Extended Forecast Discussion NWS Weather Prediction Center College Park MD 140 PM 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... ...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/Tennessee Valley... ...Guidance/Predictability Evaluation... Models and ensembles continue to show very good agreement that a highly anomalous upper ridge will remain parked over much of the West through next week. The ridge may slowly shift eastward with time (though still maintain impressive strength) in response to a compact upper low lifting along the Pacific Northwest coast and eventually pushing into British Columbia as an open wave. Models show some minor timing/strength variability with this feature, though a general blend should help to mitigate those differences. 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. The ECMWF/GFS have been most consistent in showing this feature drifting southward into the Midwest, with some timing differences later in the period. The 00z CMC however drags this low more westerly into the Northern Plains, which is inconsistent with the rest of the guidance. 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 associated with the upper trough filters into the region later next week. The WPC forecast used a blend of the deterministic models (with more emphasis towards the 00z ECMWF/06z GFS) for days 3-4. Removed the CMC thereafter in favor of increasing contribution from the ensemble means to help smooth out some of the less predictable detail differences. This maintained fairly good continuity with the previous WPC forecast as well and only minor changes were needed to the surface progs. ...Weather Highlights/Hazards... A highly anomalous upper ridge over the Northwest will prolong the dangerous and oppressive heat over the region. Temperatures will soar 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 excessive heat in a pattern with growing drought and fire threats and minimal recovery overnight will also increase the heat stress. Conditions may improve slightly for the Pacific Northwest later in the week as upper troughing pushes into southwest Canada, but excessive heat will continue across interior portions of the Northwest into next weekend. 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. Heavy rainfall axis may drop slowly southward later next week as an upper low potentially organizes over the Midwest states. 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 3-4/Tuesday and 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. Santorelli/Kong 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