Extended Forecast Discussion NWS Weather Prediction Center College Park MD 434 PM 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 but 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... Guidance agrees that the large scale pattern should return to a more typical summer regime by next Tuesday after a fairly amplified evolution from late this week through the weekend. During the first half of the period expect an eastern U.S. upper trough (likely with an embedded low) to amplify downstream from a strong but slowly weakening upper ridge dropping from southern Canada into the Plains/Midwest. Behind the descending upper ridge, flatter mean flow will likely help to eject the eastern trough by early next week. Meanwhile expect an upper ridge initially over the far southern High Plains/Rockies to retrograde slowly toward the Southwest U.S. Weak upper energy to the east of this ridge may settle/consolidate somewhat over the southern Plains by next Monday or Tuesday. Models and ensembles are still in the process of refining the forecast of the eastern U.S. upper trough. Through the 00Z/06Z cycles there appeared to be some progress toward convergence with better agreement toward an upper low closing off and tracking over or near the Upper Ohio Valley by this weekend. GFS/CMC runs represented a farther east solution than some earlier runs while the 00Z ECMWF was slower than its prior two runs but similar to the run from 36 hrs ago. This left the 00Z UKMET on the slow side of the spread. Not surprisingly the new 12Z model runs display some adjustments, in particular trending faster with the upper low's ejection after early Saturday. Interestingly the new GEFS mean holds back its overall trough more like the 00Z ECMWF mean. Confidence is fairly good with the general idea of southern Canada/northern tier U.S. flow aloft becoming flatter and more progressive, supporting the consensus that establishes a mean frontal boundary across the northern tier next week. However typically low predictability with shortwave details will keep confidence lower for exact frontal position and wave details on a day-to-day basis. Based on the available guidance through the 06Z cycle, the updated forecast started with a blend of the 00Z ECMWF/00Z-06Z GFS/00Z CMC for about the first half of the period to reflect increasing consensus toward a closed low within the eastern upper trough. Then the blend incorporated some 06Z GEFS/00Z ECMWF mean input as details diverged for the eastern trough as well as flow extending from the North Pacific across southern Canada and northern U.S. ...Weather Highlights/Hazards... The historic heat wave over the Pacific Northwest should be less intense than it has been over recent days but triple-digit heat is still likely each afternoon across some interior areas through the holiday weekend and into early next week. A portion of this heat will spread into the northern Plains and continue into next week. Best potential for afternoon temperatures to reach the century mark at some locations in Montana and the Dakotas should be during Friday-Sunday before a front dips into the northern tier. In terms of anomalies, the very gradual moderating trend will bring high temperatures from the Northwest into northern Plains down to about 10-15F above normal by next Tuesday after starting the period at 15-25F above normal on Friday. 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 likely each day over the central to southern Rockies through the medium range period. Low level upslope flow could enhance activity over some areas. 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 forecast to be centered over the Great Lakes at that time directs moisture into much of the East Coast along and ahead of a frontal boundary. It appears that the East Coast will enter a period of cloudy and wet weather just in time for the July 4th holiday weekend with cool daytime temperatures and warm/humid nights. Rainfall amounts over the northern Mid-Atlantic and vicinity will be very sensitive to exactly how the upper trough and possible embedded low evolve. A closed low would increase the potential for some areas of enhanced rainfall over this area. Confidence is somewhat higher that areas of heavy rainfall will be possible along the trailing front, from the southern Mid-Atlantic across the South and into southeastern Texas. The front should stall near the Gulf Coast for a time before lifting north and dissipating--increasing the potential for a period of cell training/repeat activity that would pose a flooding threat. Clouds/rainfall will yield multiple days of below normal highs (generally minus 5-12F anomalies) from the southern High Plains into portions of the South and East. High pressure behind the front should also provide moderately below normal morning lows to areas from central Plains and Midwest into the Ohio and Tennessee Valleys, especially Saturday-Sunday. Next week's northern tier front may produce one or more bands of meaningful rainfall but details for totals/timing/location are uncertain at this time. Rausch/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 Southern Plains, Fri, Jul 2. - Heavy rain across portions of the Northeast, the Mid-Atlantic, the Lower Mississippi Valley, the Central/Southern Appalachians, the Tennessee Valley, and the Southeast, Fri-Sat, Jul 2-Jul 3. - Heavy rain across portions of the the Lower Mississippi Valley and the Southern Plains, Sat-Tue, Jul 3-Jul 6. - Flooding possible across portions of the Central/Southern Plains, the Middle Mississippi Valley, the Ohio Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Central/Southern Plains, the Middle Mississippi Valley, and the Great Lakes. - Flooding likely across portions of the Middle Mississippi Valley. - Excessive heat across portions of the Pacific Northwest and the Northern Great Basin, Fri-Mon, Jul 2-Jul 5. - Excessive heat across portions of the Northern Plains, Fri-Sat, Jul 2-Jul 3. - Much above normal temperatures across portions of the Pacific Northwest, California, the Northern Great Basin, and the Northern Rockies, Fri-Mon, Jul 2-Jul 5. - Much above normal temperatures across portions of the Northern Rockies, the Northern Plains and the Upper Midwest, Fri-Sun, Jul 2-Jul 4. 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