Extended Forecast Discussion NWS Weather Prediction Center College Park MD 223 PM EDT Tue Sep 14 2021 Valid 12Z Fri Sep 17 2021 - 12Z Tue Sep 21 2021 ...Overview and Guidance Evaluation/Preferences... The synoptic pattern will be amplifying during the extended period with troughing over the West and ridging over the East. As the week progress so will the trough, shifting from the western states into the central U.S. The flow in between will help move along frontal systems across the Plains and New England. There is an area of convection near the Bahamas in the short term that the models have organizing by or before the day 3 period and persists through day 7 as it tracks further out over the Atlantic Ocean. The National Hurricane Center is monitoring this disturbance as a possible tropical system so refer to NHC's Tropical Weather Outlooks for additional information. The intensity and placement of QPF will be dependent on the strength and path of this feature, however at this time the heaviest rainfall should remain offshore, perhaps well offshore. The clustering of the models is relatively good initially, but they continue to struggle with the overall evolution of a few key features that by day 5- particularly the timing of the trough axis and individual shortwave perturbations within the mean trough begin to arise as it enters the Western U.S. The ensemble means have a better handle on this especially for days 6 and 7 therefore were utilized heavily. WPC favored an increasing blend towards the ensemble means days 5-7 with some modest contributions from the ECMWF and GFS for added flow definition. This blend of guidance maintained continuity with the previous WPC forecast. ...Weather/Hazard Highlights... Tropical cyclone Nicholas will likely be recent memory by this start of this forecast but remnant tropical moisture may stick around over the Gulf states but with low confidence in the details. Back along the gulf coast, this moisture may also interact with a potential convergence boundary to produce additional mainly light to moderate rainfall across parts of southeast Texas and Louisiana. Localized instances of flooding may be possible, depending on resulting heavy rain impacts as Nicholas moves through that region during the short range. Fronts tracking through the northern tier of the country will bring areas of rain, possibly moderate to heavy across portions of the Upper Mississippi Valley and Great Lakes. With greater confidence, areas of moderate to locally heavy rainfall will likely reach the Pacific Northwest Friday into the weekend with some snow possible in the highest elevations of the Washington Cascades and into the Northern Rockies. The majority of rainfall with the possible system off the East Coast should remain offshore but areas from the Mid-Atlantic into New England should continue to monitor forecasts closely due to the western periphery of the moisture shield being very sensitive to exact track. The amplified troughing over the West will allow for cooler temperatures over the Northwest. Daily highs will average 10-15F below normal beyond Friday 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 Southern/Central Plains to the Northeast within a building ridge aloft. Warmest anomalies of possibly +10-20F should shift from the Northern Plains on Saturday into the Upper Midwest and Great Lakes Sunday-Monday. 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