Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EDT Mon Jun 21 2021 Valid 12Z Thu Jun 24 2021 - 12Z Mon Jun 28 2021 ...Heat intensifies late week over the interior Northwest/West toward record levels... ...Heavy rain threat for the mid-section of the country late this week... ...Guidance/Predictability Evaluation... The WPC medium range product suite was derived from a composite of best clustered solutions of the 06 UTC GFS/GEFS mean, 00 UTC ECMWF/EC mean and the 00Z CMC/CMC mean for days 3-4 (Thu-Fri) in a pattern with above average continuity and predictability. From Day 5 onward, models continue to exhibit considerable spread as well as run-to-run variability downstream from a Rex blocking pattern on the West Coast. This calls for placing more weights than usual toward the ensemble means beginning on Day 5 and through Day 7, as the ensemble means remain compatible with one another through Day 7. The deterministic model solutions from Day 5 onward show more spread than they were yesterday in general. Notable differences include the amplitude/evolution of the hot ridge over the West, but especially with longitudinal focus of downstream trough amplification over the central U.S. as energies dig downstream from the Rex block. The 12Z UTC model guidance does not seem to offer much clarifications either. ...Weather Highlights/Hazards... An upper low just off northern California is forecast to meander offshore later this week as a hot ridge builds inland through the Northwest. This will allow temperatures to 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. Record highs are likely from the Pacific Northwest to the northern Rockies, with afternoon temperatures peaking well over 100F to near 110F are forecast for the interior sections of the West. Little to no rainfall is expected to ease the high heat throughout the broad region. Downstream, robust upper trough digging from south-central Canada will solidify a wavy and fortified cold front to move steadily over the central to eastern U.S., inviting moisture return by later week. Well organized showers and storms will significantly increase rainfall/runoff potential along and ahead of the front from the Midwest to the Mid-South, Southern Plains and across the Ohio Valley and Appalachians to the Northeast. 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 Hazards: - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, and the Ohio Valley, Sat-Sun, Jun 26-Jun 27. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Ohio Valley, and the Upper Mississippi Valley, Thu-Fri, Jun 24-Jun 25. - Heavy rain across portions of the Mid-Atlantic, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Northeast, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Sat-Mon, Jun 26-Jun 28. - Flooding possible across portions of the Ohio Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, and the Ohio Valley. - Excessive heat across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Fri-Mon, Jun 25-Jun 28. - Excessive heat across portions of the Central Great Basin, the Northern Plains, the Northern Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Sun-Mon, Jun 27-Jun 28. - Excessive heat across portions of the Central Plains and the Southern Plains, Thu-Fri, Jun 24-Jun 25. - Excessive heat across portions of the Central Plains, Thu, Jun 24. 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