Extended Forecast Discussion NWS Weather Prediction Center College Park MD 229 AM EDT Tue Aug 10 2021 Valid 12Z Fri Aug 13 2021 - 12Z Tue Aug 17 2021 ...Major Northwest heat wave to challenge daily high temperature records into the weekend... ...Potential Tropical Cyclone Six brings heavy rain threats to the Southeast and Mid-Atlantic this weekend... ...Overview... The guidance continues to show a strong upper level ridge building over the Western U.S. while deep troughing lifts from the Great Lakes into the Northeast through next weekend. The strong ridge over the West will support another dangerous heat wave with temperatures over some areas expected to reach or exceed daily records for both highs and especially warm lows. By early next week, the pattern may flip as amplified troughing/possible closed low enters the West Coast and ridging builds over the Eastern U.S.. Additionally, NHC continues to monitor Potential Tropical Cyclone Six which may approach Florida as a tropical storm this weekend bringing heavy rainfall to Florida and eventually parts of the Southeast. ...Guidance/Predictability Evaluation... There is fairly good agreement regarding trough details across the Great Lakes into the Northeast early period, though the 18z GFS remains on the faster side of continuity. Prefer a blend towards the ECMWF/UKMET/CMC which is also more in line with the ensemble means. Out west, the ridge should break down by Sunday as amplified troughing moves ashore. Guidance struggles late in the period with possible upper low formation across the Northwest but may be trending towards at least a more amplified trough. The GFS has been most consistent with showing some sort of upper feature the past couple of runs. The WPC blend trends towards the ensemble means days 6-7, with some inclusion of the ECMWF and GFS for added system definition. Guidance has also come into better agreement regarding Potential Tropical Cyclone Six and its eventual track towards the U.S.. Most models agree it may graze southern FL as it moves into the eastern Gulf, but struggle with timing (ECMWF fastest, CMC/GFS slower). Please refer to the guidance from NHC regarding the details and official forecast track of Potential Tropical Cyclone Six. ...Weather Highlights/Hazards... The heat wave across the Northwest will continue into Friday and Saturday with max/min temperatures of 10-20+ above normal likely to reach or exceed daily records both for high temperatures and warm overnight lows. Friday should be the warmest with temperatures slightly cooler (though still much above normal) for Saturday. This heat wave should be relatively short lived and less extreme compared to the all-time record event in late June, but still of major significance for the Portland-Seattle corridor. By early next week, temperatures should rebound below normal across the Northwest as upper level troughing enters the region. Elsewhere, as the upper ridge axis shifts east, monsoon activity over the Desert Southwest should increase again bringing a localized flash flood risk. A cold front pushing southeast from Great Lakes on Friday, should reach the East Coast and Southeast this weekend. This 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 exact amounts and locations are still very uncertain at this time and may take until the short range period to resolve. Farther south, daily scattered storm chances will also exist over the Deep South within a warm and moist environment. Florida should see rainfall increase Friday into the weekend due to Potential Tropical Cyclone Six (which may be a tropical storm by then). Regardless of development, heavy rainfall across Florida is likely, with tropical moisture tracking into the Southeast and southern/central Appalachians early next week. Much above normal temperatures are likely across the Northeast and Mid-Atlantic on Friday prior to the frontal passage, with hot weather also extending into the Southern states. Combined heat and humidity should support heat index values into at least the low 100s for many, on Friday, with the cold front likely to suppress the heat to the lower Mississippi Valley and areas along the central and western Gulf this coming weekend. Across the Northern and Central Plains, temperatures should be near or a few degrees on either side of normal through much of the extended range period. 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