Extended Forecast Discussion NWS Weather Prediction Center College Park MD 138 AM EDT Wed Jun 23 2021 Valid 12Z Sat Jun 26 2021 - 12Z Wed Jun 30 2021 ...Dangerous Heat Wave to build Daily to Monthly Record Temperatures across the Northwest/West... ...Heavy Rain Threat to stretch from Southern Rockies/Plains to Midwest/Mid-South, Ohio Valley and the 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, 12 UTC UKMET/ECMWF, 12 UTC NAEFS/ECENS means, the 01 UTC National Blend of Models and WPC continuity in an overall pattern with above average continuity and predictability. Forecast spread and run-to-run variability has improved compared to the last few days and a composite seems to mitigate much of the lingering and less predictable small-mid scale variances. Newer 00 UTC guidance so far remains in line with the WPC composite. However, recent Canadian runs are displaced east of most guidance with placement of the central U.S. trough and western Atlantic ridge, so they remain discounted. ...Weather Highlights/Hazards... A highly anomalous upper ridge will build inland over the Northwest this weekend and the heat wave will expand over the West into next week. Temperatures will soar to 20-30 degrees above normal from Washington/Oregon to the northern Great Basin/Rockies. Numerous daily to some monthly records are likely from the Pacific Northwest and interior California to the north/central Great Basin and Northern Rockies, with temperatures peaking over 100F to 110+F for interior sections of the West. Little to no rainfall is expected to ease this excessive heat this forecast period, growing drought and fire threats. Downstream, a robust upper trough will dig and settle into the north-central U.S. to solidify and reinforce a wavy and slow moving front to stretch from the south-central to northeastern U.S. this period. This will invite 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 Rockies/Plains northeastward across the Midwest/Mid-South, Ohio Valley and the Northeast. Episodes of cell training and flooding are likely across some of these areas with overall progression slowed as broadly sandwiched between the heat wave out West and a strong western Atlantic ridge to the east. Also, there is also some signal for tropical moisture to feed into southern Florida to fuel convection early-mid next week. 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