Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Jun 28 2021 Valid 12Z Thu Jul 01 2021 - 12Z Mon Jul 05 2021 ...Historic heat wave over the Northwest becomes less intense and continues through the 4th of July weekend... ...Heavy rain threat shifts south from southern Rockies/Plains to near the Gulf Coast by the weekend... ...Guidance/Predictability Evaluation... Models and ensembles agree that the highly anomalous upper ridge currently located over the Pacific Northwest and British Columbia will weaken slightly and shift east into the northern High Plains during the medium range period as an upper wave/trough moves into British Columbia and across southwest Canada. Meanwhile, ensemble means continue to indicate a positively-tilted trough will drift across the Great Lakes region by late this week, where deterministic model solutions tend to call for a closed low to form under this highly amplified synoptic pattern. There is increasing model agreement that a slow-moving frontal boundary across the mid-section of the country will begin to push southward when the upper trough/low over the Great Lakes is forecast to dip further south late this week. 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 and into early next week. However, models continue to show considerable uncertainty regarding the details. Solutions range from a wavy front becoming stalled across the eastern U.S. to a more consolidated low pressure system moving into the Northeast by the holiday weekend. Given the uncertainties, the Day 5-7 forecasts were derived mainly from the consensus of the 12Z EC mean, 18Z GEFS, and the 12Z CMC mean. The 18Z GFS was not included beginning on Day 5 since it began to place a low pressure center over the Ohio Valley that lagged far behind its the ensemble mean position over the Northeast. For Day 3-4, a general blend of the 18Z GFS/GEFS mean, 12Z ECMWF/ECENS means and some contributions from the 12Z CMC/CMC mean was adopted. This maintained fairly good continuity with the previous WPC forecasts. ...Weather Highlights/Hazards... The historic heat wave over the Pacific Northwest is expected to be less intense by midweek as the highly anomalous upper ridge is forecast to weaken slightly and shift eastward. Nevertheless, high temperatures across the interior sections will remain 15 to 25 degrees above normal through the holiday weekend and into early next week. Little rainfall is anticipated 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. into midweek. 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 Friday 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 with cool daytime temperatures and warm and humid nights. Cool days and warm nights are expected for the Deep South as well where the heavy rain threat continues into early next week. In contrast, dry and hot weather will return across the northern Plains and should continue into early next week under the weakened but still anomalous upper ridge. 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