Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 AM EDT Thu Sep 26 2024 Valid 12Z Sun Sep 29 2024 - 12Z Thu Oct 03 2024 ...Pattern Overview and Guidance/Predictability Assessment... Latest guidance is in broad agreement this weekend. A blocky split flow pattern over much of the country should be highlighted aloft by a warming Rockies/Southwest ridge, a low/trough out from the Canadian Maritimes, and a cutoff low meanering out from the Mid- South/Ohio Valley states. Moist Post Tropical Cyclone Helene weakens into Sunday/Monday as per NHC near Tennessee/Kentucky as entrained into the cutoff low. Slow feature translation should support a pesky rainfall/runoff threat in proximity of the low and eastward within a lingering moisture feed into soil drenched south-central Appalachians terrain. Forecast spread increased significantly next week within a more progressive and energetic northern stream flow from the northeast Pacific through the Northwest U.S. and downstream. Guidance varies with overall timing and in particular with the extent of potential southeastward digging/amplitude of upper trough energies downstream across the central and eastern U.S.. The pattern was increasingly out of sync and uncertain next week, but newer 00 UTC model guidance has converged to a degree with closer to middle of the road amplitude in line with an ensemble composite. Given lingering uncertainties, prefer an ensemble means and NBM based forecast approach through medium range time scales that maintains reasonably good WPC continuity and fits well with latest NHC Helene guidance. ...Weather/Hazards Highlights... Moisture from what will be the remnants of Helene, in combination with a lingering upper low, will result in continued showers and some thunderstorms from Kentucky/Tennessee eastward to the Mid- Atlantic region into Sunday/Monday, albeit much lighter in widespread intensity compared to the deluge of rainfall expected prior to this with Helene. After evaluation of recent 12/18/00 UTC guidance, depicted a WPC Excessive Rainfall Outlook Marginal Risk area there for Day 4/Sunday and Day 5/Monday. Lingering tropical moisture may fuel scattered instances of heavy rainfall with local runoff concerns. Even though heavy rainfall during this period should not be as widespread, the extremely saturated soils will be an aggravating factor in terms of the flooding potential. Temperatures are expected to remain warm across much of the Western U.S. and into the north-central U.S. for the upcoming weekend, with highs running up to 15-25 degrees above average for portions of the Dakotas and into eastern Wyoming. Excessive heat will once again make weather headlines for the lower elevations of the Desert Southwest with highs approaching 110 degrees for the normally hotter locations of southeast California and southern Arizona. Most of the Eastern U.S. should be near average for highs, and above average for overnight lows owing greater cloud cover and humidity. 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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