Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1053 PM EDT Tue Sep 14 2021 Valid 12Z Sat Sep 18 2021 - 12Z Wed Sep 22 2021 ...Overview and Guidance Evaluation/Preferences... The synoptic pattern during the medium range period will be amplifying as ridging builds over the east and a trough digs into the West this weekend and shifts eastward into the central U.S. by early to middle of next week. By Tuesday or Wednesday of next week, the models have started showing the potential for an upper level low to close off over the northern High Plains, but show considerable uncertainty both on timing and strength. The 18z GFS is a little faster/farther north and the 12z CMC is still on the slow side (as it as been the past couple of days). At this time the 12z ECMWF lies somewhere in the middle. Even still, there's been enough run to run inconsistencies in all the models that it's hard to latch on to one deterministic model over another. Thus, WPC prefers a blend towards the ensemble means for days 6-7. Did maintain some modest contributions from the ECMWF/GFS even through the whole period just to add some definition to the washed out means. Overall, this also maintains good continuity with the previous WPC forecast. ...Weather/Hazard Highlights... Tropical cyclone Nicholas will likely be recent memory by the start of this forecast but some of the models indicate the potential for remnant tropical moisture to interact with a lingering convergence boundary bringing additional light to moderate rainfall to parts of the Gulf Coast, in regions which have already or may receive heavy rainfall as Nicholas moves through. The details on the specifics however remain very uncertain. To the north, rainfall will impact the Eastern U.S. this weekend ahead of a front progressing through the region, while showers and storms linger over the Southeast for a couple more days within an area of weakness in the upper levels. Out West, an atmospheric river set up should be in place as the period starts on Saturday bringing continued heavy rainfall to parts of the Pacific Northwest, with some snow possible in the higher elevations. As the trough amplifies and shifts inland early next week, so should the moisture and rainfall changes with the best chance for heavy precipitation along the favorable terrain. The amplified troughing over the West will allow for cooler temperatures over the Northwest. Daily highs will average 10-15F or more below normal beyond this weekend with gradual expansion into the Northern Rockies as surface high pressure settles in behind the front. A warming trend in temperatures is expected from the Plains to the Northeast within a building ridge aloft. Warmest anomalies of possibly +10-20F should shift from the Northern/Central Plains this weekend and into the Upper Midwest and Great Lakes Monday-Tuesday. Santorelli 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