Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 PM EDT Sun Sep 7 2025 Valid 12Z Wed Sep 10 2025 - 12Z Sun Sep 14 2025 ...Overview... The overall weather pattern during the upcoming medium range period begins with amplified troughs on both the East and West Coasts with ridging over the Central U.S.. At least initially, these will be slow to move out but energy ejecting out of the West should help to finally dislodge the pattern some. In terms of sensible weather, wavy frontal systems will focus rain chances from the Northwest/West to the Rockies/Plains with gradual flow translation. Heavy rain potential remains present from Florida through the coastal Carolinas and possibly southeast New England as deepened moisture interacts with a wavy western Atlantic front. This activity will focus on the leading edge of a cool Canadian airmass insurgence for the central and eastern U.S.. ...Guidance/Predictability Assessment... The latest guidance seems generally well clustered the first half of the period on this overall pattern with the exception of the CMC. The CMC shows more interaction between the western U.S. trough energies and an additional shortwave dropping south from western Canada. This leads to the overall western U.S. pattern shifting eastward faster than the consensus. There is agreement that energy from the Western trough will eject eastward by the weekend, but for timing and amplitude, prefer the ECMWF and GFS coupled with the ensemble means. Another deep trough will reach the West Coast late weekend with some minor timing uncertainties. Elsewhere, the Eastern U.S. trough should generally hold through the period but by next weekend some more significant uncertainties arise attributed to energy dropping southward from eastern Canada. The ECMWF was the outlier here showing a deep closed low moving into the lower Great Lakes/New England region next Sunday. Generally favored a blend of the deterministic models Day 3 and 4, removing the CMC completely from the blend by Day 5. For days 6 and 7, increased weighting of the ensemble means to 70 percent to account for greater pattern and detail uncertainties. This approach still maintains good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... A persistent and wavy frontal system over the Gulf and Florida also extends just off/up the Eastern Seaboard. This provides a main focus for tropical moisture to pool, leading to daily enhanced and diurnally driven rainfall potential associated with scattered thunderstorms. A nearly stationary vortex over the eastern Gulf which may eventually merge with the front over the Gulf could help trigger and push some organized bands of thunderstorms also onshore into western Florida Peninsula into mid- late this week. Saturated grounds from pre- cursor rainfall warrant a Marginal Risk of Excessive Rainfall Outlook area to be kept across the Florida Peninsula for Wednesday and Thursday. Farther north, daily rainfall amounts from the coastal Southeast/Carolinas and up the coast offers uncertainty for how much convection occurs onshore/offshore on any given day. Small shifts in the wavy frontal boundary could produce significant differences in onshore rainfall amounts. The amplified and slowed pattern nature may lend favor to onshore flow trends and behavior. An upper trough/closed low working over the Northwest/West will support multiple days with potential rainfalls into mid-late week, shifting slowly with time from the Pacific Northwest/northern California to the north-central Great Basin/Intermountain West. This cool season type of rainfall should be marked by moderate rates that are generally not enough to cause flooding concerns. The lowered heights aloft will yield cooler temperatures supportive of some snow for the highest elevations. Monsoonal flow generally wanes into this week for the West/Southwest, but lingering moisture and instability fueling showers remain possible. Activity focus may increase through later week as some renewed lower latitude moisture channels between the transitioning western U.S. trough and the central U.S. ridge. Showers and storms will increasingly spread from the Rockies to the north-central Plains to monitor for possible runoff issues late week into next weekend given locally growing instability and boundary focus/height falls. Santorelli/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