Extended Forecast Discussion NWS Weather Prediction Center College Park MD 128 AM EDT Tue Sep 14 2021 Valid 12Z Fri Sep 17 2021 - 12Z Tue Sep 21 2021 ...Overview and Guidance Evaluation/Preferences... Models and means continue to show amplification of the large scale pattern from late this week to the start of next week, with amplified troughing building over the West and eventually shifting into the Central U.S. by next Tuesday while downstream, ridging builds across the Eastern U.S.. The flow in between will help move along frontal systems across the Plains and New England. Meanwhile guidance continues to signal potential for an area of low pressure to develop and track northward off the East Coast but continues to show the heaviest rainfall should remain offshore, perhaps well offshore. The guidance shows good agreement on this overall pattern for days 3-4 so a general model blend should serve as a good starting point. After that, uncertainty in timing of the trough axis and individual shortwave perturbations within the mean trough begin to arise as it enters the Western U.S. on Sunday and marches eastward to the Central/Northern Plains by Tuesday. The CMC is slow/west of the better consensus, while the GFS is a little bit faster (but still within the general spread). The guidance continues to jump around for timing run to run so an intermediate approach still seems best. Based on this assessment, 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 approach maintained good continuity with the previous WPC forecast for days 3-6. ...Weather/Hazard Highlights... Remnant moisture of whatever is left of tropical cyclone Nicholas (if anything) by the start of the medium range period may enhance rainfall over parts of the eastern U.S., 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. Northern tier fronts may focus areas of rain with locations over the Upper Mississippi Valley and Great Lakes tending to see the best possibility of some locally moderate to heavy activity. With greater confidence, expect areas of moderate to locally heavy rainfall should reach the Pacific Northwest Friday into the weekend with some snow possible in the highest elevations of the Washington Cascades. 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 will promote a cooling trend over the Northwest will bring highs down to 10-15F below normal from Friday onward and with gradual expansion into the northern Rockies as surface high pressure settles in behind the front. Meanwhile, downstream, expect a warming trending in temperatures 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. 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