Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Sep 15 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. There has been a signal for an upper level low to become closed over the northern High Plains however there continues to be considerable uncertainty both on timing and strength. The 06Z GFS was a bit faster than the 00Z run but overall had similar tracks to the 00Z ECWMF and the CMC. The preferred model blend was heavier with the 00Z ECWMF and 00/06Z GFS but also included some of the 00Z CMC initially, with increasing weighting of the 00Z ECWMF ensemble and GEFS mean. This help preserve run to run continuity. ...Weather/Hazard Highlights... A tropical airmass will linger over the Gulf, Gulf states and the Southeast after Nicholas fizzles out, allowing for additional light to moderate rainfall to parts of the Gulf Coast, in regions which have already received heavy rainfall. 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. Widespread rain, likely heavy at times, will spread across the Northwest with some snow possible in the higher elevations. This moisture and widespread precipitation is expected to shift to the Intermountain West and far western portions of the High Plains as the trough amplifies and shifts inland early next week. The amplified troughing over the West will allow for cooler temperatures over the Northwest. The afternoon daily maxes are forecast to 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. There will also be a warming trend in temperatures 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. Campbell 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