Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Sep 27 2023 Valid 12Z Sat Sep 30 2023 - 12Z Wed Oct 04 2023 ...Overview... An amplified trough is forecast to close off into a large upper low over the western U.S. while an expanding ridge over the mid-section of the country will tend to nudge the East Coast trough farther offshore. The threat of enhanced rainfall should subside over southern New England on Saturday as the western U.S. trough amplifies and spreads areas of moderate to possibly locally heavy rain from the Great Basin on Saturday to northern Rockies and northern High Plains Sunday to early Monday. High-elevation snows can be expected over portions of these same areas on the backside of this system. Meanwhile, convective rains are forecast on Saturday over the southern High Plains, but they are expected to linger into early next week and slide slowly southward across the Florida Peninsula. ...Guidance Evaluation/Predictability Assessment... The guidance continues to show very good overall agreement through the medium-range. The deterministic guidance shows higher uncertainty regarding the placement of the moderate precipitation across the western U.S. this weekend, with the CMC being the fastest while the GFS and ECMWF solutions are comparable. The ensemble means show a closer agreement on the precipitation distribution though. Meanwhile, the ECMWF continues to offer the most amplified solution regarding the low pressure system just off the Mid-Atlantic coast on Saturday, which would keep the heavy rain and gusty wind threat for southern New England as the medium-range period begins on Saturday. The 12Z CMC also indicates a stronger low that edges closer to the southern New England coast as well. Ensemble means agree quite well that the large cutoff low over the western U.S. will deamplify into a positively-tilted trough early next week and slowly slide eastward into the Rockies by midweek next week. Much of the uncertainty is seen over the southwestern U.S. regarding whether and where a closed low might form near the base of the deamplifying trough. The WPC medium-range forecast package was based on a 40% blend of the 00Z ECMWF/00Z EC mean, with 40% from the 00Z GFS/06Z GFS/GEFS, and 20% from the 00Z CMC/CMC mean. This blend kept good continuity, The days 4-7 QPF was a blend of the GFS, ECMWF, Canadian, bias-corrected QPF and 14z NBM. ...Weather/Hazards Highlights... A front drops through the Florida Straits as a frontal wave forms to their east, progressing towards Bermuda before stalling/wrapping back towards a mid-level closed low which forms under the base of a strong mid-level height anomaly. For Day 4-5/Saturday into Monday morning, the signal for heavy rainfall across portions of the FL peninsula continues due to instability from the very warm subtropical north Atlantic and Gulf of Mexico, precipitable water values over 2.25", the 850 hPa slice of the front shifting slowly southward across the FL Peninsula while helping to focus activity, and enhanced east-northeast flow just behind it advecting some cooler air aloft all potentially leading to efficient showers and thunderstorms, so Marginal Risk areas are in place both days. The risk area covers regions where rainfall this past week has been a bit above average, leaving the areas more sensitive than usual. A Marginal Risk area remains in place for the southern High Plains for day 4/Saturday into Sunday morning as model agreement remains good for convection-induced heavy rain potential across westernmost TX and near an amplifying lee trough in NM. A digging western trough will close into a cut off low this period which will focus moderate precip over the Intermountain West through northern Rockies, and northern High Plains. Snow at higher elevation becomes increasingly favored with time across the West as the upper level low develops and it cools off aloft. Portions of central Montana appear to have some instability available to their east and enough moisture to wonder about excessive rainfall, the QPF signal is not strong enough for a Marginal Risk at this time for either days 4 or 5 (Saturday into Monday morning). Increased ridging over the center of the country and troughing across the West, will slowly shift temperatures of 10 to 15 degrees above normal east across the Central Plains through the Great Lakes with temperatures below normal expanding from the Northwest to across the West. Below normal temperatures over the Mid-Atlantic States can be expected into Friday before moderating on approach of the ridge axis. Kong/Roth 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