Extended Forecast Discussion NWS Weather Prediction Center College Park MD 347 PM EDT Thu Sep 28 2023 Valid 12Z Sun Oct 01 2023 - 12Z Thu Oct 05 2023 ...Pattern Overview and Guidance Evaluation/Predictability Assessment... Model and ensemble solutions seem reasonably well clustered overall Sunday into Tuesday, but diverge more significantly into next midweek. A closed low/amplified upper trough over the West/Southwest is expected to slowly eject northeastward to the Rockies and gradually weaken with time, pending later forecast period influx of uncertain northern steam energies over the Plains midweek. Higher elevation snows over the Intermountain West and Rockies into early next week should taper off as the system weakens. Height falls that materialize over the Plains through next midweek may combine with surface system instability/moisture focus to support an emerging convetcive rainfall pattern, but forecast spread and run to run guidance continuity has been less than stellar at these longer time frames. Downstream upper ridge building shifting from the central to eastern U.S. will bring above normal temperatures, especially for the northern tier of the regions. However 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 weekend heavy rain threat across the Florida peninsula by early next week. Accordingly, the WPC medium range product suite was primarily derived from a composite of the 12 UTC GFS/ECMWF/UKMET/Canadian models Sunday into Tuesday in a pattern with good predictability before switching onward to a blend of best remaining compatible guidance from the GFS/ECMWF and GEFS/ECMWF ensemble means as uncertainty increases. WPC product continuity was well maintained with this plan. 12 UTC guidance remains in line. ...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. In particular, enhanced low level onshore wind/moisture influx in this pattern flow may favor heavy rains for the West Palm Beach area as winds are directed westward from the northern Bahamas. Meanwhile, 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. 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. Schichtel/Roth Hazards: - Heavy rain across portions of Florida, Sun, Oct 1. - Heavy rain across portions of the Central Plains, Tue, Oct 3. 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