Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Sat Sep 16 2023 Valid 12Z Tue Sep 19 2023 - 12Z Sat Sep 23 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 may continue to produce precipitation chances in northern New England on 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 a likely closed upper low drops through 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 this system. ...Guidance Evaluation/Predictability Assessment... There was good enough agreement the first half of the period as the trough exits the Northeast and troughing begins to amplify over the West. A general deterministic model blend was used. By Thursday and beyond though, while there is general agreement in the presence of what could be a deep upper low, there are a lot of differences in timing/strength/placement of this feature over the West. Models exhibit a lot of run to run variability of how far south this low gets, among other details, mostly related to another amplified shortwave upstream dropping through the East Pacific which may kick the low east into the Plains next weekend. 18z/Sep 15 GFS noticeably much quicker to do this and was not preferred late period. Ensemble means are extremely washed out with this feature, as there is a lot of spread in the individual ensemble members. This provided a challenge for the late period WPC forecast blend. Relied heavily on the ensemble means for the day 6 and 7 forecast, given uncertainty, but did include minor amounts of the ECMWF and CMC (which were closest in placement) for some added definition. This also sticks close to previous WPC continuity, which is a safe place to be at this point. ...Weather/Hazards Highlights... 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. The amplifying trough over the West by mid week should bring some precipitation to the Pacific Northwest/northern Rockies. 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 of moisture from the western Gulf during this time, heavy rainfall may become organized, particularly around mid-week. A Marginal Risk area maintained for parts of Kansas and Oklahoma for the Day 4/Tuesday Excessive Rainfall Outlook. Rainfall should spread northward as the associated front moves towards the region. Moisture and instability are favorable ahead of this, but still plenty of uncertainty on how organized rainfall may be and placement of highest amounts. Added a marginal risk from eastern OK/KS northward into the Upper Mississippi Valley for the Day 5/Wednesday ERO. An area of high temperatures generally 10-15F above normal will shift from the Northern Plains on Tuesday 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 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