Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1218 PM EDT Mon Aug 13 2018 Valid 12Z Thu Aug 16 2018 - 12Z Mon Aug 20 2018 ...Pattern Overview and Guidance/Predictability Assessment... The beginning of the period (Thursday morning) shows some combination of shortwave energy lifting out of the northern Plains into a second feature/shortwave elongating across the Upper Midwest. A surface wave ahead of this should gradually deepen as it progresses from the Ohio Valley into New England Thursday and Friday. Overall, the guidance is in relatively good agreement with this feature. It is interesting to note the ECMWF and its ensemble mean were noticeably slower with the shortwave placement on day 3, but by day 4 and beyond, seems to come into better agreement with the rest of the model suite. Out west, models continue to show weak height falls spilling into the Pacific Northwest on day 3, interacting with amplification in the northern stream flow. This should ultimately lead to some level of amplified flow across the middle of the country by this weekend. The latest run of the GFS (06z on Aug 13) has backed off on the more amplified trough noted previously, and now looks closer in line with the rest of the guidance. Whatever feature does emerge within the flow regime will advance downstream and should be at a position near the Mississippi River by Day 7/August 20 (Monday morning), as suggested by the ensemble means. In its wake, a possibly anomalous ridge builds just to the west across the eastern Pacific into western Canada. While most pieces of guidance do show some degree of anomalous heights, models continue to show disagreement with how strong these may be. The latest runs of the GFS continue to be the most amplified, but this may not be a bad thing as a more amplified pattern upstream (with whatever remains of T.S. Hector phasing into a larger upper west of Alaska) would support a more amplified ridge downstream. As for model choices with this cycle of the medium range progs, through day 5, there was enough operational support and agreement to constitute a majority operational 06Z GFS/00Z ECMWF blend. Beyond, normal timing and intensity differences arise within the smaller scale features so a larger influence of the ensemble means (06Z GEFS/00z ECENS) was used. Did put slightly more weight on the latest GFS/GEFS mean than the ECMWF/ECENS by day 7 to show that slightly more amplified ridging across western Canada. The overall blend for this cycle fit well with previous shift continuity so only minor changes were needed to the surface fronts and pressures and 500 progs. ...Weather Highlights and Hazards... With the expectation of a deep upper trough pushing across the Northern Rockies into the Northern/Central Great Plains during the Day 5-7, August 18-20 time period, much cooler conditions can be anticipated. With expected highs only in the 60s across the higher terrain and 70s to near 80 across the lower elevations, such readings would be around 10 degrees below average. Before this takes place, it should be quite warm across the north-central U.S. through Friday as high temperatures sit in the low/mid 90s. Elsewhere, a building upper ridge along the West Coast will allow temperatures to soar each day with most anomalous numbers over the Pacific Northwest. By Day 6-7, Portland and Seattle may push into the 90s which would be 10 to 15 degrees above mid-August climatology. Multiple rounds of showers and thunderstorms are likely from the middle of the country eastward into the Great Lakes and New England. The best focus will initially be near the lifting surface wave while vast warm sector convection should prevail to the south. With the flow expected to amplify this weekend across the Rockies, robust QPF amounts are noted in many model solutions with a particular focus across the Central Plains into the Middle Mississippi Valley. Across the Four Corners region, active monsoonal flow will aid in daily thunderstorm chances with heavier amounts potentially over southern Arizona where higher precipitable water air is to reside. Santorelli/Rubin-Oster 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