Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Dec 12 2021 Valid 12Z Wed Dec 15 2021 - 12Z Sun Dec 19 2021 ...Guidance/Predictability Assessment... Guidance clustering and forecast spread/predictability through medium range remains better than normal for the majority of the synoptic systems of concern. Nevertheless, the CMC and CMC mean have been taking the next Pacific system on a track farther north toward the Pacific Northwest than the GFS and the more recent ECMWF runs. The most recent (12Z) GFS and the ECMWF runs nudged the low track farther north though. By next weekend, models show some spread on how amplified the upper trough associated with this system will get as it dips near the southwestern border of the U.S. Meanwhile, models are showing better agreement/definition on a frontal wave to develop quickly near/south of the Great Lakes next weekend and then track rapidly during next weekend. The WPC medium range forecast products were based on a blend of 40% from the 06Z GFS/GEFS, 40% from the 00Z ECMWF/EC mean and 20% from the 00Z CMC/CMC mean, with more weights toward the ensemble mean starting on Day 6. This blend yielded a solution quite compatible with the previous WPC forecast package and seems well supported by GEFS/NAEFS/ECMWF ensembles. ...Weather/Hazards Highlights... A vigorous upper trough will lift northeastward from the Southwest midweek, bringing widespread mountain snow/rain through the Great Basin and northern/central Rockies. As the system lifts through the Plains Wed-Thu, rapid cyclogenesis across the north-central U.S. will lead to an expansive area of strong winds, together with a swath of rain/snow through the north side of the low track from the northern Plains and upper Midwest to the upper Great Lakes. Ahead of the deep cyclone, an upper ridge will build over the central U.S. where anomalous warmth will be most pronounced midweek with record high temperatures upwards to 30-40F above normal. Modest lead rains that should fall on top of recent upper Midwest snows together with the record warmth could lead to flooding issues on Wednesday. Anomalous warmth ahead of the cold front, with positive anomalies upwards to 15-25F, will reach the East late week. The trailing portion of the front will likely stall across the Deep South late this week, with an axis of enhanced rainfall forecast to set up from the southern Plains through the Mid-South/OH Valley. A frontal wave developing near/south of the Great Lakes by next weekend should bring a swath of wintry precipitation most likely across northern New England. Meanwhile, the rainfall axis across the South will tend to push southward toward the Gulf Coast region during the weekend as a high pressure system builds across the Plains toward the East Coast. Upstream on the heels of the aforementioned lead storm, another moisture surge accompanied with moderate to heavy precipitation is forecast to reach northern California and Oregon Wed/Thu ahead of the next Pacific system despite some uncertainty with the low track. Models show general agreement that this system will gradually weaken as it gradually dips into southern California by Friday. By next weekend, guidance signal is also growing that moisture from yet another Pacific system will modestly impact the Pacific Northwest. Kong/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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