 30 2020 ...Overview and Model Guidance Evaluation/Preferences... Fairly progressive flow aloft will contain a series of features with varying amplitude, bringing one or more episodes of active weather to parts of the country during the Sat-Wed period. The vigorous upper trough responsible for the heavy precipitation/wind expected over the East late this week should lift through the Northeast on Sat. An upstream shortwave initially responsible for precipitation over the northern half or so of the West will bring low pressure into the central U.S. by day 4 Sun. Beyond then confidence for how this system will ultimately track/evolve over or near the Northeast decreases, given specifics of northern stream flow interaction that are yet to be resolved. The next system will amplify toward California Sun-Mon and continue inland thereafter, likely bringing precipitation to some areas in southern parts of the West that have seen very low totals so far this season and eventually spreading moisture into the Plains/Mississippi Valley. Guidance comparison for the 12Z/18Z cycles yielded a rare case of the 12Z ECMWF mean being out of sync relative to most other models/means for the relative amplitude of leading energy emerging from the West (deeper than consensus and then slow) and the more amplified system heading into/through the Southwest (flatter than consensus). Thus its solution was given minimal consideration until late in the period when it started to compare somewhat more acceptably. For the California/Southwest system the average of models and ensemble means has been quite steady over the past couple days, with individual runs generally still shuffling around a common solution. Thus an operational model consensus early-mid period transitioning to a multi-run model/ensemble approach provides a good depiction of this system. The new 00Z ECMWF challenges the recent stability though, shifting to the fast side of the spread. Beyond the changes in the new ECMWF, during the latter half of the forecast there is uncertainty regarding the track/timing of the possible embedded upper low, which will ultimately affect the surface evolution by the time the system reaches the Plains. Complexity of dynamics affecting the system crossing the eastern half of the country early in the week has led to more spread and variability over recent days. Latest adjustments have been toward somewhat stronger northern stream involvement, favoring a surface low track into the Great Lakes by early Mon and then into southeastern Canada/New England with the potential for redevelopment near the New England coast followed by an eastward or northeastward track. A blend among the 12Z/18Z GFS and last two ECMWF runs through 12Z along with the 18Z GEFS mean represented the most common themes of guidance. By Tue this represents only a mild northward nudge of the manual forecast over the past 12-24 hours. Northward trend of the 00Z CMC late in the period has added to support for a low track in the northern portion of the solution envelope from earlier cycles. ...Weather/Threats Summary... The Northeast will trend drier this weekend though with some lingering lake effect snow likely continuing into Sat. Best potential for meaningful accumulations will be downwind from Lake Ontario. The first system to affect the West will bring moderate amounts of rain and higher elevation snow this weekend to areas from the Pacific Northwest into northern and perhaps central California, with lesser totals reaching into the northern half of the Rockies. The upper level energy that continues eastward will likely combine with Canadian flow to support central/northeastern U.S. low pressure from Sun onward, bringing generally moderate precipitation from the Ohio Valley into the Northeast with lighter/more scattered activity farther south. By Tue there is still a fair degree of uncertainty over the evolution of this system near and east/northeast of New England. Therefore confidence is not very high for the precise precipitation/wind details. Any snow should be over northern latitudes and into the central Appalachians. At least before arrival of the new 00Z ECMWF, overall confidence was generally higher for the system forecast to head into California around the start of next week. Flow aloft may be favorable for terrain enhancement of precipitation for a period of time over southern California. During Mon-Tue expect rain and higher elevation snow to spread across the southern two-thirds of the West. This should be welcome precipitation given the relatively dry conditions that have persisted over portions of this region so far this season. By late Tue-Wed precipitation should develop and expand over the central U.S. with a range of precipitation types. System progression will bring some variability for temperatures, with the general trend for a majority of the lower 48 to see near to somewhat below normal highs by the early/middle part of next week. The central/southern Plains should see highs 10-20F above normal during the weekend followed by a cooling trend behind a cold front. Areas near the East Coast will start the weekend on the chilly side with the most extreme anomalies for highs likely to be over Florida on Sat (15F or more below normal at some locations). The eastern U.S. will moderate after Sat. The Upper Midwest will see some highs at least 10F below average around Sun-Mon. Expect highs to be mostly 5-10F below normal within the cooler air forecast to spread into/across the West and eventually into the Plains. Rausch 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 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