Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 AM EDT Tue Sep 10 2019 Valid 12Z Fri Sep 13 2019 - 12Z Tue Sep 17 2019 ...Overview and Guidance Evaluation/Preferences... An upper-level ridge is forecast to remain entrenched from the lower Mississippi Valley to the Southeast through much of the medium range. Meanwhile, a stream of relatively vigorous upper shortwaves are forecast to traverse the CONUS northern tier. Farther south, an easterly wave is forecast to approach Florida, and move west into the Gulf of Mexico by the weekend, traversing the southern periphery of the expansive upper ridge. Model consensus was relatively good at the start of the medium range (Fri), and a blend of the 12Z ECMWF/CMC/18Z GFS served as a good starting point initially for the low pressure system crossing the Upper Midwest/Great Lakes, and a leading shortwave reaching the Pacific Northwest on day 3. After that, the GFS has continued to struggle with the evolution of additional shortwave energy, and the potential amplification of a large scale upper trough across the western U.S. days 5-7 (Sun-Tue). Prior runs of the GFS have continued to show much flatter flow off the Pacific Northwest into early next week, keeping any significant height falls confined relatively far north, while the ECMWF and CMC have consistently amplified a large upper trough (or perhaps even a closed low) across the northwestern U.S. The 00Z run of the GFS (which was not available for this forecast package), has finally come around to the ECMWF/CMC idea, now showing a broad upper trough amplifying across the western U.S. Sun-Tue. The forecast for days 4-7 (Sat-Tue) was based heavily on a blend of the 12Z ECMWF/CMC, with gradually increasing weight placed on ensemble means (ECENS/NAEFS) through time. This solution should bring a strong cold front across the Great Basin/northern Rockies Sun-Mon, emerging into the Northern Plains by Mon night-Tue. ...Weather Highlights/Threats... Scattered showers and thunderstorms will be possible from the Southern/Central Plains to the Upper Midwest, Ohio Valley, and Great Lakes Fri-Sat as the surface low pressure system moves from the north central U.S. into Canada, with the cold front trailing into the Mississippi/Ohio Valleys and the Southern Plains. Farther south, showers and thunderstorms are expected to become fairly widespread across the Gulf Coast states and the Southeast from late this week into early next week in association with an easterly wave propagating from the tropics across Florida and into the Gulf of Mexico. This system may produce localized areas of heavy rainfall. Meanwhile, the upper trough digging into the Northwest by Sun-Mon is expected to bring an increase in precipitation, especially to the mountain ranges of the Pacific Northwest and northern Rockies. Sufficient cold air may accompany this upper trough to bring the first round of snow to the higher peaks across the region. Finally, an increase in monsoonal moisture across the Southwest Sun-Mon is expected to bring scattered showers and thunderstorms to portions of the Four Corners region. High temperatures are forecast to be 5 to 15 deg F above average from the Ohio Valley to the Southeast and lower Mississippi Valley on Fri, with many areas reaching well into the 90s. Some slight moderation in temperatures is possible by the weekend, but highs are expected to remain 5 to 10 deg above average beneath the upper ridge. The upper trough reaching the western U.S. Sun-Mon will bring much cooler temperatures, with highs 5 to 15 deg below average expected to overspread the Northwest and northern Great Basin. Meanwhile, temperatures ahead of the trough will warm across the Plains and the Upper Midwest, where highs may reach 5 to 15 deg above average by early next week. Ryan 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