Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Tue Jul 31 2018 Valid 12Z Fri Aug 03 2018 - 12Z Tue Aug 07 2018 ...Pattern Overview and Guidance/Uncertainty Assessment... Latest guidance continues to show that a strong upper ridge south of the Aleutians will lead to an amplified eastern Pacific trough by this weekend. Best consensus has this trough persisting off the West Coast through early next week but with developing uncertainty over where the main core of trough energy will be at that time. Meanwhile a MS Valley upper trough already in the process of weakening as of early Fri will continue to weaken rapidly while an initial Bermuda high will likely build into the Southeast during the weekend and then possibly continue to retrograde into the Plains. Between the Pacific trough and eastern ridge, expect the main axis of jet energy to flow from the northern half of the West across the Northern Tier states and southern Canada. For the developing eastern Pacific trough, the ECMWF mean has been the most stable solution over the past 2-3 days--and has positioned the trough fairly close to where recommended by teleconnections relative to the upstream ridge. Eastward GEFS trends over the past couple days have brought its mean in line with the ECMWF mean. Individual models and ensemble members still offer a fair amount of spread over where the trough and possible embedded upper low will be on a particular day. The 12Z GFS/ECMWF are closest to the aforementioned means but the 18Z GFS strayed back to the west (a tendency of earlier GFS runs). On the other hand some prior ECMWF runs had been a bit east of the means. In varying ways the new 00Z GFS and recent CMC runs indicate potential for faster ejection of trough energy at least due in part to approach of upstream energy. Such detail has low predictability 6-7 days out in time thus recommending the ensemble means and 12Z GFS/ECMWF for the deterministic forecast. However it is with the understanding that a cycle of ejection and reloading is also a plausible scenario that would lead to a similar mean trough from the multi-day mean perspective. The fast and low amplitude nature of mean flow across the northern U.S./southern Canada leads to ongoing difficulty in resolving details of shortwaves aloft and associated frontal boundaries affecting the northern U.S.. There is still fair clustering for a leading system over/north of the northern Plains Fri-Sat followed by divergence on exactly how the leading energy aloft will evolve and on details of upstream energy that flows into the western U.S.. Guidance is generally signaling that by late in the period and beyond some of this trailing energy will ultimately settle into a modest eastern North America trough that would be consistent with the pattern upstream. Given the relative agreement of the 12Z GFS/ECMWF with the 18-12Z GEFS/12Z ECMWF means for the eastern Pacific trough as well as decent alignment elsewhere (with the 12Z UKMET/CMC differing over one or more areas) those runs served as the operational input for the updated forecast. Ensemble means constituted a small minority early in the period and reached a total half weight by day 7 Tue. ...Weather Highlights/Threats... By the latter half of the period the amplifying upper trough off the West Coast has the potential to bring more moisture into parts of the Pacific Northwest/Vancouver Island than might typically be expected for the first half of August. Rainfall amounts will be sensitive to exact details aloft and individual model solutions are currently quite varied for multi-day totals. Also over the West, diurnally favored convection with some locally moderate-heavy activity will continue over the Four Corners states. Farther eastward expect areas of enhanced rainfall from the Gulf Coast northeastward at the start of the period to become lighter and more scattered with time as ridging aloft builds inland from the Atlantic. At the same time the area extending from the northern Plains into the Great Lakes will become a prominent focus for periods of potentially heavy/strong convection. The system reaching the northern Plains late this week will provide an initial focus for activity. Then the orientation of flow aloft will encourage persistence of a mean frontal boundary over or near this region, leading to the possibility of repeat activity/training at some locations. Expect high temperatures to be below normal (mostly single-digit anomalies) during the period over the Northwest while farther south readings should slowly decrease to or a fraction below normal. Min temps will generally be warmer relative to climatology. To the east of the Rockies there will be gradually increasing coverage of above normal temperatures. Highest anomalies with a few pockets exceeding 10F above normal (somewhat more likely for min temps) should be within an axis from the northern-central Plains into the Northeast. Below average highs over the Southeast to start the period should trend toward normal levels as clouds/rainfall become less organized. 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