Extended Forecast Discussion NWS Weather Prediction Center College Park MD 233 AM EDT Tue Sep 07 2021 Valid 12Z Fri Sep 10 2021 - 12Z Tue Sep 14 2021 ...Overview... The medium range period will feature a trend toward quasi-zonal flow as an upper high over the Four Corners region weakens into the southern Plains and Southeast. Troughing will reload over the Northeast as a couple systems traverse the U.S./Canadian border. The pattern remains rather dry with generally near to above normal temperatures. ...Guidance Evaluation and Preferences... Models and ensembles showed decent longwave agreement with much more spread in the shorter wavelength features. Perhaps the ECMWF has been more consistent than the GFS or Canadian/UKMET, but even it has wavered in how it handles the Pacific flow across the CONUS. Thus, continued to favor a blended solution of the recent deterministic guidance to start, with increasing weight to the ensemble means (GEFS and ECMWF EPS) by the end of the period as uncertainty continues to grow. Trend has been toward a deeper trough in the northern Rockies early next week, but the east-west (fast-slow) spread continues, with the GFS quickest and Canadian slowest. This left the ECMWF mostly in between nearer to the ensemble means. Per the latest NHC outlooks, a tropical system is likely to form in the East Pacific over the next day or so but is forecast to track offshore/west of Baja California. In the Gulf of Mexico, tropical development remains uncertain but by the medium range it could be on its way out into the Atlantic. Please see the NHC outlooks for more information. ...Weather/Hazard Highlights... Rainfall will focus along and ahead of a cold front (perhaps fused with some additional tropical moisture) into the Deep South and Florida to start the period. Daily showers/storms are likely for much of the FL peninsula to the south of the boundary as it washes out. Some moisture will lift northward on the backside of the upper high into the northern Rockies, with generally light amounts. With shortwaves and frontal systems traversing across the north-central to northeastern U.S., periods of rain showers can be expected there. By around next Monday, tropical moisture could spread some organized rain toward western parts of the Gulf Coast (including south Texas), but uncertainty remains high. Temperatures will be near to above normal over most areas, especially over the Front Range on Friday with highs 10-20 degrees above normal. This area of warmth will move into the southern/central Plains over the weekend and toward the Middle Mississippi and Ohio Valleys next week. Temperatures in the Great Lakes/Northeast will be near to a bit below normal via northwest flow behind the cold front Friday, moderating a bit over the weekend as the surface high settles over the region. The next cold front may bring in another shot of slightly cooler than normal temperatures across the northern tier next Mon-Tue. Fracasso 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