Extended Forecast Discussion NWS Weather Prediction Center College Park MD 134 AM EST Mon Dec 16 2019 Valid 12Z Thu Dec 19 2019 - 12Z Mon Dec 23 2019 ...Atmospheric River for part of the West Coast later this week... ...Overview... Upper troughing south of the Gulf of Alaska on Thursday will slowly move eastward as undercutting waves send periods of rain and mountain snow to the Pacific Northwest southward to Northern California through the weekend. Northern stream across the US/Canadian border will remain quasi-zonal with a drier WNW flow and little precipitation east of the Rockies and north of the Mid-Mississippi and Tennessee Valleys. A somewhat unusual system in the southern stream may dip through the Gulf/Florida this weekend with an uncertain future by next Monday. ...Guidance/Predictability Assessment... For the West, consensus approach sufficed to deal with some timing/trajectory uncertainties, with the exception of the 12Z Canadian (not preferred downstream as well). GFS remained on the slightly quicker side by Sat as heights begin to lower, but was not unreasonably even at the edge of the ensemble spread in light of the past few days of runs. Trended toward 50/50 ensembles/deterministic by the end of the period as there remains a difference between the ECMWF-based and GFS-based ensembles related to Pacific troughing near the Papahanaumokuakea Marine National Monument. For the eastern half of the CONUS, forecast revolves around how to handle the southern stream upper low (assuming enough northern/southern stream separation). Trend seems to favor a more separated and southerly track, eventually encouraging cyclogenesis in the Gulf around Saturday. Though the models have wavered quite a bit, the ensembles provide a much steadier, though blurrier, forecast. Thus, continue to stick with what the pattern may likely support with compensating lower latitude negative height anomalies (the western one off Baja CA). This is supported by teleconnections from the dipole couplet over Banks Island (northern Canada) but not as much from other anomaly centers (e.g., troughing stretched in the eastern North Atlantic and strong upper ridging over the Sahara). Given that the 12Z GFS and ECMWF were relatively close to their ensembles (with their typical fast/slow relative biases, respectively), blended approach with their ensemble means provided an adequate starting point. ...Weather Highlights/Threats... Lead system on Thursday should weaken over BC with another right behind it, focused into WA and northern OR with that round of precipitation. As trailing upper troughing sharpens, moisture plume should lift northward Friday as the cold front approaches before sinking southward through coastal OR/NorCal as the front moves onshore Saturday. Several inches of precipitation are expected but over a 2-3 day period with a likely break in between for a given location. Daytime temperatures will be near to above average overall with a trend cooler as the front moves inland this weekend. East of the Rockies, temperatures will be above average with dry conditions and upper ridging overhead. Quick cold shot will skirt through the Northeast on Thursday (10s/20s for highs and -0s/0s/10s for lows) with a moderation thereafter. Coastal/Gulf system will provide some rainfall for the Southeast, depending on its evolution, with snow possible on its northern fringe in marginally colder air at generally higher elevations. 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