Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Thu Sep 26 2024 Valid 12Z Sun Sep 29 2024 - 12Z Thu Oct 03 2024 19Z Update: The 12Z model guidance suite maintains good overall synoptic scale agreement across the continental U.S. on Sunday and into Monday. The guidance still shows the upper trough arriving over the Great Lakes by Tuesday, with the CMC developing a stronger trough/closed low over southern Ontario by Wednesday compared to the other guidance. That trends also continues going into Thursday with the CMC indicating a stronger low solution. Timing and evolution becomes less clear with the next trough beginning to develop over the northwestern U.S. late in the forecast period midweek, so the ensemble means were gradually increased to about 40% by next Wednesday/Thursday, while still maintaining good overall continuity from the previous forecast. The previous forecast discussion is appended below for reference. /Hamrick ------------------- ...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 meandering 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 central Kentucky/Tennessee eastward to the central Appalachians and Mid-Atlantic region into Monday, albeit much lighter in intensity compared to ongoing current rainfall. The Marginal Risk area for the Day 4 period was expanded some to include more of Virginia, Maryland, and southern Pennsylvania given a stronger model QPF signal. There was also an extension northward into central PA for Day 5 with the existing Marginal Risk area, while maintaining the outlook area over the central Appalachians as the remnant low is slow to move out of the region. 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 through early in the week. Hamrick/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