Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Sep 30 2023 Valid 12Z Tue Oct 03 2023 - 12Z Sat Oct 07 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 and pushing into the central U.S. in moderated form as the week progresses, while warmer than normal temperatures across the northeastern quadrant of the CONUS will become more limited in area by late week. 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 Tuesday and focused in the southern Plains by Wednesday. Some rainfall that will likely be lighter looks to push into the eastern U.S. later in the week. ...Guidance Evaluation/Predictability Assessment... Model guidance has at least been locked in with showing a fairly deep trough to push through much of the CONUS through next week, and show similar timing with its axis moving across the Rockies Tuesday into the central U.S. Wednesday. Shortwaves within the trough are somewhat more questionable but within reasonable spread. The early part of the forecast period was based on a blend of the 12/18Z GFS and the 12Z ECMWF, CMC, and UKMET that all seemed within reason initially. By the latter half of the week, models diverge with a few aspects of the trough's evolution as it tracks east. For multiple model runs, the GFS suite has been indicating more ridging across western Canada than the ECMWF/CMC that have been stronger with shortwave energy suppressing that northeastern side of the ridge, which has implications for the axis and depth of the trough downstream. The incoming 00Z GFS and GEFS are finally showing a little more evidence for the shortwave, relatively more comparable to other guidance. The shortwave energy serves to broaden the trough by late week. Thus a narrow trough like the 12Z GFS was showing has become more of an outlier especially in seeing the newer 00Z guidance. Meanwhile the base of the trough is another area of uncertainty, as some model runs including the 12Z CMC and UKMET as well as the 18Z GFS (in a more phased way) hold some energy back in the Southwest into late week. This has seemed like the minority scenario for the past day or two, but the 00Z CMC and GFS have come in indicating this as well, maintaining that possibility. The ECMWF has been indicating ridging there. Thus the WPC forecast transitions gradually to a more ensemble mean-heavy blend by the latter part of the week due to the increasing model spread as the forecast period progresses. This serves to minimize the individual model differences but does maintain the larger scale upper trough with ridging on either side. ...Weather/Hazards Highlights... The trough and cold front emerging into the Plains will support widespread rain and thunderstorms in the central U.S. on Tuesday. A broad Marginal Risk in the Excessive Rainfall Outlook covers most of the region, but guidance seems to have narrowed in on heavier QPF along the surface low track in the northern Plains/Midwest. The frontal low along with a potent shortwave and some instability in the vicinity should provide the ingredients for heavy rainfall, and a Slight Risk has been delineated for eastern North Dakota/northeastern South Dakota/northwestern Minnesota. Then by Wednesday, the northern side of the cold front may be moving more progressively through the Midwest for some lighter rain, while heavy rain and thunderstorms become more widespread in the southern Plains where the right entrance region of the jet will provide ample lift and plenty of instability is in place just ahead of the front. Thus a Slight Risk is in place there in the Day 5/Wednesday ERO, as despite the drought conditions, high rain rates that could overcome the dryness are likely. The cold front should gradually push through the south-central U.S. Thursday and Friday, pressing rain chances southward with it. Meanwhile showers are forecast to track across the Ohio/Tennessee Valleys around Thursday and into the East by Friday along and ahead of the front. Not too much precipitation looks to occur elsewhere, though there could be some lingering snow over higher elevations of the West in association with the upper trough crossing the region through midweek. Some rain in the western Atlantic could provide a few showers to eastern Florida, but otherwise looks to stay mainly offshore. In terms of temperatures, the amplified trough will lead to cooler temperatures than average while the ridge to the east will support above normal temperatures. The Interior West should see cooler temperatures than average especially through midweek, with 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 through around midweek, warming to near normal for the latter part of the week. Meanwhile warmer than average temperatures by around 15-20F above normal are forecast for the Midwest into the Great Lakes and the Interior Northeast, with highs in the upper 70s to low 80s. As the cold front and upper trough march east into the latter half of the week, high temperatures will cool to below average across the central and then east-central U.S., limiting the above normal temperatures to the Northeast. Another upper ridge coming in behind the trough should raise temperatures along the West Coast to above average. 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