Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Sat Apr 10 2021 Valid 12Z Tue Apr 13 2021 - 12Z Sat Apr 17 2021 ...Central Rockies/High Plains Heavy Snow Threat and additional rains for the central Gulf Coast... ...Pattern Overview and Guidance/Predictability Assessment... The medium range period begins Tuesday with broad troughing stretching across the northern tier of the country with two main upper low features: one across the Upper Great Lakes, and another dropping into the Northwest. After Tuesday, the Great Lakes low should slide slowly eastward (influenced by a deep closed low meandering/retrograding from the Canadian Maritimes) while the Western U.S. low settles over the Great Basin for much of the week within an otherwise blocky pattern. In general, the latest suite of guidance shows fairly good agreement on the large scale pattern, but forecast spread is growing on the details. For the Great Lakes upper low, most of the guidance shows it shifting eastward and weakening with time and may be trending a bit faster than previous model runs. The future of this is also dependent on another deep upper low/lows slowly retrograding uncertainly from offshore the Canadian Maritimes towards New England before finally lifting north and away from the coast. After day 3-4, the details get very uncertain as the GFS/CMC are the quickest to eject the Great Lakes low eastward, while the ECMWF keeps some energy back across the Great Lakes. The ensemble means lie somewhere in the middle and are probably a good starting point at this time. Out West, the deterministic guidance shows better agreement that energy drops in from the Northwest and settles into the Great Basin region as a closed low for much of the week. By day 6-7, there is some uncertainty as to how quickly upper ridging to the north weakens which would help push the low eastward. The ECMWF/CMC hang on to a closed Western U.S. low into next weekend, while the GFS would suggest the trough pushes eastward into the Plains. Again, prefer a preference towards the better clustered ensemble means for days 6-7 which would suggest the energy hangs back a little longer than the GFS shows. The WPC medium range product suite was primarily derived from a majority blend of the deterministic GFS/ECMWF/CMC for days 3-4, quickly transitioning to mostly ensemble means thereafter. Maintained some smaller contributions from the ECMWF/CMC late period just for some added definition and details to the otherwise washed out means. The maintains good WPC continuity from the previous shift as well. ...Weather/Hazards Highlights... The combination of an amplified upper trough/closed low over the West and cold Canadian High Pressure (with temp anomalies 10-20 degrees below normal) will set the stage for a potentially heavy upslope snow event over the Central Rockies/parts of the High Plains. Some deformation and terrain enhanced precipitation is also possible over the Great Basin underneath of the upper low itself. Dependent on system progression later in the week, a moderate rain focus may shift into the Central Plains as well, but there is lingering uncertainty at these longer time frames. Meanwhile, a wavy frontal boundary will settle near the Gulf of Mexico which should focus periods of moderate to possibly heavy rainfall across much of the South and back into parts of the Lower Mississippi Valley/Southern Plains. Amounts don't look particularly high, although some of this region (especially parts of the Central Gulf Coast states) have seen much above normal rainfall over the past few weeks so any additional rain may increase flooding concerns. Farther north, showers may also linger across parts of the Northeast mid week as upper level energy slides 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, 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