Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM 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 probable over the Northwest/West... ...Anomalous heat for New England Monday into Wednesday... ...Persistent Heavy Rain Threat to stretch from Southern Rockies/Plains to Ohio Valley and eventually the Northeast... ...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 before opening up. All models agree that the upper ridge will then gradually weaken/flop over across southern Canada toward the latter half of next week as the compact upper low lifts northward and edges into the Pacific Northwest as an open wave. An amplifying positively-tilted trough with an embedded closed center moves little across the Midwest, Plains, into the Great Lakes during this period due to the orientation of the upstream ridge -- thought the 00z ECMWF was too far east with this feature and that the 06z GFS/weaker form of the 00z Canadian was a better way to go, model wise. 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 north and strong subtropical ridge off the East coast, which leads to anomalous heat in Maine/New England and 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 a blend of the 06z GFS/00z Canadian/00z UKMET/00z ECMWF early on before increasing ensemble mean inclusion mid to late next week. The remainder of the grids showed a more even blend of deterministic and ensemble mean guidance through the period, as always. ...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 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 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 southern Plains to the northeastern U.S. through the medium-range period. This will invite deep moisture to continue streaming in 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 -- 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 develop and persist from the Northeast to the Mid-Atlantic where afternoon temperatures are forecast to peak well into the 90s away from the immediate coasts during next week. 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 next week. Roth/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