Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Wed Aug 01 2018 Valid 12Z Sat Aug 04 2018 - 12Z Wed Aug 08 2018 ...Pattern Overview and Guidance/Uncertainty Assessment... The forecast over the eastern Pacific and western North America after day 4 Sun has become dramatically more uncertain over the past 24-36 hours. During the weekend there is relative agreement that an upper trough with embedded low will be amplifying over the eastern Pacific. At this time frame there is moderate scatter in closed low position (00Z ECMWF a little northeast, GFS southwest) but these differences could prove critical mid-late period. Before today the primary question mark was how quickly the initial upper low/trough energy might eject in response to upstream flow. Now the recent GFS runs and 00Z CMC offer the potential for upstream flow to be sufficiently flat that the Pacific low would linger for a longer time, while western U.S. ridging becomes quite strong/westward instead of settling over the southern Rockies/High Plains. At the same time the GFS/00Z CMC scenario would have a progressive northern stream shortwave reaching western-central Canada by days 6-7 Tue-Wed. On the other hand the ensemble means have been fairly consistent in advertising less flow separation over the eastern Pacific, with the only question being the exact longitude of the overall trough. Yesterday's 00Z ECMWF appears to have been a relative hiccup as it was farther east/less amplified with the trough versus other runs. The 12Z ECMWF/CMC were not far from the ensemble mean scenario but did stray a bit faster with inland ejection late in the period. Adding to the confusion, the 12Z parallel (FV3) GFS was very close to the means but the 18Z run changed to the pattern of recent operational GFS runs. This highlights the Aleutians/Gulf of Alaska as an area of high sensitivity in relation to determining the ultimate flow evolution over the eastern Pacific/western North America. Teleconnections relative to the positive height anomaly associated with the strong northern Pacific/Aleutians ridge early-mid period have thus far been supportive of the ensemble mean scenario. For this reason and to maintain continuity until there is emphatic support for an alternate solution--which could be fairly soon given 00Z guidance trends--the preference based on 12Z/18Z solutions transitioned from an operational model blend early in the period toward a majority of 18Z GEFS/12Z ECMWF ensemble means with a small lingering weight of the 12Z ECMWF/CMC by days 6-7 Tue-Wed. Meanwhile the forecast looks less sensitive over the eastern half of the continent. Regardless of differences for the eastern Pacific into western North America evolution mid-late period, nearly all guidance transitions from low amplitude west-southwesterly Northern Tier flow to an eastern North America mean trough that ends up suppressing Atlantic ridging that builds into the Southeast early-mid period. The uncertainty for this part of the forecast will lie with individual shortwave impulses that will affect surface fronts/waves. Relatively small scale of the shortwaves will lead to low predictability at extended time frames. The guidance selection made for the upstream forecast provided a reasonable starting point for depicting the most common elements of the pattern evolution over central/eastern parts of the country. ...Weather Highlights/Threats... Increasing spread and continuity changes in recent guidance are lowering confidence in exactly how much moisture reaches the Pacific Northwest ahead of the upper trough/low forecast to evolve over the eastern Pacific. The currently preferred solution would have some rainfall arriving toward the end of the weekend/early next week but latest adjustments in 00Z runs are increasing the potential for later arrival with lower amounts. Likewise the temperature forecast for portions of the West next week has become more uncertain. Established consensus has suggested areas west of the Rockies should tend to be near to below normal for daytime highs and coolest versus normal over the Northwest. However there is the possibility of warmer temperatures. Elsewhere, expect diurnally driven showers/thunderstorms along the terrain in the Southwest/Four Corners region through most of the period. From the weekend into early next week a mean frontal boundary with embedded waves will be a focus for periods of locally heavy/strong convection from the northern Plains into the Great Lakes. Toward midweek the gradual development of upper troughing over the East should push the best organized activity into the Appalachians and vicinity. Before that occurs, westward protrusion of the Bermuda high into the Southeast will promote a drier trend over the East after a wet short range period. There will still be some scattered showers/storms encouraged by daytime heating. Areas along the Gulf Coast may see one or more days of rainfall as well. For temperatures, above normal anomalies will be fairly widespread east of the Rockies. Warmest readings versus normal should exist for morning lows from the upper MS Valley into New England, with some pockets of very warm highs from portions of the Plains into Northeast on one or more days. 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