Extended Forecast Discussion NWS Weather Prediction Center College Park MD 205 AM EDT Thu Sep 28 2023 Valid 12Z Sun Oct 01 2023 - 12Z Thu Oct 05 2023 ...Overview... A closed low/amplified trough in the West will slowly move eastward and weaken with time. Higher elevation snows early should taper off as the system weakens. Ridging expands across the Plains/East bringing above normal temperatures, especially for the northern tier of the regions. Under the base of the Eastern Ridge, an upper low forms off the Southeast coast, which pushes a front through the Florida Straits and ends the heavy rain threat across the peninsula early next week. ...Guidance Evaluation/Predictability Assessment... The guidance continues to show very good overall agreement through the medium-range, though detail issues remain. To deal with those issues, the WPC medium-range 500 hPa heights, fronts, winds, and pressures were derived from an even split of the 27/12z ECMWF, 27/12z Canadian, 27/12z UKMET, and 27/18z GFS from Sunday into Sunday into Tuesday before swapping out the UKMET for increasing amounts of the 12z NAEFS/ECMWF ensemble means mid next week. This blend kept reasonable continuity. The days 4-7 QPF was a blend of the 27/18z GFS, 27/12z ECMWF, 27/12z Canadian, and 01z NBM. The remainder of the grids were more heavily based on the 01z NBM. ...Weather/Hazards Highlights... A front drops through the Florida Straits as a frontal wave forms to their east, progressing towards Bermuda before the low stalls/tries to wrap back towards a mid-level closed low which forms under the base of a strong mid-level height anomaly. For Day 4/Sunday 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 a Marginal Risk areas is in place. 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 is now in place for portions of the southern High Plains for day 5/Monday into Tuesday morning as there is the signal for convection-induced heavy rain potential across westernmost TX and near a front/lee trough/dry line in southeast NM. Snow at higher elevations of the West begins to fade as the upper trough weakens and edges eastwards with time. While portions of Montana appear wet early on, the QPF signal does not appear to be significant enough for a Marginal Risk on days 4-5/Sunday into Tuesday morning. Troughing across the West will initially lead to coolness -- initially 10-20F below average for high temperatures across the Southwest (near 80F) and Great Basin (near 60F) before moderating -- while ridging to the east will allow portions of the Northern Plains, Great Lakes, and Northeast to be 10-15F above average during the afternoon (highs in the 70s and 80s). The drought-stricken western and central Gulf will be hottest, with highs in the low to mid 90s. 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