Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Nov 13 2023 Valid 12Z Thu Nov 16 2023 - 12Z Mon Nov 20 2023 ...Pattern Overview and Guidance Evaluation/Predictability Assessment... It remains the case that the upper pattern across the Lower 48 will feature a slowly-moving upper low off the coast of California that gradually works inland next weekend, outpaced by northern stream upper ridging moving into southwestern Canada. In the east, a trough in the Southeast will move eastward with its surface reflection carrying a low pressure center across Florida. This system may merge/interact with a frontal boundary moving through the Great Lakes to form a deeper area of low pressure near or just offshore the Mid-Atlantic/Northeast next weekend. Much of the rest of the middle of the country will be dry with mild temperatures for mid-November. The WPC medium range product suite was primarily derived from now more broadly well clustered mass field guidance of the latest GFS/ECMWF/UKMET/Canadian and GEFS/ECMWF ensemble means along with the National Blend of Models and WPC continuity. Applied most blend weight to the models for better detail/trends Thursday into Saturday, before applying greater focus to the ensemble means as forecast spread slowly but steadily increases through longer time frames. ...Weather/Hazards Highlights... The recent slower and drier trend in the guidance over California has prompted removal of all Marginal Risk areas. Though rainfall is likely over much of the Sunshine State, the evolution of the system may translate the bulk of the heavier rain east of the Florida Peninsula by Thursday. However, kept the WPC Excessive Rainfall Outlook (ERO) Marginal Risk area over Atlantic coastal areas for the Day 4/Thursday ERO given easterly flow and a slow relaxation of well above normal precipitable water over the region until the system fully passes. For the West/Southwest, the slow approach of the closed southern stream upper low will only gradually bring in bouts of light to moderate/terrain enhanced rain and mountain snow that expand eastward in time. The delay in the height falls likely lessens the stronger moisture flux into the area as the system fills before moving ashore, but most areas will see at least measurable rainfall (especially coastal areas) outside some of the shadowed deserts. As the amplified upper system finally crosses the Divide, return flow out of the Gulf of Mexico will help increase moisture over the Plains then Mississippi Valley, with rain expanding in coverage late weekend into early next week. At the same time, another Pacific system, but in the northern stream, will progress inland from the Northwest to the north-central Rockies for the weekend along with a quick swath of rain and mountain snow before ejecting into the north-central states to lift limited moisture into the region. Meanwhile, amplified northern stream upper trough energy slated to sweep from the northern Plains Thursday through the Northeast this weekend along with an associated and wavy frontal system. Limited moisture and precipitation over the Midwest/Great Lakes may be enhanced Friday into Saturday from the Mid-Atlantic/Carolinas up through the Northeast in feed off the Atlantic. Model QPF varies from modest to heavy amounts, especially for New England as dependent on uncertain system details and strength/interaction with a lead coastal low set to lift up the East Coast. Schichtel 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw