Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Fri Sep 27 2024 Valid 12Z Mon Sep 30 2024 - 12Z Fri Oct 04 2024 ...Overview... A broad upper trough swinging across the Plains early next week will finally dislodge the persistent eastern U.S. upper trough/low responsible for multiple days of rainfall enhanced by Helene's moisture. The new pattern will favor progressive mean flow across the northern tier, pushing along a couple frontal systems, and southern tier ridging (though with possible breaks in places). The Mid-Atlantic/Northeast will gradually trend drier in the later half of next week while precipitation chances trend up in Florida and along the Gulf Coast. Fronts associated with northeastern Pacific systems could bring light rain to the northern Pacific Northwest. Expect the most notable temperature anomalies next week to be on the warm side of normal over the southern half of the West on a consistent basis and more briefly over parts of the central U.S. ...Guidance/Predictability Assessment... Model guidance is in good agreement Monday through Wednesday, with an upper trough swinging across the Plains. This pattern will push a cold front across the Central U.S. and into the East while a wave evolves ahead of it along a front lingering near the East Coast. A storm tracking toward the Alaska Panhandle should bring another front into the Northwest late Tuesday or early Wednesday. Model solutions gradually diverge next Thursday and Friday as the models struggle with the evolution of the upper trough as it moves over the East and with an upper low/trough moving from the Gulf of Alaska down the Canadian West Coast. The CMC seemed to fall the furthest from the general model consensus/means, and is the only model taking the western low/trough southwest into the Pacific instead of down the Canadian Coast, which impacts the downstream evolution of the upper level pattern. The GFS and the ECMWF dynamical and AI models seem to fall more in line with the consensus with the low/trough entering the Northwest on Thursday, upper ridging over the Intermountain West, and weak troughing over the central/eastern U.S. Late in the period, guidance also differs regarding signals for tropical development in the northwestern Caribbean/Gulf of Mexico, with GFS/CMC runs showing greater potential than other guidance. Remaining solutions that have some indication of low pressure over the region suggest it may be weaker and over the southwestern Gulf by next Thursday/Friday. For the WPC afternoon forecast, a blend of the available deterministic models was used for Monday/Tuesday, with more weight placed on the GFS and ECMWF than the CMC and UKMET. Ensemble means from the GEFS and ECENS were added in increasing amounts for Wednesday through Friday while the CMC and UKMET were gradually phased out of the blend. ...Weather/Hazards Highlights... The lingering upper low over the East and accompanying moisture from Helene will continue to produce some showers and thunderstorms over portions of the central Appalachians and Mid-Atlantic into Monday, though much lighter in intensity compared to the ongoing/short-term rainfall. There is a Marginal Risk area depicted on the Day 4/Monday Excessive Rainfall Outlook (ERO) that includes portions of Pennsylvania, Maryland, West Virginia, Virginia, and DC. Heavy rainfall during this period should not be as widespread but wet conditions from prior days of rainfall may be an aggravating factor in terms of the flooding potential. By Day 5/Tuesday, the guidance develops a more spread for finer details at the surface and aloft, leading to a more diffuse signal for where any pockets of enhanced rainfall may exist. Therefore, the Day 5 ERO depicts no risk areas at this time. Beyond Tuesday, some rain may continue northeastward from the Appalachians/Mid- Atlantic but with low confidence in the coverage and amounts. Elsewhere, Florida and eventually parts of the Gulf Coast farther to the west may see some rainfall at times. Northern parts of the Pacific Northwest may see a couple episodes of light rain, one with a front reaching the region on Tuesday and another around Friday. Temperatures will remain quite warm to hot across the southern half of the West next week, especially Monday through Wednesday when high temperatures are forecast to be 10-15 degrees above average, with numerous daily records possible. More transient anomalous warmth will be likely over parts of the Central U.S. ahead of cold fronts, including areas from the Upper Midwest southwestward on Monday and in Plains on Wednesday (and lingering to some degree into Thursday over southern areas). The Northwest and northern High Plains will tend to be near to below normal. The East will tend to see highs vary somewhat but will generally be near normal. Rausch/Dolan 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