Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1140 PM EDT Sun Sep 12 2021 Valid 12Z Thu Sep 16 2021 - 12Z Mon Sep 20 2021 ...Heavy Rain From Tropical Storm Nicholas May Continue Into Thursday... ...Overview and Guidance Evaluation/Preferences... Weak flow around Nicholas may allow heavy rain from the system/its remnants to continue over portions of the western Gulf Coast into Thursday but the system is expected to be absorbed/dissipated by late in the week. Follow the National Hurricane Center advisory products for the latest information on Nicholas. General agreement among 12Z global deterministic/18Z GFS of a deep central Canadian low shifting east with the next low/trough approaching the Pacific Northwest Saturday. The 12Z ECMWF diverges from consensus and its ensemble mean with a much less amplified wave than the GFS/CMC and the ensemble means by Day 5. There is decent ensemble mean agreement particularly with the 18Z GEFS and 12Z ECENS means Days 5-7 with the deepening mean trough aloft along the West Coast late this week into the weekend while western Atlantic/eastern U.S. ridging (or at least well above normal heights) deflects the westerlies across the northern tier U.S. and southern/eastern Canada. Another feature to note is a wave/weak area of low pressure possibly forming north of the Bahamas by Thursday and track northward, off the East Coast. The 18Z GFS remains the strongest with possible tropical development of this system with great divergence in overall guidance. A fair degree of uncertainty persists for specifics of flow aloft reaching the East Coast from late week into the weekend but the majority scenario has enough westerly flow to push the system and its rainfall away from most if not all of the Northeast. This system requires close monitoring as minor changes could have significant implications for any rain that may fall across an already saturated coastal Mid-Atlantic/Northeast. ...Weather/Hazard Highlights... Nicholas could maintain its definition long enough to support a heavy rain threat through mid to late week, though there is uncertainty in the eventual track of the system, how long it will maintain tropical characteristics and heavy rainfall threat before becoming absorbed into the weak flow and dissipate. Continue to monitor the latest NHC advisory products. Remnants of Nicholas may allow some enhanced rainfall over the eastern US depending on the track of the remnants and the associated plume of tropical moisture. Mostly lighter rainfall looks to accompany frontal passages through the central parts of the CONUS through this weekend. With the next frontal progression, a slower and more wavy front could produce heavier rainfall over the Upper Mississippi Valley/Great Lakes while rainfall intensity would be less with a faster front. Meanwhile moderate to locally heavy rainfall looks to reach the Pacific Northwest Friday into the weekend with some snow possible in the highest elevations of the Washington Cascades. Evolution of the large scale pattern will favor cooler temperatures settling over the West Coast states, with some highs 10-15F below normal over the Northwest by the end of the week/next weekend, along with steadily increasing coverage of plus 10F and greater anomalies for max/min readings over the Plains and east-central U.S. Locations to the south of an initial northeastern U.S. front will also see well above normal temperatures from midweek onward. Any clouds/moisture with the system forecast to track over the western Atlantic will be a wild card with respect to high temperatures near the East Coast. WPC 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