Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Apr 15 2023 Valid 12Z Tue Apr 18 2023 - 12Z Sat Apr 22 2023 ...Overview with Guidance/Predictability Assessment... Recent guidance has been showing similar themes and trends for some aspects of the forecast, but still with a fair amount of embedded uncertainty underneath a what is forecast to be a blocky pattern from Siberia through most of Canada. The most common idea is for leading western U.S. trough energy to form an upper low near the U.S.-Canadian border, potentially leading to a Rex Block configuration for a time, while upstream energy feeds into the larger scale trough that ultimately reaches the Plains/Mississippi Valley. Ensemble means have been getting deeper with the anchoring upper low that may reach the Upper Midwest/Upper Great Lakes toward the end of the period, providing an incremental increase in confidence for that feature. However there is still a lot of uncertainty over how individual surface waves may evolve over the course of the period. Meanwhile recent consensus trends had been for somewhat more upper ridging to approach the West Coast by Friday-Saturday, but the new 00Z GFS/CMC/ECMWF runs all show the possibility that energy from an opening central Pacific system could pass through the ridge--ultimately leading to flatter mean flow than depicted in most of the 12Z/18Z guidance. The updated forecast incorporating that 12Z/18Z guidance started with a blend of 12Z ECMWF/18Z GFS/12Z CMC early in the period and then included up to half weight of the 18Z GEFS/12Z ECMWF means by days 6-7 Friday-Saturday. UKMET runs continue to be more aggressive than other guidance with eastern Pacific ridging, in turn leading to more amplified western-central U.S. troughing and a farther south track of the embedded upper low. The forecast pattern will support rain and mountain snow from the northern-central West Coast east into the Rockies, with increasing coverage and intensity of rainfall over parts of the Plains and east-central U.S. mid-late week. ...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 lead to a planned introduction of 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 in association with a diffuse southern stream shortwave. While there had been an increasing QPF trend in some guidance, newest runs vary considerably for how much rain this convection may produce. The combined moisture and instability may support some locally heavy activity but with confidence in specifics remaining too low to depict an 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. 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