Extended Forecast Discussion NWS Weather Prediction Center College Park MD 239 PM EDT Fri Sep 15 2023 Valid 12Z Mon Sep 18 2023 - 12Z Fri Sep 22 2023 ...Overview... An upper trough and surface cold front will move off the East Coast early next week while the tail end of the front lingers over the Florida peninsula and northern Gulf of Mexico. This system will produce precipitation chances in New England on Monday and Tuesday and provide focus for daily showers and thunderstorms in Florida through the week. In the West, unsettled weather is forecast to develop by mid-week as upper level troughing amplifies over the region. A couple of frontal systems will move across the Northwest and emerge in the Central U.S. during the second half of the week, which will create precipitation chances across the Northwest, Rockies, and Plains. Below average temperatures will filter into the West in the wake of the frontal systems. ...Guidance Evaluation/Predictability Assessment... Guidance showed good agreement on the upper level pattern through the medium range period. Upper level troughing will exit the East Coast early in the week, followed by a ridge moving over the eastern U.S. mid-week, and troughing will amplify over the West Coast during the second half of next week. A deterministic model blend on the GFS, ECMWF, CMC, and UKMET were used for days 3-5, but model differences increase during the later half of the period. The GFS advances the western trough inland faster than the ECMWF and CMC, but the ensemble means seem to agree on a slower solution. To smooth out differences in the timing and position of the western trough, the GEFS and ECENS means were added to the model blend for days 6 and 7. ...Weather/Hazards Highlights... The risk for heavy rainfall leading to flash flooding is forecast to be low early next week. A frontal boundary 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 amounts that may fall. Shorter term guidance may ultimately help to refine any areas that could at least merit a Marginal Risk. Wrap around precipitation on the backside of a surface low exiting Maine may produce some moderate rainfall on Monday in New England, and given already wet soils in that region, it wouldn't take much to create at least localized flooding concerns. Still, there's too much uncertainty in the location of heavier rainfall amounts to warrant adding a Marginal Risk for the Day 4/Monday Excessive Rainfall Outlook. The amplifying trough over West next week should bring some precipitation to the Pacific Northwest/northern Rockies and eventually the Plains. High elevations in the northern Rockies could 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 across the south-central Plains Tuesday-Thursday. With a prolonged fetch from the western Gulf during this time, heavy rainfall may become organized, particularly around mid-week. A Marginal Risk area has been added for parts of Kansas and Oklahoma for the Day 5/Tuesday Excessive Rainfall Outlook. An area of high temperatures up to 15F above normal will shift from the Northern Plains on Monday to 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 Montana to southern California next Wednesday to Friday. Otherwise, most temperatures from the South to the East will be near seasonable values through much of the week. Santorelli/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, 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