Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1252 AM EST Fri Nov 30 2018 Valid 12Z Mon Dec 03 2018 - 12Z Fri Dec 07 2018 ...Cold pattern more typical of mid-winter to grip much of the contiguous U.S. next week... ...Overview... A cold pattern is in store for much of the CONUS during the medium range as mean upper-level troughing persists from the northern plains/Upper Midwest to New England. Upstream support for this pattern shows no signs of waning, with a highly amplified and multi-center blocking pattern extending from Siberia across Asia to the North Pacific, which will maintain anomalous ridging from western British Columbia into Alaska. The flow resulting from this pattern will favor continued southward transport of polar/arctic air through Canada and into the CONUS. An active northern stream flow regime will result in a dominant storm track from central Canada southward into the central/eastern U.S., with the potential for cyclogenesis off the Eastern Seaboard. With the flow across the North Atlantic much less blocked than that farther west, systems should become more progressive through time as they move east, with limited threat for significant coastal storms. Additionally, cutoff southern stream energy from the Pacific will affect the West Coast and Southwest during the period. ...Guidance Evaluation/Preferences... Models initially show similar solutions on day 3 (Tue), but spread increases steadily through the period as models struggle with a rapid train of northern stream shortwave energy and their interaction with lower amplitude southern stream energy. A multi-model deterministic blend (ECMWF/GFS/UKMET) was a sufficient basis for the forecast during days 3-5 (Tue-Thu). Timing/intensity differences among the models were relatively small with shortwave energy skirting the north central U.S. during this time, as well as a cold front pushing off the Eastern Seaboard. With the cutoff Pacific energy nearing the West Coast during this time, models have come to somewhat better consensus after struggling, with a general trend toward a slower and more amplified solution noted. After Thu, model consensus begins to degrade quite quickly, with handling of northern stream energy diving south across central Canada seemingly changing with each model cycle. The ECMWF has shown particularly poor run-to-run consistency in this portion of the forecast. Models show general agreement that one of the more significant shortwaves should be crossing Alberta and Saskatchewan on Wed, but solutions diverge sharply by Thu as to whether the wave moves east across the Great Lakes (as shown by the GFS and prior ECMWF runs), or becomes highly amplified across the northern/central plains, allowing heights to rise along the Eastern Seaboard. As mentioned above, D+8 anomalies suggest that flow should gradually become more progressive as systems leave the strong blocking pattern prevalent across the the North Pacific. Given this, and little support from the ensemble means, the deterministic ECMWF was not favored during days 6-7. One aspect of the forecast showing relatively higher confidence during the late medium range is the southern stream system. Models show general agreement that this upper wave should cross the Southwest and approach the southern plains by late next week. Some timing differences exist, but ensemble means appear well representative of a consensus solution here. Given the relatively poor consistency in the guidance for most areas, the forecast was shifted nearly exclusively toward the ECENS/NAEFS means during days 6-7. This solution keeps lower heights in place across the eastern U.S. than shown by the ECMWF. ...Weather Highlights/Hazards... As described above, the flow pattern will favor southward transport of polar/arctic air into the CONUS through much of the medium range. In a relatively rare occurrence, nearly the entire CONUS will see below average temperatures by late next week, with areas from the northern/central plains to the Upper Midwest/Great Lakes likely at the core of the cold air. High temperatures across those areas will range from 10 to perhaps more than 20 deg F below average through much of next week. After a brief warm up along/ahead of a frontal boundary, below average temps (5-10 deg F) will return to the Eastern Seaboard by Tue-Wed, with temps dropping further by later week as additional rounds of cold air arrive. The Rockies/Great Basin will not be immune to the cold either, with high temperatures 5-15 deg below average through much of next week, the result of a strong polar surface high remaining locked in place. In terms of precipitation, this will be a relatively dry pattern for much of the northern tier, as low pressure systems traversing the northwesterly flow remain relatively moisture-starved. A wave of low pressure along the front across the Southeast on Mon-Tue should focus more widespread rains for portions of Florida and the coastal Southeast before quickly moving out into the Atlantic Ocean. The slow-moving Pacific energy will increase rain and mountain snows along the West Coast by Tue/Wed before spreading precipitation into the Southwest and portions of the southern Great Basin by/Rockies by Thu and the southern plains by Fri. A number of model solutions suggest the potential for heavy rain across the southern plains and lower Mississippi Valley by next Fri, but given the time range (day 7), confidence is modest at best. Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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