Extended Forecast Discussion NWS Weather Prediction Center College Park MD 227 PM EDT Mon Aug 09 2021 Valid 12Z Thu Aug 12 2021 - 12Z Mon Aug 16 2021 ...Major Northwest heat wave to challenge daily high temperature records into the weekend... ...Tropical moisture brings a heavy rain threats to the Southeast and Mid-Atlantic this weekend... ...Overview... Deterministic guidance maintains good agreement and continuity for the overall pattern of a rather amplified ridge drifting east across the West Coast and a deep low over lifting north from Hudson Bay Thursday through Saturday. There are then some timing and strength differences for the next through entering the Pacific Northwest/BC on Sunday into Monday. A tropical wave approaches Florida or the eastern Gulf of Mexico this weekend with preceding tropical moisture reaching into the Mid-Atlantic. The strong ridging over the West will support an extreme heat wave that affects the Northwest and parts of the Great Basin/California into or through the weekend with temperatures over some areas expected to reach or exceed daily records for both highs and especially 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, particularly ahead of an approaching cold front and any tropical low center that develops. ...Guidance/Predictability Evaluation... Acceptable agreement with the 00Z ECMWF/CMC/UKMET and 06Z GFS was sufficient to allow a simple blend of these through Day 5. The GFS continues to be much more amplified with the next wave into BC Saturday into Sunday which draws a low off the Pacific Northwest inland while the ECMWF/CMC leave a low offshore. Based on this difference and related downstream differences, went heavier toward a GEFS/ECENS mean blend for Days 6 and 7. Regarding a tropical wave approaching the CONUS from the Antilles this weekend, there is decent agreement to having a cyclone develop and track toward FL or the eastern Gulf of Mexico. ...Weather Highlights/Hazards... The Northwest heat wave should produce several days with max/min temperatures of 10-20F above normal across the Pacific Northwest through Saturday night. Anomalies for daytime highs should be the greatest around Thursday/Friday when a few locations, perhaps centered over Portland, OR could reach 20-25F above normal. Dozens of locations across the Northwest and into northern CA/NV should see readings reach/exceed daily records for high max and mins. 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 still be of major significance for the Portland-Seattle corridor. As the upper ridge axis shifts east, monsoon activity over the Desert Southwest should increase again which brings a localized flash flood risk. The current guidance consensus indicates Arizona as the main threat area. A cold front pushes southeast from the northern Plains/Great Lakes Thursday night, reaching the East Coast and Southeast this 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, particularly Saturday as tropical moisture meets the front from the central Appalachians to the TN Valley and then east to the Mid-Atlantic and Southeast on Sunday. Farther ahead of the front, daily scattered storm chances will also exist over the East and Deep South within a warm and moist environment. Florida sees 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 continues to monitor this system for further tropical development in the next few days. Very warm to hot conditions prevail to the east and south of the cold front through Friday. The highest anomalies of plus 10-15F are most likely for morning lows from the Great Lakes into the Northeast Thursday night-Friday night with several more daily records possible Friday morning. The combined heat and humidity should support heat index values into at least the low 100s for many lower elevations on Friday from the Mid-Atlantic to eastern TX. High heat index values are suppressed by the cold front Saturday and Sunday to over the lower Mississippi Valley along with areas along and inland from the Gulf Coast. Jackson 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