Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1115 AM EDT Tue Sep 04 2018 Valid 12Z Fri Sep 07 2018 - 12Z Tue Sep 11 2018 ...15 UTC Update... Models/ensemble have not changed significantly compared to the overnight forecast issuance. Model spread is relatively low early in the medium range and a blend of mostly deterministic guidance including the ECMWF and GFS was used as a forecast starting point. From day 5 (Sun) onward, the GFS was faster than the consensus with the remnants of Tropical Storm Gordon across the Ohio Valley/Great Lakes. Additionally, a bimodal distribution of solutions takes shape across the Pacific Northwest by days 6-7. Upper trough amplification appears likely either in the vicinity of the Pacific Northwest or perhaps offshore, downstream of anomalous (+4 standard deviations in 500 hPa heights) upper ridge amplification in the Bering Sea. The ECMWF/ECENS continue to be the strongest with the upper ridge across the Bering, and also strongest with pushing height falls into the Pacific Northwest by early next week. The GFS/CMC and NAEFS keep the strongest height falls a bit farther west over the North Pacific. A fairly high degree of consistency was noted in the ECMWF/ECENS solutions, although underdispersion in the ECENS could be a problem given the rather tightly clustered ensemble members around the mean. Given these considerations, a trend toward heavier ensemble mean weighting was shown from day 5 onward, with approximately 60/40 weighting toward the ECENS/ECMWF camp and GFS/NAEFS camp, respectively. Ryan Previous Discussion (issued at 0658 UTC) ...Pattern Overview and Guidance/Predictability Assessment... For about the first half of the period a blend of the 12Z ECMWF and varying weights of the 18Z GFS/12Z CMC represented the best ideas of guidance through the 18Z cycle of guidance. The majority total weight of the ECMWF/CMC accounted for recent tendencies of the GFS to lose definition of T.S. Gordon leading into the start of the medium range period. GFS runs do show a better defined low by the weekend in response to the shortwave energy progressing across the Plains/MS Valley. The blend (with minor adjustments for the official NHC forecast and yesterday's WPC-NHC coordinated track into Mon) yielded good continuity as faster ECMWF trends/slower GFS trends with the Plains into Great Lakes surface low have narrowed the guidance spread. The new 00Z GFS has trended slower than the 18Z run so there is now even less spread than before. Farther upstream there is decent agreement into the weekend with some lingering question marks on the finer details. Guidance still shows a very strong Bering Sea ridge aloft (likely aided in part by the extratropical evolution of Typhoon Jebi well to the west) supporting a northeast Pacific mean trough. A shortwave off the Pacific Northwest coast as of early Fri will progress inland and support southern Canada low pressure with a trailing front. At the same time energy dropping south from Alaska will close off another Pacific low. The remainder of the forecast transitioned toward somewhat more 18Z GEFS/12Z NAEFS influence relative to the ECMWF/ECMWF mean by day 7 Tue as the models/ensembles show increasingly divergent ideas for larger scale flow aspects, with conflicting considerations recommending an intermediate solution. A greater proportion of guidance indicates that the Bering Sea ridge and a northern Pacific upper low should be at a farther south latitude than depicted in the ECMWF/ECMWF mean. In addition teleconnections relative to the ridge's corresponding positive height anomaly center in the ECMWF scenario would suggest flatter mean flow over the western U.S. versus the trough depicted in the ECMWF. Teleconnections relative to either the GFS/GEFS or ECMWF ridge favor flat or modestly cyclonic flow over eastern North America--another nod to the non-EC solutions. At the same time the favored mean flow over the northeast Pacific could ultimately allow for slightly greater progression than the GFS/GEFS by the end of the period. Thus the aforementioned blend provided the best composite of these considerations by next Mon-Tue. ...Weather Highlights/Hazards... Expect weakening tropical cyclone Gordon to reach near the south-central Plains late this week and then progress northeastward as an extratropical low. Moisture from Gordon and interaction with a wavy front initially stalled just to its north as of Fri will ultimately produce a threat for heavy rainfall over some locations from the south-central Plains through the Midwest and Great Lakes/Northeast. While there is still uncertainty with specifics, there is somewhat better guidance agreement on timing/rainfall coverage than 24 hours ago so confidence is gradually improving. There may also be an enhanced rainfall focus over the Mid-Atlantic near the stalled front. Meanwhile persistent moisture over the southern tier states should allow for periods of rainfall, while energy aloft and a surface trough may enhance rain at times over Florida. The shortwave aloft approaching/crossing the Pacific Northwest late this week/weekend should produce a period of light-moderate precipitation over that region and then eventually the northern Plains/upper MS Valley. The next Pacific system should bring some moisture into the Pacific Northwest early next week but only with medium to lower confidence regarding precise timing and intensity/southward extent. High pressure building into the Northeast will bring a pronounced cooling trend to that region. The southern Mid-Atlantic/Southeast will remain south of the stalled surface front and should see above normal high temperatures most days, while a broader portion of the East will see warm min temperatures in the moist flow ahead of Gordon and its extratropical evolution. The central Plains into Midwest will see well below normal highs Fri-Sat followed by a trend to near normal temperatures. The overall pattern should favor slightly below normal highs over the Northwest after Fri while the southern half of the West should see above normal readings for most of the period. 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