Extended Forecast Discussion NWS Weather Prediction Center College Park MD 134 AM EST Sun Jan 10 2021 Valid 12Z Wed Jan 13 2021 - 12Z Sun Jan 17 2021 ...Overview... The best consensus of guidance is fairly stable in showing amplification of the western mean ridge aloft after midweek along with corresponding deepening of a large scale trough over eastern North America. This trough will replace an elongated multi-stream mean trough from eastern Canada into northern Mexico. Another Pacific shortwave rounding the mean ridge will likely reach the Northwest by early next weekend and flatten downstream flow enough to start pushing away the late week trough over the East, with elongated troughing eventually becoming established over the Rockies by late next weekend. Expect heaviest precipitation to be in the Northwest early in the period as shortwaves/frontal systems head into and around the mean ridge aloft. Elsewhere, some areas of precipitation (mostly snow) looks to spread into the Great Lakes and vicinity by late week and the Northeast by next weekend. ...Guidance/Predictability Assessment... The pattern transition toward larger scale features later in the week should improve predictability relative to earlier time frames when smaller shortwaves that are typically not well resolved a few days out in time play a greater role in the forecast. Still there are some differences of note through the forecast. The 18z/Jan 9 GFS and the 12z/Jan 9 CMC are slower/stronger than the 12z/Jan 9 ECMWF with shortwave energy reaching the Great Lakes by Wednesday and the Northeast on Thursday, but run to run continuity with this feature is poor across most of the guidance. GFS runs continue to be inconsistent with the shortwave reaching the Northwest by next Saturday, sort of flip-flopping between noticeably flatter and more amplified than other models. ECMWF runs have been less erratic but show a gradual stronger trend. There's some differences in the models on how quickly the amplified late-week eastern US trough gets kicked eastward. The 18z/Jan 9 run of the GFS is noticeably flatter/faster than the ECMWF/CMC which show a more amplified/elongated trough slower to exit the East Coast next weekend. The ensemble means would favor something closer to that of the ECMWF. A composite blend of the mainly operational models best represented the guidance early in the period, while toning down the most uncertain details as described above. A trend towards more ensemble mean guidance (with some contributions from the GFS/ECMWF) later in the period helped mitigate late period uncertainties while maintaining good continuity from the previous WPC forecast. ...Weather/Threats Highlights... The atmospheric river setup in the Northwest during the short range should be diminishing by the start of the medium range (Wednesday), but some moderate to locally heavy rain and mountain snow should continue to focus along the favored terrain of the coastal ranges and the Cascades. Exact location of highest totals is still dependent on lower predictability shortwave specifics better resolved in the short range. There should be a brief break as upper ridging amplifies followed by an episode of what should be less extreme precipitation as another shortwave and frontal system pass through. The Olympics and northern Cascades could see locally heavier activity though. Some of the moisture from these events will also extend into the northern Rockies. Showers will be possible across parts of Florida along a trailing stationary front, with low-confidence in the specifics dependent on shortwave energy that may cross the southern tier of the lower 48. The deepening upper trough and leading frontal system reaching the East by Thursday to Friday should bring a combination of synoptic and lake effect/enhanced snows from the Great Lakes into the northern/central Appalachians. There could be some rain in the southern part of the moisture shield along the leading front. Parts of the Great Lakes/Northeast may continue to see some snow into weekend. Continue to expect well above normal temperatures across much of the northern tier through much of next week into the weekend with areas from Montana into the Upper Midwest likely to see the greatest anomalies. Greatest coverage of plus 20-30F anomalies will likely be Wednesday into early Thursday. Some of this warmth will also extend into the central Plains. Upper ridging will bring a warming trend to the Western U.S. mid-late week with highs up to 10-15F above normal and possibly daily record highs at a few locations over California and the Southwest. The upper trough/front reaching the East late this week will bring highs down to near or modestly below normal levels by next Saturday. 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