Extended Forecast Discussion NWS Weather Prediction Center College Park MD 130 AM EDT Wed Aug 11 2021 Valid 12Z Sat Aug 14 2021 - 12Z Wed Aug 18 2021 ...Major West/Northwest heat wave to challenge daily high temperature records through Saturday and weakening by Sunday... ...Tropical Storm Fred to bring a heavy rain threat from Florida this weekend and the Southeast/Southern Mid-Atlantic early next week... ...Pattern Overview... Latest models and ensembles continue to show a strong upper level ridge continuing over the Western U.S. on Saturday, with amplified troughing moving into the region by Sunday and beyond which should suppress the ongoing heat wave. Meanwhile, an upper level trough will move through from the Great Lakes to the Northeast Saturday-Monday, with heights beginning to rise again across much of the East by later in the medium range period. Tropical Storm Fred will likely affect Florida this weekend bringing heavy rainfall, and then also into parts of the Southeast and southern Mid-Atlantic early next week as tropical moisture feeds inland and interacts with a stalled leading frontal zone. ...Guidance/Predictability Evaluation... Across the board, the guidance shows above average agreement through much of the period both with respect to incoming troughing across the West and the exiting trough in the Northeast. Out West, the 12z CMC gets a little fast late period so it was excluded from the blend. A total deterministic model blend worked well for days 3-4, but after that WPC prefers a blend of the more agreeable ECMWF/GFS and the ensemble means which helps mitigate the typical timing and detail differences of various smaller scale features. The models have also come into better agreement regarding the eventual track of Tropical Storm Fred. Most guidance tracks the system north just off the western Florida coast and eventually into the Southeast early next week. There remain some minor timing uncertainties, which should work themselves out as it gets closer in time. Please refer to NHC guidance for the latest details and official forecast track of Fred. ...Weather Highlights/Hazards... The heat wave that will focus across the Northwest will continue into 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. Sunday temperatures should still be above normal, but not nearly as extreme as previous days. By Monday, troughing looks to quickly move into the region so temperatures should actually rebound below normal across the Northwest. Some portions of the Northern Rockies could be 10-15 degrees below normal next Tuesday into Wednesday. Above to much above normal temperatures will shift eastward Sunday and Monday into parts of the Northern Plains as well. Tropical Storm Fred should be impacting much of the Florida peninsula by the start of the period on Saturday. Heavy rainfall and gusty winds are likely through the weekend as Fred tracks very near to the state. At this time, the heaviest rainfall looks to be along the Gulf Coast of Florida Saturday and Sunday, but heavier rain bands reaching the central and eastern parts of the state are likely, especially if the storm were to track a little bit farther east of its current projected path. Tropical moisture will also spread into the Southeast and southern Mid-Atlantic early next week, and well ahead of Fred. This moisture may interact with a leading stalled frontal boundary and favorable Appalachian terrain offering the potential for a pre event. 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 should reach the East Coast and Southeast this weekend focusing periods of showers and thunderstorms with areas of locally moderate to heavy rain likely. Exact amounts and locations remain somewhat uncertain at this time and may take until the short range period to resolve. Daily scattered storm chances will also exist back into the lower Mississippi Valley and Southern Plains within a warm and moist environment. In addition, combined heat and humidity should support heat index values into at least the low 100s for many across the lower Mississippi Valley and areas along the central and western Gulf this coming weekend. 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