Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Sat Jan 09 2021 Valid 12Z Tue Jan 12 2021 - 12Z Sat Jan 16 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. Expect heaviest precipitation during the period to be in the Northwest as shortwaves/frontal systems head into and around the mean ridge aloft. Elsewhere, precipitation should be exiting the East early in the period with some areas of precipitation (mostly snow) spreading into the Great Lakes and vicinity by late week. Shortwave energy ejecting from the southern Plains/northern Mexico could produce one or more areas of precipitation over parts of the South/Florida but with very low confidence for how much or where if any occurs. ...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. As of early day 3 Tue recent guidance trends have been more sheared with shortwave energy supporting a weak system which the guidance average now depicts as a mere wavy front. Upstream elongated trough energy over the southern Plains into Mexico continues to be problematic in terms of strength and timing of ejection. Confidence in any particular solution remains below average for this. Meanwhile differences persist for energy rounding the upstream ridge. The 00Z CMC/GFS looked particularly fast with leading energy that they brought into the Great Lakes after midweek. The 06Z/12Z GFS have returned toward consensus that places more emphasis on trailing amplifying flow. GFS runs have been inconsistent with the shortwave reaching the Northwest by next Sat, 00Z/06Z versions noticeably flatter than other models and the 12Z run more amplified than the average. ECMWF runs have been less erratic but show a gradual stronger trend. An operational model composite represented the best ideas of guidance early in the period while toning down the most uncertain details. A trend toward nearly even weight of the 06Z GFS/GEFS mean and 00Z ECMWF/ECMWF mean maintained consensus and continuity well over most areas later in the forecast. ...Weather/Threats Highlights... Heaviest rain and mountain snow in the Northwest will likely extend into midweek, with strong westerly flow aloft focusing moisture into the favored terrain along the coastal ranges and 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 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 extend into the northern Rockies. Precipitation should be exiting the Eastern U.S. as the medium range period begins on Tuesday. The associated weak system will bring a trailing front through/south of the Florida peninsula, with showers possible at least along the front and possibly later as the front settles south of the state. Any mid-late week rainfall will depend on lingering moisture and low-confidence specifics of 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 from Tuesday 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. A cool start on Tuesday across the South should moderate back to normal by Wednesday and beyond. The upper trough/front reaching the East late this week will bring highs down to near or modestly below normal levels by next Saturday. Rausch/Santorelli 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