Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Jun 30 2021 Valid 12Z Sat Jul 03 2021 - 12Z Wed Jul 07 2021 ...Triple-digit heat continues over the interior Northwest while spreading into portions of the northern Plains through the 4th of July weekend... ...Threat of heavy rain lingers near the Gulf Coast through the weekend into next week... ...Guidance/Predictability Evaluation... Guidance continues to show difficulties in handling a fairly amplified trough together with its interaction with a frontal boundary across the eastern U.S. through the holiday weekend. The GFS has since abandoned the idea of closing off the positively-tilted trough over the Ohio Valley much farther to the west than what the ECMWF and CMC have been indicating. This has improved the agreement among model solutions. Nevertheless, even the more consistent ECMWF solutions continue to exhibit more run-to-run variability than desirable. Therefore, it is best not to lean toward a particular deterministic solution but to incorporate more of the ensemble means. A general compromise based on the 12Z EC mean, 18Z GEFS, the 12Z CMC mean, together with some of their deterministic components yielded an intermediate solution that is most compatible with continuity. This basically keeps a positively-tilted trough with a hint of a closed off low over the northeastern quadrant of the country through the holiday weekend. The axis of the upper trough has been nudged faster toward the south and east with each successive forecast cycle. Models agree that the upper trough should lift northeastward into the Canadian Maritimes on Monday. This will allow the large scale pattern to return to a more typical summer regime by next Tuesday as a slowly weakening upper ridge drops from southern Canada into the Plains/Midwest. 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. 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. The WPC medium-range forecasts were based on a general compromise of the 12Z EC mean, 18Z GEFS, the 12Z CMC mean, together with some of their deterministic components. ...Weather Highlights/Hazards... The historic heat wave over the Pacific Northwest will 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 Saturday. 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, cloudy and wet weather is forecast to replace a heat wave across the Northeast to the Mid-Atlantic by the holiday weekend as the upper trough/low forecast to be centered over the Great Lakes directs moisture into much of the East Coast. Cool daytime temperatures and warm/humid nights will be likely following the passage of a cold front. 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. Recent model trend appears to favor a faster progression of the upper trough, leading to partial clearing skies possible over portions of the East Coast later in the July 4th weekend. Confidence is somewhat higher for areas of heavy rainfall to impact portions of the southern Mid-Atlantic, the Deep South and into southeastern Texas along the trailing front. 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. Kong/Rausch 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. 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