Extended Forecast Discussion NWS Weather Prediction Center College Park MD 114 AM EST Fri Jan 08 2021 Valid 12Z Mon Jan 11 2021 - 12Z Fri Jan 15 2021 ...Overview... The guidance for flow aloft continues to show the establishment of an amplified western ridge/eastern trough pattern with various uncertainties for embedded features within multiple streams. Consensus shows the upper ridge strengthening during Tue-Thu while it will likely take until next Thu and beyond for the eastern trough to deepen as a result. The two main areas of notable precipitation will be over the Northwest in association with multiple shortwaves/frontal systems and over portions of the South/East with a system tracking from the northern Gulf of Mexico into the western Atlantic during the first half of the period. Parts of the northern tier will see well above normal temperatures through the period while the South will be below normal early next week. ...Guidance/Predictability Assessment... Guidance shows overall good agreement with the large scale pattern, but continues to struggle with the smaller scale/less predictable features and details. For the northern Gulf/western Atlantic system, models have trended towards a slower and more closed upper low over/near the Southeast on Monday, lifting off the Mid-Atlantic coast by mid-week. After this, there is still some spread in timing and intensity of northern stream flow details from the Plains into the Northeast, so specifics for the East coast remain uncertain. Out West, there's excellent agreement on the large scale setup, with most differences related to details which have low predictability or are within typical model error for the time frame. The 12z CMC solution is the only notable outlier, which brings a suspiciously strong shortwave through the Northwest Tuesday into Wednesday. The WPC progs favored a deterministic model blend between the 18z/Jan 7 GFS, the 12z/Jan 7 ECMWF, and the 12z/Jan 7 CMC...with more weighting towards the GFS/ECMWF. After this, replaced the CMC in favor of the latest runs of the ECENS/GEFS means, increasing weighting of the means from day 5 onward to help mitigate the detail uncertainties outlined above. This solution results in very good continuity with the previous WPC forecast. ...Weather/Threats Highlights... The system in the northern Gulf on Monday may produce locally moderate to heavy precipitation over portions of the central Gulf Coast states. There remains some considerable uncertainty for the exact northward/westward coverage and intensity of precipitation farther east, corresponding to guidance spread for strength/track/timing of low pressure into the Atlantic. The northwestern part of the moisture shield may contain some snow, but general lack of cold air in place suggests the best potential for any snow to be over higher elevations and/or at night. Taken as a whole though, the new 12z models continue to suggest decreasing precipitation potential for the Northeast. In the West, rain and high elevation snow over the Pacific Northwest should be heaviest early next week, with highest totals over Washington and along favored terrain of the coastal ranges/Cascades. Some moisture should eventually reach the northern Rockies, but expect precipitation to trend lighter by Wed-Thu as the upper ridge near the West Coast strengthens. The system reaches the Upper Great Lakes by Thu-Fri and could bring the potential for mainly lake effect snows within the northerly/westerly cold flow on the backside of the front. Highest temperature anomalies in the northern tier should extend from Montana through the Dakotas into Minnesota. Morning lows will likely be at least 10-20F above normal most days and possibly 20-30F above normal by Wed-Thu. Highs may not be as quite as extreme versus average but plus 20-30F readings are still possible mainly Tue into Wed. A Pacific cold front should start a cooling trend on Thu though temperatures still look to remain above average from the northern/central Plains eastward. Meanwhile warmth over the West will likely become more pronounced by midweek as the upper ridge strengthens, leading to decent coverage of plus 10F or greater anomalies for highs by next Thu. Within the cool pattern over the South, areas from the southern Plains into Lower Mississippi Valley should see highs 10-20F below normal on Monday, with a moderating trend across the South the rest of the week. Santorelli/Rausch 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