Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Jun 25 2021 Valid 12Z Mon Jun 28 2021 - 12Z Fri Jul 02 2021 ...Historic and Dangerous Heat Wave with monthly/all-time record temperatures possible over the Northwest/West... ...Heavy Rain Threat to stretch from Southern Rockies/Plains to Ohio Valley... ...Guidance/Predictability Evaluation... Models and ensemble continue to advertise a highly anomalous upper ridge to reach peak intensity over the Pacific Northwest early next week while a compact upper low in the Pacific is forecast to move toward the northern coast of California. The ECMWF and the CMC are similar in terms of keeping the compact upper low closer to the coast and slower to lift northward than the GFS and GEFS solutions. The proximity of this upper low would appear to keep the heat in the central California valleys at bay as in the ECMWF and CMC solutions. All models agree that the upper ridge will then gradually weaken toward the latter half of next week as the compact upper low lifts northward and edges into the Pacific Northwest as an open wave. Meanwhile, global models continue to indicate a broad upper ridge will remain the dominating synoptic feature over the eastern U.S. well into next week while a positively-tilted trough drifts across the northern Plains into the Great Lakes. A broad and persistent southerly flow will direct tropical moisture from the Gulf of Mexico northward through the Deep South and interact with a slow-moving frontal boundary across the mid-section of the country. This will increase the chance for heavy rain to develop from the southern Plains to the Ohio Valley through much of next week. In addition, models show some signal for a tropical wave/low to move westward from south of Bermuda to reach the southeast U.S. coast during the early part of next week. The WPC medium range product suite was derived from a composite of best clustered solutions of the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/ECENS means and the 12 UTC CMC/CMC mean in an overall pattern with above average predictability days 3-5 (Monday-Wednesday), followed by shifting blend focus toward the still compatible ensemble means by days 6/7 amid gradually increasing forecast spread. ...Weather Highlights/Hazards... A highly anomalous upper ridge is forecast to reach peak intensity over the Northwest early next week before showing signs of weakening later 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 to be tied or broken from the Pacific Northwest and interior California to the north/central Great Basin and northern Rockies, where temperatures peaking over 100F to 110+F are forecast for the lower elevations. Little rainfall is expected to ease this excessive heat in a pattern with growing drought and fire threats. Downstream, a positively-tilted 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. through the medium-range period. This will invite deep moisture to return from the Gulf of Mexico. 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 by the blocky synoptic pattern sandwiched between the heat wave out West and a strong western Atlantic ridge to the east. Last but not least, tropical moisture may feed into southern Florida from the Atlantic to fuel convection early-mid next week. Kong/Schichtel 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