Extended Forecast Discussion NWS Weather Prediction Center College Park MD 149 AM EDT Sat Aug 11 2018 Valid 12Z Tue Aug 14 2018 - 12Z Sat Aug 18 2018 ...Pattern Overview and Guidance/Predictability Assessment... A pair of slow moving cut-off upper lows will initially situate over the mid-Atlantic and Southern/Central Plains, respectively. Both features should gradually shift downstream as the entire synoptic-scale flow budges eastward. The latter system affecting the central U.S. is forecast to eventually become absorbed within amplified flow across the Great Lakes on Day 5/6, August 16/17. Relative to yesterday's forecast, this becomes the key feature aloft with recent ECMWF runs backing off on a significant upper low across New Brunswick down into Maine. Farther upstream, models remain uncertain on the poleward extent of ridging across western North America late in the period. It does appear nearly certain a 594-dm mid-level ridge will re-establish its semi-permanent summer home across the Desert Southwest. After exhibiting a brief faster trend with the exiting closed low over the mid-Atlantic, the guidance have trended slower as anticipated. Weak upstream height falls should eventually nudge the system offshore in the following days entering the Canadian Maritimes by late Thursday. Shifting focus to the longwave trough sitting over the Great Lakes and points southward, there is plenty of ensemble spread with regards to placement and magnitude. Additionally, run-to-run continuity is lacking suggesting future shifts in the newer guidance. Perhaps not surprisingly, the 00Z GFS/CMC diverge from previous solutions which casts further doubt on anything beyond an ensemble approach from Day 5/Thursday onward. Elsewhere, the unknown phase of the synoptic pattern across western North America remains an issue. However, multi-day ECMWF ensemble mean comparisons show the prominent ridge a bit flatter in recent runs. All the while, the same comparison for the GEFS/NAEFS solutions seem to maintain a quasi-zonal pattern with perhaps a weak longwave ridging signal across the Pacific Northwest into western Canada. One thing that appears nearly certain is a return of the 594-dm mid-level ridge to the southwestern U.S. This has shown up in the past several runs of the GFS/ECMWF. With reasonable operational model agreement through Wednesday, went with a myriad of solutions although leaned more heavily on the 12Z ECMWF and 18Z/12Z GFS. With uncertainty building across many key features on the map, added heftier contributions from the 12Z ECMWF/NAEFS ensemble means. By early next weekend, this blended approach would support a weak negative height anomaly across the Great Lakes and its vicinity while a mean ridge sets up across a large chunk of the western U.S. ...Weather Highlights and Hazards... A mild pattern remains in place from the Pacific Northwest eastward through the Upper Intermountain/Northern Rockies into the north-central U.S. General departures from climatology should be around 5 to 10 degrees accompanied by continued dry conditions. However, the 00Z GFS does bring a swath of heavy rainfall into the picture late Thursday/early Friday while the 12Z ECMWF keeps the area dry. With wet weather in store for the Southern/Central Plains, this should keep readings slightly below average. Models show some healthy 24-hour amounts anywhere from Oklahoma/Kansas east-northeastward into Missouri/Illinois with numbers in the 1 to 2 inch range. Otherwise, daily chance for thunderstorms are likely anywhere east of the Mississippi River given the slow moving longwave trough combined with a robust warm sector. Across the Desert Southwest, monsoonal convection should break out each afternoon/evening. The 00Z GFS shows very high amounts although these appear contaminated by convective feedback. 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