Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Wed Jul 29 2020 Valid 12Z Sat Aug 01 2020 - 12Z Wed Aug 05 2020 ...Heavy rainfall threat from the Ohio Valley to the Lower Great Lakes this weekend... ...Potential Tropical system threat for Florida/Southeast U.S. this weekend into early next week... ...Overview and Guidance/Predictability Assessment... The medium range period features renewed troughing across the Eastern half of the country as troughing is also established off the West Coast and surface waves make their way inland across the Northwest. In between, upper ridging should build across the Intermountain West/Rockies/High Plains as a subtropical high settles over the Southwest. Additionally, what is currently Potential Tropical Cyclone Nine may impact the Bahamas, Florida, and into the Southeast U.S. as a tropical storm by this weekend and into early next week. Overall, the model guidance continues to show better than average clustering with respect to the large scale pattern, and so a complete deterministic model blend worked well through day 4. Beyond day 5, the usual variance continues with smaller scale details, especially concerning energy entering the Northwest U.S., but incorporation of the ensemble means helped to mitigate these differences. The largest forecast concern for the medium range period continues to be with what is currently Potential Tropical Cyclone Nine, located just west of the Leeward Islands this morning. During the extended range, the latest forecast track from the NHC has this system developing into a tropical storm and tracking up the Florida Peninsula and into the Southeast U.S. Saturday-Wednesday. The models and ensembles however continue to show significant uncertainty in the details and exact track, which ultimately is highly influenced by both the eventual strength of the system and steering currents from the subtropical Atlantic high in place. Forecast confidence on this system is low at best, and so adjustments to the track are likely. Please refer to the products and advisories issued by the NHC for the latest information regarding PTC Nine. ...Weather/Hazard Highlights... Low pressure in the Tennessee Valley along a wavy frontal boundary looks to track northward into the Ohio Valley/Northeast this weekend. The boundary will provide focus for numerous showers and thunderstorms across the Mid-Atlantic back to the Midwest and northward into the Lower Great Lakes. Meanwhile, heavy rainfall and gusty winds associated with Potential Tropical Cyclone Nine (likely a Tropical Storm by the medium range) may begin to affect Florida and the Southeastern U.S. this weekend. As the system tracks northward, it should interact with the frontal boundary to the west which could ultimately spread heavy to possibly excessive rainfall northward into the Carolinas/Mid-Atlantic region early next week. Meanwhile, upper level high pressure anchored over the Southwest U.S. and building ridging northward should support moderately anomalous summer heat across the Intermountain West/Rockies/High Plains and the Southwest. Daytime highs in the 90s to near 100 degrees are possible (110 to 120 across the desert Southwest) Saturday and Sunday which are roughly 5 to 10 degrees above average. From the Plains to the Midwest, daytime highs should hover around 5 to 10 degrees below normal through the entire period as upper level troughing becomes established over the region. Santorelli Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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