Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM EDT Fri Apr 09 2021 Valid 12Z Mon Apr 12 2021 - 12Z Fri Apr 16 2021 ...Central Rockies/High Plains Heavy Snow Threat as well as additional heavy rainfall possible across parts of the South next week... ...Pattern Overview and Guidance/Predictability Assessment... The medium range period (Monday-Friday) will feature a fairly blocky pattern as strong upper level ridging in Canada and a deep closed low meandering off the Canadian Maritimes inhibits much movement of features over the lower 48. One upper low should weaken as it moves slowly across the Great Lakes through much of next week while the Canadian Maritimes upper low rotates and retrogrades slowly westward. Meanwhile, energy drops into the Western U.S. forming a closed low by Tuesday which looks to sit over the Great Basin area for several days before possibly beginning to drift eastward next Friday. Latest model and ensemble guidance shows greater than average agreement on the overall pattern, but continue to differ on the details, especially the second half of the period with respect to multiple low pressure areas across the Great Lakes/Northeast. Out West, guidance does agree upper low energy should linger much of the week, but there are questions on how quickly it begins to shift eastward on day 7. Both the latest runs of the ECMWF/CMC want to hold back the energy to along the West Coast, while the GFS is the quickest to eject it eastward (owing mostly to a weakening of upper ridging to the north). The ensemble means, although much more subdued, do tend to hold the energy back more than the GFS (but maybe not as much as the ECMWF). The WPC medium range product suite leaned heavily on the deterministic guidance days 3-4 (ECMWF/GFS/CMC) and transitioned towards majority ensemble means thereafter. Continued smaller contributions from the ECMWF/CMC for added detail and system definition. This approach fit well with previous shift continuity. ...Weather/Hazards Highlights... Northern stream closed low energies over the Upper Midwest/Great Lakes early in the week should keep the threat for showers in the forecast across the Northeast with wrap back snow possible across parts of the far northern Plains/Upper Mississippi Valley. A wavy trailing front will settle over the South/Gulf Coast region and offer potential for another round of heavy rainfall from the Lower Mississippi Valley to parts of the Southeast next week. While there are differences in exact rainfall amounts, this region has seen much above normal precipitation over the past few weeks already, so any additional rainfall could present flood concerns. Moderate to locally heavy rainfall is also possible farther upstream into the Southern Plains as uncertain ejecting southern stream impulses locally focus lift and moisture. Temperatures across the east will begin slightly above normal before transitioning to closer to average by later in the week. An amplifying upper trough/closed low over a cooled West next week should bring some terrain enhanced precipitation over the Great Basin, but the greater threat is for prolonged periods of heavy upslope snow over parts of the Central Rockies/High Plains as ample Canadian surface high pressure digs and settles southward. Temperatures within this airmass appear to be 10 to 20 degrees below normal, especially from eastern Montana to the Central High Plains. 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