Extended Forecast Discussion NWS Weather Prediction Center College Park MD 239 AM EDT Wed Sep 27 2023 Valid 12Z Sat Sep 30 2023 - 12Z Wed Oct 04 2023 ...Overview... An amplified trough is forecast to develop over the western U.S. while increasing ridging shifts into the East. Convective rains are possible near a lingering front along southern New England coast Friday into Saturday but with a high degree of uncertainty. Meanwhile, portions of Minnesota could receive heavy rain ahead of a front where it has been wetter than average this past week. As the amplified trough is forecast to close off into a large upper low in the western U.S., higher elevation snows can be expected over portions of the northern Rockies, while moderate to possibly locally heavy rains are forecast for the lower-elevations from areas of the Great Basin to portions of Montana Sunday and Monday. ...Guidance Evaluation/Predictability Assessment... The guidance shows very good agreement. The 500 hPa heights, pressures, fronts, and winds were based on an even blend of the 12z Canadian, 12z ECMWF, 12z UKMET, and 18z GFS early before swapping out the UKMET for the 12z NAEFS and 12z ECMWF ensemble means. This blend kept good continuity, The days 4-7 QPF was a blend of the 18z GFS, 12z ECMWF, 12z Canadian, and 01z NBM. The remainder of the grids were primarily 01z NBM based, with some automatic adjustments made to adjust to the QPF. ...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 for the southern High Plains for day 4/Saturday into Sunday morning as the 18z/00z GFS have not given up on the heavy rain potential for convection to develop across westernmost TX and near a possible 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. Roth/Kong 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