Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Jun 22 2021 Valid 12Z Fri Jun 25 2021 - 12Z Tue Jun 29 2021 ...Dangerous Heat Wave to build record temperatures across the Northwest/West... ...Elongated Heavy Rain Threat to stretch from Southern Plains to Northeast... ...Guidance/Predictability Evaluation... The WPC medium range product suite was primarily derived from a composite of best clustered solutions of the 18 UTC GFS/GEFS mean and 12 UTC UKMET/ECMWF/ECENS mean Friday into Saturday in a pattern with above average continuity and predictability. The models exhibit considerable spread as well as run-to-run variability over the weekend with and downstream from a Rex blocking pattern on the West Coast. The ensemble means remain compatible through early next week and were used in composite with similarly consistent guidance from WPC continuity and the 01 UTC National Blend of Models. This composite develops and maintains north-central U.S. upper troughing a bit on the amplified side of the full envelope of guidance, seemingly consistent with upstream blocking. Newer 00 UTC guidance overall continues these general trends. ...Weather Highlights/Hazards... A highly anomalous upper ridge will build inland over the Northwest by the weekend and the heat wave will expand for much of the West into next week. Temperatures will climb to 15-25+ degrees above normal from Washington/Oregon to the Northern Rockies, and more generally 5-15 degrees above normal along and west of the Rockies. Records are likely from the Pacific Northwest to the north/central Great Basin and northern Rockies, with temperatures peaking over 100F to near 110F forecast for interior sections of the West. Little to no rainfall is expected to ease this excessive heat. Downstream, robust upper trough digging from south-central Canada will solidify a wavy, fortified and slow moving cold front to stretch from the south-central to northeastern U.S. this weekend and early next week, inviting deep moisture return and pooling. Well organized showers and storms will significantly increase rainfall and runoff potential along and ahead of the front with an elongated and focused zone from the Southern Plains northeastward across the Midwest/Mid-South, Ohio Valley and Appalachians to the Northeast. Episodes of cell training and repeat cells are likely. Schichtel 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