Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Tue Aug 10 2021 Valid 12Z Fri Aug 13 2021 - 12Z Tue Aug 17 2021 ...Major West/Northwest Heat Wave to Challenge Daily High Temperature Records into the Weekend and also Late Week over the East... ...Potential Tropical Cyclone Six Heavy Rain Threat from Florida this weekend to the Southeast/Southern Mid-Atlantic early next week... ...Pattern Overview... Latest models and ensembles continue 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 the 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 enters the West Coast and ridging builds over the East. 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 parts of the Southeast and southern Mid-Atlantic into early next week along the slower than average track and as tropical moisture feeds inland well in advance into a stalling lead frontal zone and southern/central Appalachian topography. ...Guidance/Predictability Evaluation... Guidance remains in decent agreement regarding Potential Tropical Cyclone Six. Most guidance tracks the system near FL by this weekend as it moves into the eastern Gulf of Mexico before lifting inland into the Southeast early next week. Six offers a maritime and inland heavy rainfall/runoff threat. Please refer NHC guidance regarding the details and official forecast track of Potential Tropical Cyclone Six. Overall, The WPC medium range product suite was primarily derived from a blend of generally best clustered guidance from the latest GFS/GEFS mean and ECMWF/ECMWF ensemble mean along with the 13 UTC National Blend of Models. Nudged blend weights from the models increasingly toward the ensembles gradually over time consistent with slowly growing forecast spread in a pattern with near average predictability. ...Weather Highlights/Hazards... The heat wave that will focus 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. 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 western upper ridge axis shifts east with advent of upper troughing over the Northwest, monsoon activity over the Desert Southwest should increase and spread northward, 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 as tropical moisture spreads inland well in advance of Six offering the potential for a pre event. 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/Schichtel 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