Extended Forecast Discussion NWS Weather Prediction Center College Park MD 242 AM EDT Mon Aug 09 2021 Valid 12Z Thu Aug 12 2021 - 12Z Mon Aug 16 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. 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, while a tropical wave could bring enhanced precipitation to parts of Florida by next weekend. ...Guidance/Predictability Evaluation... Main issues to note are regarding details around individual shortwaves through the mean troughing over the Great Lakes and the Northeast. The GFS and CMC become much faster with the trough axis than a lot of the guidance next Sunday and beyond. The ECMWF is slower but also more in line with the ensemble means. Out west, there remains spread and variability for how quickly an incoming shortwave to the Northwest erodes the ridge late period, with again, the GFS on the fast side of the envelope. The WPC blend consisted of a mix of the deterministic runs for the first half of the period, leaning more heavily on the ECMWF and the ensemble means late period for the reasons outlined above. This approach also maintained good continuity with the previous shift. ...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 Thursday 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 and Monday the highs may moderate somewhat as the ridge gets eroded by incoming shortwave energy, but morning lows should remain quite warm. 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 details on exactly where remain very uncertain. 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 tropical wave/surface low and some upper level energy moving into the area from the Bahamas/Cuba. The National Hurricane Center is currently monitoring this system for potential tropical development in the next few days. 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 Thursday-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 Northeast later in the 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. 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