Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Jun 29 2021 Valid 12Z Fri Jul 02 2021 - 12Z Tue Jul 06 2021 ...Historic heat wave over the Northwest becomes less intense and continues through the 4th of July weekend... ...Heavy rain could impact portions of the Deep South, Southeast and the East Coast ahead of an upper-level trough late this week and through the weekend... ...Guidance/Predictability Evaluation... Global models continue to show a great deal of uncertainty in predicting the details associated with a positively-tilted trough over the northeastern portion of the country through the medium range period. Model guidance has split into two camps since yesterday as the GFS now tends to close off an upper low over the Ohio Valley while the ECMWF tends to lift the trough faster into eastern Canada during the weekend. The CMC on the other hand, adopted an intermediate solution. The ensemble means are more agreeable with one another--which basically lift an amplified positively-tilted trough very slowly across the Northeast. This pattern would generally bring damp/wet weather into the East Coast by the holiday weekend but with important details yet to be resolved. Somewhat more certain is that the threat of heavy rain ahead of a cold front should push southward across the Deep South through the weekend. Meanwhile, areas of heavy rainfall are also possible along the East Coast but they will depend on the uncertain details associated with the interaction between the positively-tilted trough and the frontal boundary. The guidance is much more agreeable in predicting the slow weakening and eastward progression of the anomalous upper ridge over the Northwest and into the northern Plains. Given the uncertainties in the eastern U.S., the medium-range forecasts put more weights on the ensemble means than usual, and were based on the consensus of the 12Z ECMWF/EC mean, 18Z GFS/GEFS, and the 12Z CMC/CMC mean. This solution yielded 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 but triple-digit heat is forecast for each afternoon across the interior section through the holiday weekend and into early next week. Meanwhile, the heat is expected to spread into the northern Plains and continue into next week as afternoon temperatures could reach the century mark at some locations. Some rain showers over the northern Rockies may offer slight relief to the heat but they will do little to ease the growing drought concern in these areas. Farther south, some late-day monsoonal type showers and thunderstorms are expected each day over the central to southern Rockies through the medium range period. Meanwhile, a heat wave across the Northeast to the Mid-Atlantic during the short range period 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. Areas of heavy rain could also move into the East Coast late this week ahead of the front but they will depend on the details of the trough-front interactions that are yet to be resolved by models. Cool days and warm nights are expected for the Deep South as well where the heavy rain threat continues into early next week. Episodes of cell training and flooding are likely across some of these areas. This will be in stark contrast with the dry and hot weather expected to persist over the northern Plains through the medium range period. 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 Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Sat-Mon, Jul 3-Jul 5. - Heavy rain across portions of the Mid-Atlantic, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Northeast, the Southern Appalachians, the Southeast, and the Ohio Valley, Thu-Fri, Jul 1-Jul 2. - Flooding possible across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of the Central Plains, the Great Lakes, the Middle Mississippi Valley, and the Southern Plains. - Flooding likely across portions of the Southern Plains. - Excessive heat across portions of the Central Great Basin, the Northern Plains, the Northern Rockies, California, the Northern Great Basin, and the Pacific Northwest, Thu-Mon, Jul 1-Jul 5. - Excessive heat across portions of the Upper Mississippi Valley and the Northern Plains, Fri-Sat, Jul 2-Jul 3. 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