Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Jun 26 2021 Valid 12Z Tue Jun 29 2021 - 12Z Sat Jul 03 2021 ...Historic and Dangerous Heat Wave with monthly/all-time record temperatures probable over the Northwest/West... ...Anomalous heat for New England into Wednesday... ...Persistent Heavy Rain Threat to stretch from Southern Rockies/Plains to the Midwest and later into the Ohio Valley... ...Guidance/Predictability Evaluation... Models and ensembles agree that a highly anomalous upper ridge over the Pacific Northwest will begin to weaken by around Tuesday as a compact upper low just offshore is forecast to lift northward as an open wave and edge closer toward the Pacific Northwest coast. The ECMWF and CMC have been slower than the GFS regarding the forward motion of the upper low. The 18Z and 00Z GFS runs have slowed down the upper low to agree much better with the ECMWF and CMC. By later next week, models generally show the weakened upper ridge will drift further inland across the northern Rockies into the northern High Plains but with still impressive strength. Across the northern tier states, a positively-tilted trough will likely drift very slowly eastward with increasing model agreement for a closed center to form over the Great Lakes toward the end of next week. Meanwhile, 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. Downstream, this leads to an anomalously strong subtropical ridge off the East Coast to extend northward into the Canadian Maritimes. A heat wave is forecast for much of New England into the Mid-Atlantic into the middle of next week before cooler air associate with the upper trough filters into the region later next week. Meanwhile, models continue to indicate the possibility of 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 pressures, 500 hPa heights, and winds were comprised of the 18Z GFS/GEFS, the 12Z ECMWF/EC mean, together with smaller contributions from the 12Z CMC/CMC mean. A more even blend of deterministic and ensemble mean guidance was adopted early on before increasing ensemble mean inclusion mid to late next week to handle the uncertainty. ...Weather Highlights/Hazards... A highly anomalous upper ridge at peak intensity over the Northwest early next week is forecast to weaken slowly later next week. Temperatures will soar to 20 to locally more than 30 degrees above normal from Washington/Oregon to the northern Great Basin/Rockies. Numerous daily and monthly high temperature records -- including for the states of WA, OR, and ID -- 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 between 100F and the upper 110sF are forecast for the lower elevations. All-time record highs are probable across portions of OR and WA. Little rainfall is expected to ease this prolonged excessive heat in a pattern with growing drought and fire threats. Downstream, a positively-tilted upper trough drifting slowly across the north-central U.S. will solidify and reinforce a wavy and slow moving front from the southern Plains to the northeastern U.S. through the medium-range period. Deep moisture streaming in from the Gulf of Mexico will continue to help organize showers and storms which 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. Drought conditions across the Upper Midwest and Great Lakes in particular should ease if not be outright eliminated over the next week. 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. Under the northward displaced Bermuda High, a heat wave appears to persist into mid-week from the Northeast to the Mid-Atlantic where afternoon temperatures are forecast to peak well into the 90s away from the immediate coasts. The hottest days/most anomalous readings will be across New England from days 3-5/Monday through Wednesday next week before a front slips southward and rain increases in coverage and intensity. Last but not least, tropical moisture/disturbances should move from the Atlantic into FL/the Southeast underneath the displaced Bermuda High to fuel convection and some gusty winds early next week. Kong/Roth/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