Extended Forecast Discussion NWS Weather Prediction Center College Park MD 226 AM EDT Fri Sep 15 2023 Valid 12Z Mon Sep 18 2023 - 12Z Fri Sep 22 2023 ...Overview... Upper troughing and a surface cold front will slide through the East early next week, with some wrap around rains possible across New England. The associated front will linger across Florida through much of the week providing a focus for daily showers and storms. In the West, guidance continues to advertise a significant pattern shift after initial ridging weakens and moves east, with an amplifying trough/possible closed low bringing wet weather across the Northwest/Rockies late in the week. This should bring a pronounced cooling trend to most of the West. ...Guidance Evaluation/Predictability Assessment... On the large scale, the guidance show fairly good agreement on this overall pattern. Some minor differences in the details, but was able to utilize a total deterministic model blend for days 3-5. After this, there are some larger differences that arise out West with the arriving trough. There's considerable run to run continuity amongst the GFS, ECMWF, and CMC regarding depth and timing of this trough/closed low and so it seemed prudent to trend towards the better agreeable ECENS and GEFS means for the days 6 and 7 blend. The resulting WPC forecast is slightly deeper than previous shift continuity out West, but that seems to be the latest trend in the guidance. ...Weather/Hazards Highlights... A frontal boundary stalling over the Florida Peninsula will maintain a fairly wet pattern much of the week. 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. Wrap around precipitation on the backside of a surface low exiting Maine may produce some moderate rainfall on Monday, and given already wet soils up in that region, it wouldn't take much to create at least localized flooding concerns. Still, there's enough uncertainty to preclude even a marginal risk for the Day 4/Monday ERO. Upper troughing expected to amplify over the 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. An area of high temperatures up to 15F above normal shifts from the Northern Plains on Monday, reaching 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 and Friday. Otherwise expect most temperature anomalies from the South to the East to be closer to or a few degrees on either side of normal 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