Extended Forecast Discussion NWS Weather Prediction Center College Park MD 223 AM EDT Sun Aug 08 2021 Valid 12Z Wed Aug 11 2021 - 12Z Sun Aug 15 2021 ...Northwest heat wave likely to bring temperatures that challenge daily records from midweek onward... ...Overview... Most of the guidance agrees that upper level ridging will consolidate from the Northwest U.S. into the Southern Plains during the extended range period, while upper troughing (and embedded shortwaves) moves through the Upper Great Lakes and Northeast. These features should support a heat wave affecting the Northwest and parts of the Great Basin/California with temperatures over some areas likely to approach or reach daily records for highs/warm lows from midweek onward. Across the eastern third of the country, associated fronts and leading warm/moist flow will produce areas of rain and thunderstorms, some locally heavy at times. ...Guidance/Predictability Evaluation... The latest cycle of model guidance continues to show above average agreement regarding both the upper ridge across the West and South, and mean troughing settling from the Great Lakes to the Northeast. There remains some typical uncertainties in the details of individual shortwave impulses embedded within the larger scale trough, but a general blend of the guidance seems like a good starting point right now. Elsewhere, there continues to be some timing differences regarding a shortwave dropping out of the Gulf of Alaska as well as another shortwave moving towards the West Coast day 7/Sunday which could act to suppress the Western U.S. ridge a bit. Given the late period timing, opted to lean on a 50/50 blend of the deterministic models and ensemble means which gave a reasonable middle ground solution. ...Weather Highlights/Hazards... For the Northwest heat wave, daytime high anomalies of plus 10-20F or so should prevail over Washington/Oregon and surrounding areas from Wednesday through the weekend. A decent number of locations across the Northwest and into northern California may see readings approach if not reach/exceed daily records for highs and warm lows over multiple days. Current forecasts suggest this heat wave will not be nearly as extreme as the all-time record event in late June but could still be the second most significant of the summer for the Portland-Seattle corridor. Meanwhile, shortwaves/moisture tracking around the southern side of the consolidating upper ridge over the Great Basin/central Rockies will likely provide a steady increase of coverage and rainfall amounts in monsoon activity over and near the Four Corners states. As a result high temperatures over this area may decline a bit after near or slightly above normal values early in the period. To the east of the Rockies, a leading Plains into Great Lakes front (dissipating by Thursday) and a trailing front that reaches the East Coast and south-central U.S. around the start of next weekend will provide the best-defined focus for showers and thunderstorms during the period. Some areas of locally moderate to heavy rain will be possible from the Midwest and Great Lakes/Ohio Valley into the Mid-Atlantic/Northeast but it will likely take into the short-range time frame to resolve the finer details of magnitude and location of highest totals. Farther ahead of the fronts, daily scattered storm chances will also exist over the East and Deep South within a warm and moist environment. A tropical wave may also increase rainfall over southern Florida next Friday into Saturday. Very warm to hot conditions should prevail to the east and south of the two cold fronts. The highest anomalies of plus 10F or greater should be for morning lows over and near the Great Lakes especially Wednesday-Thursday plus the Northeast late in the week, though a few isolated plus 10F anomalies for highs could be possible. Even with single-digit anomalies for highs over most areas, the combined heat and humidity could support heat index values into the low 100s for many in at least the southern half of the central/eastern U.S.. Highest heat index values are likely over the lower half of the Mississippi Valley and vicinity along with areas along and inland from the Gulf Coast. The cold front reaching the East toward the end of the week will likely bring high temperatures down to within a few degrees on either side of normal over the central U.S. by Friday and farther east by the weekend. Santorelli/Rausch 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