Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Fri Sep 29 2023 Valid 12Z Mon Oct 02 2023 - 12Z Fri Oct 06 2023 ...Overview... An amplified pattern is in store next week, as an upper trough tracks across the Interior West and then into the Plains and east-central U.S., with amplified upper ridges on either side. The upper trough will support cooler than normal temperatures in the West, while warmer than normal temperatures over most of the central/eastern U.S. early in the week should moderate closer to normal as the week progresses. Moist and unstable inflow just ahead of the trough and an associated wavy cold front will likely produce an emerging convective rainfall pattern for much of the Plains Monday-Tuesday and likely focused in the southern Plains by Wednesday. Expect lighter rainfall to push into the eastern U.S. later in the week. Meanwhile a reflection of the upper low/trough now crossing the the East Coast may linger over the western Atlantic for most of the week but most of the associated rainfall should remain offshore. ...Guidance Evaluation/Predictability Assessment... During the first two or three days of the period, the primary forecast issues involve details for shortwave energy within the overall western into central U.S. upper trough along with northern Plains frontal wave development as the core of the initial western trough ejects northeastward. Most of the guidance differences for shortwave specifics and surface wave development appear fairly typical for forecasts 3-5 days out in time, favoring a blended/compromise approach among the latest operational runs. However the 00Z UKMET offered one of the lower confidence solutions in depicting more southwestward elongation of the upper trough late in its run. By late next week, individual models and ensemble members increasingly diverge within a recent 2-3 day trend of the GEFS/ECens means toward a stronger and eastward Pacific/West Coast upper ridge and more amplified east-central North American trough. There has been more consistency for western Atlantic into Canadian Maritimes mean ridging. The most basic difference starts with relative strength of the northern part of the Pacific ridge versus Gulf of Alaska into western Canada shortwave energy, with some influence just upstream as well. Latest GFS runs and the 12Z UKMET favor a stronger/eastern upper ridge, maintaining more amplitude of the central U.S. upper trough. The GEFS mean tilts in that direction. The ECMWF/CMC are stronger with the shortwave energy, yielding a farther west ridge axis that allows for this energy to flow into the mean trough and eject leading energy more quickly--ultimately resulting in a broader overall trough by Day 7 Friday. Not surprisingly the ECens mean hints at that idea. Teleconnections relative to positive height anomaly centers near the British Columbia coast and Canadian Maritimes in the D+8 multi-day mean charts favor fairly amplified east-central North American troughing by the end of the period or just beyond, which the ECMWF/GFS and their ensemble means show in principle. CMC/CMCens runs appear to be lagging the trends and teleconnection relations toward the majority scenario. However this does not provide a lot of insight as to whether the GFS/GEFS or ECMWF/ECens details would be more likely on a day-to-day basis, so preference is to transition the early-mid week model blend toward greater weight of the GEFS/ECens means relative to the GFS/ECMWF (given decreasing confidence in exact operational model specifics) along with equal consideration of the GFS/GEFS vs ECMWF/ECens clusters. ...Weather/Hazards Highlights... The central U.S. will be the primary focus for heavy rainfall/flash flood potential during the Monday-Tuesday night period covered by the Days 4-5 Excessive Rainfall Outlooks. By Day 4, moisture and instability look to pool in the southern High Plains just ahead of the cold front supported by the western U.S. upper trough. The right entrance region of the upper jet will provide ample lift which combined with the unstable conditions could produce high rain rates, possibly overcoming the ongoing dry conditions there to produce some localized flash flooding. Thus a Marginal Risk is depicted for Monday across that region, with guidance supporting minimal adjustment from prior issuance. Rainfall chances are likely to increase in the north-central U.S. as well, but uncertainty in the amounts and placement of enhanced convection still precludes introducing any excessive rainfall risk at this point. As the trough and cold front emerge into the Plains on Tuesday, rain and thunderstorms should become widespread in the central U.S., with model guidance showing embedded heavier amounts through all the Plains states into the Upper Midwest, but with little agreement in placement at this time. A large Marginal Risk area remains in place for Tuesday as a starting point. The latest cycle of guidance is starting to focus a little more on a potential axis of heavier totals in the northern Plains as ejecting energy aloft supports a northern tier frontal wave. However there is still some spread for amounts/location, and how this activity may overlap areas of wetter or drier soil conditions. Thus prefer to wait at least another cycle before introducing any embedded Slight Risk area. Plains convection should focus more over southern parts of the region by Wednesday, with some rain amounts of over an inch already widespread in the deterministic forecast. Some rain and storms should spread into the Ohio/Tennessee Valleys later in the week, with greater uncertainty in timing and amounts. In general the rainfall totals over the eastern U.S. should be lower than over the Plains. Meanwhile, some lingering showers and storms are possible over southern Florida especially early next week as a front pushes through, but with lighter amounts than in the short range. The Pacific Northwest may see a round of light to moderate precipitation Monday-Tuesday before likely drying out by midweek. Some snow may linger into early next week over higher elevations of the West in association with the upper trough crossing the region. In terms of temperatures, the early week trough atop the West will lead to cooler temperatures than average, with lows 5-10F below normal but highs around 10-20F below average. This puts high temperatures generally in the 80s for the Southwest and the 50s and 60s for the Great Basin. Meanwhile, temperatures from the Plains eastward will be near to above average on Monday, with the largest anomalies of 15-20F above normal (for highs, even more anomalous for lows) focused on the Upper Midwest to Great Lakes and the Interior Northeast, with highs in the upper 70s to low 80s. The Plains should moderate to near normal or perhaps a few degrees below normal temperatures as the week progresses, with the area of above normal temperatures becoming more limited to the Northeast by later in the week as the cold front reaches the east-central CONUS. Rausch/Tate 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