Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM 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 into the weekend... ...Overview... Most guidance maintains good agreement and continuity for the overall pattern through the period. Expect upper level ridges from the eastern Pacific and southern Rockies/High Plains to consolidate over the Great Basin and central Rockies by Friday with a strong ridge remaining in place through the weekend. Downstream the axis of a mean upper trough (with embedded shortwaves) will move from the Upper Midwest into the Great Lakes and Northeast. The strong ridging over the West should support a heat wave that affects 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 primary guidance issues continue to involve details around the periphery of the western U.S. ridge and shortwaves embedded within the mean trough that settles into the Great Lakes/Northeast. Over the West, 00Z/06Z GFS runs were generally stronger than other guidance with shortwave energy on the south side of the upper high while the new 12Z run has adjusted more to the model/mean average. Also latest GFS runs have been on the aggressive side of operational guidance in how much Pacific shortwave energy reaches the northwestern U.S. and western Canada by next Sunday. Even among remaining guidance there is some spread and variability for how quickly heights aloft may decline. The GFS scenario is within the full ensemble spread albeit in the minority while the new 12Z ECMWF erodes the ridge a bit more than the prior run. An intermediate model/mean solution looks reasonable at this time. A blend approach maintains stability for the eastern trough aloft and leading surface front details with embedded shortwave specifics having lower predictability. The updated forecast started with an operational model composite early and then trended toward a half model (more 00Z ECMWF than 00Z CMC) and half mean (06Z GEFS/00Z ECens) blend by day 7 Sunday. ...Weather Highlights/Hazards... The Northwest heat wave should produce several days with max/min temperatures of 10-20F above normal over Washington/Oregon and some surrounding areas from Wednesday through the weekend. Anomalies for daytime highs should be the greatest around Thursday-Friday when a few locations could reach 20-25F above normal. A decent number of locations across the Northwest and into northern California/Nevada may see readings approach if not reach/exceed daily records for highs and warm lows over multiple days. By Sunday the highs may moderate somewhat but morning lows should remain quite warm. Coverage and character of any smoke will provide short-term wild cards for temperature forecasts. Currently expect this heat wave to be less extreme than the all-time record event in late June but it may still be the second most significant of the summer for the Portland-Seattle corridor. Shortwaves and moisture to the south of the consolidating upper ridge over the Great Basin/central Rockies will likely promote an increase of coverage and rainfall amounts in monsoon activity over the Four Corners states and parts of the Southwest. There is still some uncertainty in the relative strength of the shortwaves and ridge to the north so the precise coverage/intensity of rain will take additional time to resolve. High temperatures over the southern Rockies/Southwest should be within a few degrees on either side of normal Wednesday followed by the warmer areas trending to modestly below normal by late week/weekend. A weakening Plains into Great Lakes front and then a trailing front that reaches the East Coast and south-central U.S. by next weekend will focus areas of showers and thunderstorms over the eastern third to half of the lower 48 during the period. Some areas of locally moderate to heavy rain are likely but it may take until the short-range time frame to refine the most favored areas for highest totals. Current indications suggest some potential from the Upper Ohio Valley and portions of the Appalachians to areas along the East Coast. The southern Mid-Atlantic/Carolinas and areas to the west could see increasing potential for training/repeat activity later in the weekend as the front decelerates. Farther ahead of the fronts, daily scattered storm chances will also exist over the East and Deep South within a warm and moist environment. Florida should see rainfall increase Friday into the weekend due to a surface trough and some upper level energy moving into the area from the Bahamas/Cuba. This feature originates from a tropical wave initially near 50W longitude. Very warm to hot conditions should prevail to the east and south of the two cold fronts. The highest anomalies of plus 10-15F are most likely for morning lows from the Great Lakes into the Northeast Wednesday-Friday with a few daily records possible. Plus 5-10F anomalies will be more common for highs but there may be a few isolated higher readings over the central Plains around midweek and to the east/northeast mid-late week. The combined heat and humidity could support heat index values into at least the low 100s for many lower elevations across the southern half to two-thirds 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. This will suppress 100F+ heat indices more toward the Gulf Coast region by Sunday. 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