Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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 east into the central and east-central U.S., with amplified upper ridges on either side. This will lead to cooler than normal temperatures in the West and warmer than normal temperatures in the eastern half of the U.S. that may moderate closer to normal as the week progresses. Moist and unstable inflow just ahead of the trough and an associated cold front should lead to an emerging convective rainfall pattern for much of the Plains Monday-Tuesday and likely focused in the southern Plains by Wednesday. ...Guidance Evaluation/Predictability Assessment... Model differences in the first half of the week mainly have to do with the timing of the trough shifting eastward. The 12Z UKMET was the main initial outlier with a slower trough than the better clustered GFS/ECMWF/CMC runs, so leaned toward the latter group for the early part of the forecast. Beginning Wednesday and especially by Thursday-Friday, models diverge further with the timing and also the depth of the trough. This is at least in part due to the potential for additional energy to dive into the northwestern side of the trough, which the ECMWF and CMC runs have been persistently showing. On the other hand, GFS runs show any energy there in west-central Canada to be weaker and track more eastward rather than dropping southeast. This keeps a more amplified ridge to the west of the trough and allows for the trough to stay deep as it tracks east. The new 00Z GFS even tends to bulge the base of the trough in response to the northern side of the ridge and show the trough's axis as slightly positively tilted even into late next week, while other guidance is neutral to negatively tilted by that point. The variability with the shortwave means the pattern is out of phase across the northern Plains among deterministic models and various ensemble members by the latter half of next week. With this possible/questionable shortwave energy looking to emerge from the Arctic, this will likely remain uncertain in future model cycles. Additionally, while not the majority, a handful of runs from multiple models hang energy back into the Southwest from the trough, providing further uncertainty. The 12Z CMC did this and the EC ensemble mean hints at it as well. Given all the uncertainty in the details of the pattern, the initially mainly deterministic model blend transitioned to favor more ensemble means with increasing lead time, to just over half by day 6 and 70 percent by day 7 in the mass fields. ...Weather/Hazards Highlights... On Monday there may be some lingering snow in higher elevations of the West with the upper trough axis overhead. Meanwhile farther south, moisture and instability look to pool in the southern High Plains just ahead of the cold front. The right entrance region of the 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 has been introduced for Monday for that region. Rainfall chances are likely to increase in the north-central U.S. as well, but uncertainty in the amounts and placement of enhanced convection 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 is in place for Tuesday as a starting point, but it is likely that one or more embedded Slight Risks are introduced in future forecasts if models start to converge on locations of heavy amounts. Then by Wednesday the heaviest convection looks to focus across the southern Plains, 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 by Thursday or so, but with uncertainty in timing. Elsewhere, 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. 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 to 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 overtakes the east-central CONUS. 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