Extended Forecast Discussion NWS Weather Prediction Center College Park MD 435 PM EDT Thu Sep 14 2023 Valid 12Z Sun Sep 17 2023 - 12Z Thu Sep 21 2023 ...Overview... An upper trough amplifying over the Midwest Sunday in the wake of Lee will reach the Eastern Seaboard Monday before lifting over the Northeast by Tuesday. The front associated with this trough should produce areas of rain with varying intensity over the Southeast and Mid-Atlantic Sunday with lake effect rain off the eastern Great Lakes Sunday into Monday. A persistent wavy front should maintain a wet pattern over the Florida Peninsula through next week. Over the West, guidance continues to advertise a significant pattern change after initial ridging weakens and progresses eastward, with a trough and leading cold front bringing some moisture into the Pacific Northwest and northern Rockies. Above normal temperatures over the interior Northwest and northern High Plains on Sunday will shift eastward across the northern tier while most of the West sees a pronounced cooling trend. ...Guidance Evaluation/Predictability Assessment... A fairly quiet weather pattern for the CONUS for this time of year develops this weekend with a trough over the East in the wake of Lee, a broad ridge drifting east over the west-central portions, then renewed troughing over the Northwest around midweek. There was decent agreement among 00Z/06Z guidance allowing the forecast to rely on deterministic inputs from the 00Z ECMWF/CMC/UKMET and 06Z GFS through Day 5, then mixing in some 00Z ECENS/06Z GEFS means for Days 6/7. The main forecast differences are with the placement and depth of the trough building over the Northwest to the Intermountain West Tuesday through Thursday. The GFS is farther east, bringing more precip to the northern Rockies while the ECMWF is a bit farther west (and the CMC even farther west and more closed/ off). However, the differences were not great enough to eliminate any from the forecast blend. ...Weather/Hazards Highlights... There is a less than 5% risk for areas of organized flash flood in the Days 4-5 Excessive Rainfall Outlooks covering Sunday through Monday night. A series of fronts stalling over the Florida Peninsula will maintain a fairly wet pattern. However moisture anomalies are only moderately above normal, flash flood guidance values are high, and models do not show much clustering for any relatively heavier rain that may fall. Shorter term guidance may ultimately help to refine any areas that could at least merit a Marginal Risk. On Sunday, there will be some lingering moisture and instability near the Rio Grande from Texas into New Mexico, but organized heavy rain is not expected. A cold front pushing through the Appalachians/Southeast Sunday into Monday may produce some locally enhanced rainfall but available moisture looks near climatology. The Florida Peninsula should continue to see periods of rain beyond Monday due to the wavy front, with heavier amounts still tending to be localized. The Southeast coast may be near the edge of rainfall that persists over the western Atlantic. Upper troughing expected to amplify over the West next week may bring some precipitation to at least the Pacific Northwest and northern Rockies with timing differences among guidance. High elevations in the northern Rockies would see some snow if the upper trough and possible embedded low are deep enough. Ahead of the western trough/low, areas of rain will likely develop up the southern Plains Tuesday-Thursday. With a prolonged fetch from the western Gulf during this time, heavy rainfall may become organized, particularly around Wednesday. An area of high temperatures up to 15F above normal shifts from the interior Northwest/High Plains on Sunday will drift east with the ridge axis, reaching only the Great Lakes by Thursday. The upper trough forecast to amplify over the West next week should promote a cooling trend over the region starting Tuesday, with high temps 5-15F below normal stretching from MT to southern CA by next Thursday. Otherwise expect most temperature anomalies to be in the single digits, including the East leaning a little below normal Sunday-Monday before moderating to near normal. Jackson 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 Hazards: - Heavy rain across portions of the Central Plains and the Southern Plains, Wed-Thu, Sep 20 -Sep 21. - Flooding occurring or imminent across portions of the Southeast. 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