Extended Forecast Discussion NWS Weather Prediction Center College Park MD 335 PM EDT Sat Apr 15 2023 Valid 12Z Tue Apr 18 2023 - 12Z Sat Apr 22 2023 ...Overview with Guidance/Predictability Assessment... As a low pressure system moves farther away into the Canadian Maritimes when the medium-range period begins next Tuesday, attention will then shift westward to the next system of concern across the northeastern Pacific into the Pacific Northwest where models continue to generally agree that a slow-moving elongated upper low will periodically send shortwave energies into and across the western U.S. under a northwest-to-southeast oriented upper jet. The continual ejection of shortwave energies out of the western U.S. into the central U.S. will tend to amplify an upper trough over the mid-section of the country late next week. Models and ensembles today have trended toward a more amplified solution with respect to this upper trough as it gradually advances eastward to the vicinity of the Great Lakes and Midwest/Mississippi Valley by next weekend. This has resulted in a better-developed surface cyclone to possibly reach the Great Lakes late next week, with the lead cold front possibly moving into the East Coast next Saturday. Meanwhile, ensemble means continue to depict an omega/Rex block pattern setting up across the higher latitudes from Alaska through much of Canada. Under this blocky pattern, the forecast across North America will be subject to a higher degree of uncertainty than normal, with further adjustments to the model solutions quite possible through late next week. The latest 12Z ECMWF (and to some degree the rest of the 12Z guidance) now depicts a much more amplified solution to this pattern across the eastern half of the country late next week. With the above considerations, the WPC medium-range forecasts begin with a blend of 40% from the 06Z GFS/GEFS, 40% from the 00Z ECMWF/EC mean, and 20% from the 00Z CMC/CMC mean Day 3 - 5, followed by a higher percentage from the ensemble means through Day 7. ...Weather/Hazards Highlights... The upper low tracking across southeastern Canada and possibly brushing the northern tip of Maine may support lingering mostly light snow to the lee of the eastern Great Lakes and rain or snow over interior New England on Tuesday. Rain and higher elevation snow will extend from the Pacific Northwest/northern California into the northern half of the Rockies as of Tuesday while a wavy frontal system emerging into the Plains should begin to increase the coverage and intensity of rainfall over the central U.S. By the day 5 time frame (12Z Wednesday-12Z Thursday), guidance is suggesting that the frontal system could slow down for a time over the Plains/Midwest as upstream energy replaces a leading wave. While finer details are far from resolved, the combination of the forecast pattern along with potential persistence/pivoting of moisture over this area continues to support a Marginal Risk area centered over Iowa and including surrounding areas in the day 5 Excessive Rainfall Outlook. Gulf inflow will likely support other areas of heavy rainfall over parts of the Mississippi Valley and east-central U.S. toward the end of the week as the cold front continues eastward. Depending on how the system evolves late in the week, some northern tier locations could see some cold sector snow. Also of note, guidance continues to show the potential for convection over the southeastern half of Texas around Tuesday to Wednesday in association with a diffuse southern stream shortwave. The GFS (especially the 06Z) depicts a superposition of two upper jets that results in much higher rainfall amounts focusing in this areas. The 12Z GFS has backed off on this heavy rain scenario. Given the recent lack of consistency in the deterministic QPFs in this area, opt not to include a Marginal for this area on the day 4 (12Z Tuesday-12Z Wednesday) Excessive Rainfall Outlook. The lower 48 will see a gradual progression of temperature anomalies over the course of the period, with an upper trough promoting an area of below normal temperatures (generally up to 10-15F below normal) over the West Tuesday-Thursday, the northern Plains during the latter half of the week, and in somewhat modified form over more of the Plains/Mississippi Valley by Saturday. The West could see a few daily records for cold highs mainly on Tuesday. Meanwhile the leading warm sector should bring highs of 10-20F above normal to portions of the Plains through midweek and then expanding warmth through most of the eastern U.S. late in the week. Continued motion of the forecast cold front should confine above normal readings to areas near the East Coast by next Saturday. Early in the period, locations over/near the central-eastern Great Lakes will see a chilly day on Tuesday with highs 10-15F below normal due to the proximity of the southeastern Canada upper low. Kong/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, 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