Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 PM EDT Sun Jun 27 2021 Valid 12Z Wed Jun 30 2021 - 12Z Sun Jul 04 2021 ...Historic heat wave over the Northwest should become less intense but continue into the 4th of July weekend... ...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 good agreement that the strong upper ridge currently located over the Pacific Northwest and British Columbia may finally begin to weaken and shift east into the northern High Plains through the medium range period as an upper wave moves into British Columbia and across southwest Canada. Meanwhile, a positively-tilted trough over the Northern Plains and Upper Great Lakes region on Wednesday will move only slightly eastward into the Northeast by the end of next weekend. There continues to be increasing model signals for the upper trough to develop into a closed low over or near the Great Lakes later this week although the ensemble means tend to retain the positively-tilted configuration, likely due to uncertainties in individual ensemble runs. 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 used a majority deterministic model blend the first half of the period with above average predictability. Thereafter, shifted the blend towards the ensemble means amid increasing forecast spread, but did include a fair amount of the GFS/ECMWF (with some manual intervention in the resulting 500mb charts) due to the good run-to-run consistency on the Great Lakes low. 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 holiday 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 next 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 just in time for the July 4th holiday weekend. 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