Extended Forecast Discussion NWS Weather Prediction Center College Park MD 503 PM EDT Wed Aug 11 2021 Valid 12Z Sat Aug 14 2021 - 12Z Wed Aug 18 2021 ...Major West/Northwest Heat Wave challenging daily temperature records into the weekend then moderates... ...Tropical Storm Fred Heavy Rain Threat from Florida this weekend to the Southeast/Appalachians/Mid-Atlantic early-mid next week... ...Pattern Overview... Models and ensembles still show a strong upper level ridge lingering over the Western U.S. on Saturday, with amplified troughing moving into the Northwest Sunday/Monday and beyond into the West 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, southern to central Appalachians and Mid-Atlantic early-mid next week as tropical moisture feeds inland and interacts with a stalled leading frontal zone draped to the north of Fred and terrain. ...Guidance/Predictability Evaluation... The NHC tracks Tropical Storm Fred to the Florida Keys by Saturday and then over the far eastern Gulf of Mexico this weekend before lifting inland into the Southeast and up the Appalachians early next week as a gradually weakening but very wet system both along/east of the track and in advance with deep moisture feed into a draped front. Fred offers a maritime and inland heavy rainfall/runoff threat. Please refer NHC guidance regarding the details and official forecast track. The WPC medium range product suite was primarily derived from a blend of best clustered guidance from the NHC, WPC continuity, the 00 UTC ECMWF/ECMWF ensemble mean and the 13 UTC National Blend of Models. Nudged blend weights to the ECMWF ensemble mean days 5-7 consistent with slowly growing forecast spread with Fred and also the extent of upper troughing to work into the Northwest/West next week. GEFS and ECMWF ensembles best support upper trough emergence over the Northwest next week in between the deeper GFS and less amplified ECMWF. ...Weather Highlights/Hazards... The heat wave that will focus across the Northwest will continue into Saturday with max/min temperatures of 10-15+ 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 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 into the Keys and far eastern Gulf of Mexico. Tropical moisture will also spread into the Southeast, Appalachians and Mid-Atlantic early-mid 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/Schichtel Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, the Central Appalachians, and the Tennessee Valley, Sun-Tue, Aug 15-Aug 17. - Heavy rain across portions of the Southeast, Sat-Sun, Aug 14-Aug 15. - Heavy rain across portions of the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Sat-Wed, Aug 14-Aug 18. - Heavy rain across portions of the Southwest, Sat-Tue, Aug 14-Aug 17. - Excessive heat across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat, Aug 14. - Excessive heat across portions of the Northern Plains, Sun, Aug 15. 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