Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Sat Aug 07 2021 Valid 12Z Tue Aug 10 2021 - 12Z Sat Aug 14 2021 ...Northwest heat wave likely to bring temperatures that challenge daily records from midweek onward... ...Overview... Most guidance expects an upper ridge tracking from the eastern Pacific into the Northwest U.S. and a separate northwestward-drifting southern High Plains ridge to consolidate into a strong ridge anchored over the Great Basin and central Rockies by next Friday-Saturday. 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. Moisture/impulses to the south of the consolidating ridge may lead to some expansion of monsoon activity. Farther east the axis of a mean trough consisting of multiple individual shortwaves should progress eastward from the northern Plains and ultimately settle over eastern Canada and the northeastern quadrant of the lower 48. Associated fronts and leading warm/moist flow will produce areas of rain and thunderstorms mainly over the eastern half of the country. ...Guidance/Predictability Evaluation... The 00Z/06Z models and means generally followed through on the recently improved agreement for evolution within the mean trough aloft reaching the eastern half of North America. A leading northern Plains shortwave should eject into Canada while upstream energy reinforces the upper trough, resulting in dissipation of the initial cold front followed by another front that pushes south/east from the northern tier during the latter half of the week. From the large scale perspective the primary difference that arises is toward Friday-Saturday when 00Z/06Z GFS runs shift the Northeast Pacific/Gulf of Alaska into western Canada upper pattern farther eastward than consensus. At least into early Saturday this does not appear to have much effect on the cold front reaching the eastern U.S. though. The 12Z GFS has made some favorable adjustments toward the majority. Also note that the 00Z/12Z CMC runs stray faster than the consensus with the eastern shortwave/front by late in the period. Guidance comparisons led to a starting blend consisting of the 00Z/06Z operational models early followed by a trend to an even model/mean mix by day 7 Saturday--a little more 00Z ECMWF than 00Z ECens and less 06Z GFS than 06Z GEFS. ...Weather Highlights/Hazards... For the Northwest heat wave, a few locations in Oregon could see highs reach 10F or more above normal already on Tuesday but greater coverage of plus 10-20F or so anomalies should prevail over Washington/Oregon and some surrounding areas from Wednesday into the weekend. A decent number of locations 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 increase rainfall over southern Florida next 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 Tuesday-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 next Saturday. Rausch/Santorelli 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