Extended Forecast Discussion NWS Weather Prediction Center College Park MD 120 AM EST Fri Jan 04 2019 Valid 12Z Mon Jan 07 2019 - 12Z Fri Jan 11 2019 ...Overview and Guidance Evaluation/Preferences... A moderately progressive flow pattern will be in place across the CONUS during the extended forecast period, with fairly amplified upper waves traversing the country every few days. No signals for any significant incursions of arctic air are seen in the guidance. A look at hemispheric D+8 500 hPa anomalies reveals strongly positive height anomalies centered between Greenland and western Europe, helping to displace cold air from high latitudes into Eurasia and away from North America. Teleconnections associated with this and other hemispheric anomaly centers suggest that we should see a tendency for upper ridging to rebuild across western North America, but with continued progressive flow across the North Pacific the ridge will likely be somewhat transient. Model guidance has struggled recently with the progressive flow across the North Pacific and with the timing and amplification of these waves as they cross the CONUS, and models still appear to be struggling with this to some degree. Ensemble spaghetti plots show a rather significant reduction in spread over the past couple ensemble cycles in the day 3-4 time period (Mon-Tue), with the ECMWF seeming to be among the more stable and well-centered of the deterministic solutions within the spread with respect to the shortwave crossing the central/eastern states during that time frame. Given these considerations, the forecast during days 3-5 was based on a multi-model deterministic blend, but weighted a bit more heavily toward the ECMWF. Guidance shows good agreement that another amplified upper trough should approach the West Coast and move inland from day 5 (Wed) onward. Model variability is a bit higher as to the specific details of this system, such as whether it will have a closed mid/upper-level circulation or will be more of an open wave. In general, the trend has been toward a more open and somewhat more progressive wave, with the ECMWF leading this trend and the GFS lagging. Thus, continued to emphasize the ECMWF more heavily relative to the GFS during the day 6-7 (Thu-Fri) time frame, with majority weight shifted toward the ensemble means (especially the ECENS). ...Weather Highlights/Hazards... The low pressure system crossing the central/eastern U.S. Mon-Wed will be accompanied by the potential for snowfall, perhaps heavy for some areas, along/north of the low track, from the northern Great Lakes to New England. Additional heavy snows will be possible across the higher elevations of the West Coast states, such as the Cascades and the Sierra, with heavy rain at the lower elevations, as the next Pacific trough/frontal system moves ashore Tue-Wed. Locally heavy rain may reach coastal areas of southern California by Wed/Wed night. As mentioned above, the large scale flow pattern will be setup such that no significant outbreaks of arctic air are expected across the CONUS during the forecast period. High temperatures are expected to be well above average through most of the period across much of the central/eastern states, with the greatest anomalies (+15 to 20 deg F) centered across portions of the central Plains and Midwest. 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