Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1127 AM EDT Wed Sep 05 2018 Valid 12Z Sat Sep 08 2018 - 12Z Wed Sep 12 2018 ...Heavy rains expected for portions of the Mid-Mississippi and Ohio Valleys late this week into early next week... ...15 UTC Update... Model/ensemble consensus throughout the medium range was slightly above average, leading to somewhat better than average forecast confidence, particularly with systems traversing the northern stream. An amplified flow regime is expected to set up across much of the northern hemisphere over the next several days in response to the extratropical transition of former Typhoon Jebi across eastern Asia. Ensemble means continue to support a highly anomalous (+4 standard deviations) upper ridge setting up across the Bering Sea by late this week, with additional flow amplification likely downstream across North America. A multi-model consensus was the favored approach to the forecast update this morning. The deterministic ECMWF/GFS/CMC solutions formed a basis for the forecast for days 3-5 (Sat-Mon). This consensus solution matched the official NHC forecast track for Tropical Depression Gordon (forecast to be post-tropical by the medium range time period), and also handled relatively minor timing/amplitude differences with shortwave features traversing Canada and the U.S. Northern Tier. The most significant differences among the guidance remain along the West Coast by days 6-7 with expected upper trough amplification downstream of the anomalous Bering ridge. In general, a trend has been noted toward the ECMWF/ECENS solutions showing stronger height falls along the West Coast, as opposed to farther west/offshore. While GEFS/NAEFS members remain farther west than the ECENS, they have trended toward it. Thus, by days 6-7 (Tue-Wed), weighting of ECENS/NAEFS means was increased to comprise a majority of the WPC forecast blend. Ryan Previous Discussion (issued at 0650 UTC) ...Pattern Overview and Guidance/Predictability Assessment... The eastern Pacific/western North American pattern will continue to be influenced by very strong Bering Sea ridging which at times may extend into Mainland Alaska. This ridge will continue to promote a broad mean trough over the northeastern Pacific. While the general scenario has been well advertised, guidance has had some difficulty in resolving exact details of the ridge. This in turn has been leading to occasionally significant spread for the character of the downstream trough and sensible weather effects over the western U.S.. Farther east the greatest focus will be on post-tropical/extratropical Gordon whose surface reflection is most likely to track northeastward through the middle MS Valley and lower Great Lakes. The low track will be around the periphery of a persistent western Atlantic/East Coast ridge aloft. Early in the period the upper ridge will continue a weakening trend from the short range time frame but then there is a strong consensus that the ridge will begin to rebuild after early Sun. Guidance available through the 12Z/18Z cycles shows much less spread over the eastern Pacific/western U.S. than what was seen 24 hours ago. Much of the convergence in guidance has been achieved by GFS/GEFS mean trends toward more amplified troughing along with more suppressed ridging aloft over southern parts of the West. Adjustments that have been made with the upstream ridge appear to support the current majority cluster that develops a decent core of negative height anomalies fairly close to Vancouver Island by the end of the forecast period. The 18Z GFS/GEFS mean and 12Z ECMWF/ECMWF mean provided a good representation of the consensus scenario. This would have one shortwave crossing the Northwest on Sat, followed by the arrival of another trough (with embedded upper low) around Mon-Tue. The most prominent trend over the past day for the surface system associated with Gordon is toward a slower and farther southward solution, in response to the strength/persistence of eastern Canada/New England high pressure. In fact the 18Z GFS/12Z ECMWF and their means suggest the possibility that with upper support eventually outpacing the surface low, there may not be a discernible feature by day 7 Wed. Based on current consensus for significant features, the forecast started with a blend of the 18Z GFS/GEFS mean and 12Z ECMWF/ECMWF mean. The early-middle part of the forecast tilted mostly to the operational runs while the remainder of the forecast leaned more toward the means. ...Weather Highlights/Hazards... Early in the weekend Gordon should merge with a front draped across the central-eastern U.S., with the extratropical wave progressing northeastward through the mid MS Valley and lower Great Lakes before potentially weakening just west of or over New England. Abundant tropical moisture interacting with the frontal system will likely generate heavy rainfall, with latest southward trends in the guidance suggesting an axis of highest rainfall totals from the mid MS valley to the upper Ohio Valley/central Appalachians. Some of this moisture should extend into the Mid-Atlantic and Northeast. Meanwhile there will also be areas of rainfall farther south, ahead of a front/trough trailing from extratropical Gordon. Florida will see diurnally favored convection with a surface trough providing some added focus during the first half of the period. A frontal system will bring a fairly narrow band of rainfall across parts of the Northern Plains and vicinity during the weekend into Mon. Approach of another front may bring a little more rainfall to this region toward midweek. The evolving pattern will likely bring multiple days of precipitation to the Pacific Northwest, with most activity expected to be on the light to moderate side. During the first half of the period expect below normal high temperatures from the southern/central Plains into Mid-Atlantic/Northeast due to a broad area of clouds/rainfall, or cool high pressure in the case of New England. By the first half of next week the evolving pattern will favor cool readings over the Northwest and an axis of moderately above normal readings from the Southwest into the Northern Plains. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml