Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Tue Sep 13 2022 Valid 12Z Fri Sep 16 2022 - 12Z Tue Sep 20 2022 ...Overview... The medium range period begins Friday with broad upper troughing with embedded shortwaves across much of the western to north-central U.S., while warm ridging is in place over the east-central CONUS. Troughing is forecast to sharpen up and deepen over the West by the weekend into early next week, though with some lingering model differences in the placement/timing. This will lead to cooler temperatures and some precipitation coming into the West. Farther east, rounds of rain are possible along the northern tier with progressive shortwaves and frontal boundaries passing through, while a meandering front should continue to allow for scattered thunderstorms across Florida and the Gulf Coast. Temperatures should be above normal underneath ridging in the central U.S. and eventually into the East. ...Guidance Evaluation/Predictability Assessment... Model guidance has fortunately come to much better consensus compared to a day or so ago with the overall pattern and particularly the evolution of the Pacific to West Coast troughing. However some model differences remain with timing of the trough's movement eastward as well as the potential for a closed low within the trough. Given that some related energy stems upstream from the uncertain Alaska/northern Pacific region and some from the recurvature of Typhoon Merbok in the central Pacific, the details may take more time to resolve. For the 12/18Z model cycle, the ECMWF remained a somewhat faster solution with less potential for a rounded and persistent closed low compared to other guidance. GFS runs were farthest west and slower as they closed off a low, while the UKMET and CMC were somewhat in between with the position. By Monday and especially Tuesday, GFS runs kept an upper low closed off in the Pacific with even a bit of a retrograde, and this solution was not favored as it did not have much ensemble member support. Any significant model differences downstream were mainly related to where ridging sets up dependent on how far east the trough pushes inland. Thus the WPC forecast used a blend of the deterministic 12Z ECMWF, CMC, and UKMET and 18Z GFS early on, lessening the weightings especially of the faster EC and slower GFS in favor of the GEFS and EC ensemble means, which showed fairly similar solutions to each other, and agreed fairly well with continuity. The incoming 00Z GFS especially and to some extent the 00Z CMC have trended a little faster toward the 12Z and now 00Z ECMWF solutions. ...Weather/Hazards Highlights... Upper troughing approaching and frontal systems making their way across the West late this week into early next week could lead to some light to moderate precipitation across parts of the Pacific Northwest/northern California and into parts of the Great Basin and Northern Rockies. With the cold troughing aloft coming in early next week, this could produce snow in the highest elevations of the Northwest. Farther south, moisture from a tropical system that may form and track near Baja California could lead to showers and thunderstorms for the Four Corners states by around Monday. Into the Northern Plains to Midwest, a couple of fronts and some shortwave energy could focus moisture and lead to rounds of rainfall there during the medium range period, perhaps reaching the Northeast early next week. Some heavy rain potential may be possible occasionally, but there is low confidence for particular areas or times when this could occur. Meanwhile, a quasi-stationary front lingering for several days across the Florida peninsula and back across the Gulf of Mexico and western Atlantic should support rounds of showers and thunderstorms. The heaviest rain continues to be forecast offshore of the Georgia/Carolina coasts and the Gulf Coast, but some rainfall could sneak onshore from southern Texas to the North Carolina Outer Banks, but with the best chance for notable rain onshore across Florida. As confidence in the pattern of troughing in the West and ridging developing farther east grows, temperature forecasts for a cooler West and warmer central/eastern U.S. are becoming more clear. Cooler than average temperatures particularly in terms of highs are expected to overtake the western U.S. by this weekend into early next week. Highs of 10-15F below average may be common in the West Coast states into the Intermountain West, with even cooler readings currently forecast centered over northern California on Sunday. Meanwhile temperatures are forecast to warm to 10-15F above average (with actual temperatures well into the 90s) centered in the Central Plains/Middle Mississippi Valley for this weekend into early next week, and warmer than normal conditions could stretch into the Midwest/Great Lakes to eastern U.S. with time. Tate 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml