Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Wed Apr 07 2021 Valid 12Z Sat Apr 10 2021 - 12Z Wed Apr 14 2021 ...Overview and Guidance Evaluation/Predictability Assessment... Models and means continue to show a blocky pattern covering portions of North America into the Atlantic, with upper ridging likely from eastern Canada through southern Greenland while a couple of upper lows/surface systems head into the Great Lakes and yet another upper low meanders near the Canadian Maritimes. Farther west, as ridging builds into western Canada (consistent with teleconnections relative to the eastern Canada/southern Greenland ridge as well as the Pacific trough to the west) solutions continue to diverge by early-mid week for possible western U.S. troughing. The forecast approach maintains good continuity from the past couple cycles, incorporating a model blend early followed by a trend toward the ensemble means and some ECMWF input. However there is the ongoing possibility for future change should guidance start clustering toward any minority ideas. During the weekend there is good agreement in principle that a compact Midwest into Great Lakes shortwave/upper low will promote fairly strong and concentrated surface low development from the Mississippi Valley into Great Lakes. There is still some spread for track though. The front anchored by this low should continue across the East with embedded low pressure continuing over the western Atlantic. Then a majority of guidance including the ensemble means would have a larger scale shortwave reaching into western North America by early Saturday, leading to an upper low that crosses the northern tier states and eventually reaches the Great Lakes. The corresponding surface low would bring another frontal system across the lower 48 during the weekend and the first half of next week. Both upper lows exhibit some sensitivity in terms of whether they stay in the Great Lakes region or some/all of their energy may get carried farther eastward by more progressive mean flow to the south. Some recent GFS/CMC runs have tended to be the solutions exhibiting the latter idea while their ensemble means have been holding back closer to the ECMWF. On the other hand the means could be lagging the models in their depiction of the Canadian Maritimes upper low. Confidence is low for precise movement of this low, with the GFS/CMC actually bringing it back to or near New England by next Wednesday versus remaining guidance that is farther east. As for western troughing, teleconnections relative to the strong positive height anomalies over or just east of eastern Canada continue to support the more modest depictions of the GEFS/ECMWF/CMC means versus the deeper trough and embedded closed low scenario that the GFS has been showing over the past couple days and now joined by the 00Z CMC. ...Sensible Weather and Potential Hazards... During the weekend expect the strengthening Mississippi Valley into Great Lakes system and its associated fronts to focus areas of moderate to heavy rainfall over portions of the eastern U.S. Heaviest rain should be over and near the central and eastern Gulf Coast. Significant totals could also be possible closer to the upper/surface low depending on training within any rain bands. The next system emerging from the West will bring mostly light rain/higher elevation snow to northern parts of that region and then some rain/thunderstorms to the eastern half of the country but in most cases with lighter amounts than expected for the preceding system. Some of the precipitation could be in the form of snow over parts of the extreme northern tier from late weekend into next week. During the early to middle part of next week the combination of cyclonic flow aloft and low level upslope flow could produce some precipitation over the central/south-central Rockies, though with considerable uncertainty over coverage and amounts. Warm flow ahead of the developing storm tracking into the Great Lakes this weekend will bring widespread lows of 10-20F or so above normal over the East Saturday-Sunday and plus 10-25F anomalies for highs in a more focused area from the Great Lakes/Upper Ohio Valley into New England on Saturday. Temperatures will drop to within a few degrees on either side of normal behind this system's cold front. The next system initially coming into western North America will spread below normal highs (minus 5-15F anomalies) across the Northwest/northern Rockies during the weekend and then into the central/east-central U.S. Monday into Wednesday. Southern parts of the West will remain above normal through the period and an upper ridge approaching from the Pacific may expand coverage of plus 10-20F anomalies for highs over northern California and the Northwest early-mid week. 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