Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 AM EDT Sun Jun 27 2021 Valid 12Z Wed Jun 30 2021 - 12Z Sun Jul 04 2021 ...Historic heat wave over the Northwest is forecast to continue into the 4th of July weekend while becoming less intense... ...Anomalous heat for New England into Wednesday... ...Heavy rain threat from southern Rockies/Plains to the Midwest midweek should shift south to near the Gulf Coast by the weekend... ...Guidance/Predictability Evaluation... Models and ensembles show that the highly anomalous upper ridge across the Pacific Northwest and British Columbia will lose some strength by midweek as a compact upper low lifts northward just off the Pacific Northwest coast and weaken into an open wave. There is good model agreement that the ridge will expand slowly eastward into the northern High Plains through the latter half of the week ahead of a positively-tilted trough initially over the northern Plains. There continues to be increasing model signals for the upper trough to develop into a closed low over the Great Lakes later this week although the ensemble means tend to retain the positively-tilted configuration. Meanwhile, a slow-moving frontal boundary across the mid-section of the country should begin to push slowly southward amid increasing model signals for the upper trough/low over the Great Lakes to dip further south. This pattern should bring the heavy rain threat progressively southward from the mid-South to the Gulf Coast while directing moisture toward much of the East Coast for the 4th of July weekend following the early/mid-week heat wave across New England and the Mid-Atlantic. The WPC medium range product suite was derived from a composite of the 18Z GFS/GEFS mean, 12Z ECMWF/ECENS means and some contributions from the 12Z CMC/CMC mean in an overall pattern with above average predictability days 3-5 (Wednesday-Friday), followed by shifting blend focus toward the still compatible ensemble means by the 4th of July weekend amid gradually increasing forecast spread. This maintained fairly good continuity with the previous WPC forecasts. ...Weather Highlights/Hazards... A highly anomalous upper ridge over the Northwest will prolong the dangerous and oppressive heat over the region. As the ridge is forecast to weaken slightly, the heat should become much less intense by Wednesday near the Pacific Northwest coast but high temperatures across the interior sections will remain 20 to nearly 30 degrees above normal through late week with some improvement possible by the 4th of July weekend. 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. 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. Episodes of cell training and flooding are likely across some of these areas before the heavy rainfall axis slowly drops southward toward the Gulf Coast by the 4th of July weekend. A heat wave across the Northeast to the Mid-Atlantic into midweek should be broken by Thursday as the upper trough/low centered over the Great Lakes directs moisture into much of the East Coast ahead of a frontal boundary. It appears that the East Coast will enter a period of damp/wet weather as we head into the 4th of July weekend. 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