Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 AM EDT Thu Apr 14 2022 Valid 12Z Sun Apr 17 2022 - 12Z Thu Apr 21 2022 ...Overview and Guidance Evaluation/Predictability Assessment.. Compared to what has been a pretty amplified and active weather pattern the past few days, relatively low amplitude mean flow should continue through about Sunday or Monday across much of the lower 48. This features shortwave energy through the Northeast, with a possible closed low development over the Canadian Maritimes, while a shortwave and associated surface low pressure system moves over the northern tier. Chances are increasing that this may bring another round of spring snow to parts of the Northern Plains on Sunday into Monday. After this, the pattern is expected to transition to a more amplified flow, as an upper trough/closed low reaching the West Coast by Tuesday should act to build a downstream ridge and amplify the northern tier shortwave as it digs through the Great Lakes and Northeast by mid next week. Meanwhile, as the Western U.S. trough energy progresses through the Northwest, it could again face amplification in response to yet another upper low/amplified trough towards the West Coast next Wednesday-Thursday. The latest 12z/18z (Apr 13) suite of guidance shows good agreement on the large scale through day 4/Monday, with plenty of lingering uncertainties in the details. A general deterministic model blend however sufficed for this period and helped to smooth out those differences. Amplification of the next system through the Northern Plains into the Great Lakes is of course highly dependent on uncertain details of the West Coast upper low/trough on Monday into Tuesday. The notable outlier here is the UKMET which is quicker to close off a low over the Northeast on Tuesday, and also farther south. As such, it was not included in the blend tonight. The GFS and GEFS mean both get fast with this energy on Wednesday as it swings through the Canadian Maritimes. Timing and detail differences become more apparent by next Tuesday as the West Coast trough shifts inland. The GFS is faster with the energy, but the ECMWF is quite a bit farther north, with the CMC forming a decent middle ground solution. These differences continue into Wednesday and Thursday too as the system slides across the Great Lakes. However, for a day 6-7 forecast and given continued run to run uncertainties, neither model presents a clear outlier. The blend for the latter periods transitioned more towards the ensemble means (ECENS mean and NAEFS) to account for the detail differences, but also continued slight majority of the deterministic GFS/ECMWF/CMC for added system definition. ...Weather Highlights/Threats... Moisture and instability interacting with a warm front lifting through the South/Southeast this weekend and approach of a cold front from the north should bring areas of moderate to locally heavy rainfall to parts of those areas, but with lower confidence in exact placement of heaviest totals. Farther north, a low pressure system into the Northern Plains on Sunday may again bring a round of accumulating spring snow to northern locations, with rainfall farther south along the attendent cold front into the Midwest. Moisture reaches the East early next week, with some potential for enhanced rainfall along the Carolina coast as well as precipitation (maybe some snow) north and west of a surface low developing off the New England Coast. Some gusty winds are also possible across the Northeast Tuesday into Wednesday. Out West, expect another episode of precipitation from the Pacific Northwest/northern California into the northern Rockies through early next week, with some enhanced totals possible over favored coastal/terrain areas. Precipitation potential may increase again into next midweek with approach of another amplified upper trough and frontal progression. The core of below normal temperatures across the northern Plains and vicinity will likely persist into early next week, with daytime highs generally 20-30F below normal, and moderating slowly with time. The system reaching the Midwest/East early next week could draw some of the chilly northern Plains air southward and eastward behind it, with expanding coverage of minus 10-20F anomalies Sunday-Tuesday. Warm anomalies across the Southern Plains and Southwest on Sunday should progress slowly north and eastward as upper level ridging builds and moves through. 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml