Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Jun 20 2021 Valid 12Z Wed Jun 23 2021 - 12Z Sun Jun 27 2021 ...Heat intensifies late week over the interior Northwest/West toward record levels... ...Mid-late week heat eases for the Plains before possibly returning during weekend over the northern Plains... ...Heavy rain threat for the mid-section of the country late this week... ...Overview... Ample upper ridging is again forecast to build/strengthen into parts of the West next week as an upper vortex along the West Coast slowly drifts offshore leading to a Rex block pattern. This will allow another stretch of warm/hot weather and record highs to expand over the Pacific Northwest southward to the Desert Southwest by late in the week. In turn, this will promote downstream troughing over the Great Lakes/Midwest as another frontal boundary pushes through the region with showers and storms. ...Guidance/Predictability Evaluation... A composite blend of the 06 UTC GFS/GEFS mean together with the 00 UTC ECMWF/ECMWF ensemble mean and some contribution from the 00Z CMC/CMC mean appears to offer a good starting point for the day 3-5 (Wednesday-Friday) forecast period in a pattern with above average continuity and predictability. Uncertainty and model spread increase across southern Canada into the northern Plains by next weekend as model solutions are trending toward digging additional energy downstream from the top of the Rex block. This calls for increasing usage of the ensemble means for Days 6 and 7. Developing model differences included details with the hot ridging over the West, but especially with the amplitude and longitudinal focus of downstream upper trough amplification over the east-central U.S. ...Weather Highlights/Hazards... An upper low just off northern California is forecast to detach from the northern stream and then meander in the same vicinity as a ridge of high pressure over the Pacific builds into the Northwest through the medium-range period. This will allow temperatures to climb to 15-25+ degrees above normal in Washington/Oregon next weekend, and more generally 5-15 degrees above normal along and west of the Rockies. Record highs are likely to expand across the area especially across the Pacific Northwest and into the northern Rockies late in the week where afternoon temperatures peaking over 100F and possibly 110F are forecast for the interior sections of the West. Little to no rainfall is expected to ease the heat throughout the region. The heat may then expand eastward into the northern Plains late next weekend. Farther east, an increasingly robust/amplified upper trough digging from south-central Canada will carry a wavy and fortified cold front southeastward into the east-central U.S. as a warm front lifts through and out of the southern Plains, inviting moisture return northward by later week. Showers and storms will significantly increase in coverage and intensity later this week along and ahead of the front and near areas of low pressure, with focuses over the Upper Midwest/Corn Belt through the Mid-Mississippi and Ohio Valleys. It appears that the Mid-Mississippi and Tennessee Valleys will be the most likely regions to see some heavy rainfall late next week. Kong/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