Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Sun Aug 19 2018 Valid 12Z Wed Aug 22 2018 - 12Z Sun Aug 26 2018 ...Pattern Overview and Guidance Evaluation/Preferences... The forecast remains on track for the well-advertised pattern change expected to occur next week into the weekend. A strong ridge aloft building between Alaska and Hawaii will likely encourage a downstream mean trough to become aligned from western Canada to near the northern California coast. Teleconnections relative to the positive height anomalies associated with the Pacific ridge provide good support for the consensus trough position. As this pattern becomes established the upper trough initially over the East will fill/lift out, and by next Sun mean ridging (with perhaps an embedded weakness or two) may extend from the western half of the Atlantic across the southern half of the eastern/central U.S. through the southern Rockies and northwestern Mexico. From day 3 Wed into day 5 Fri an operational model blend consisting of the 00Z GFS/ECMWF/UKMET/CMC either represented consensus or provided a good intermediate starting point depending on the feature. The 06Z GFS was out of sync with consensus (including the 00Z FV3 GFS and to a reasonable degree latest GEFS means) for flow details over the eastern half of Canada, leading to a more amplified and persistent upper trough over the East by late in the week. Thus the 06Z GFS was omitted from the forecast blend. Even among remaining solutions there is still a fair degree of shortwave spread within this overall weakening/departing trough. Meanwhile guidance has yet to lock onto specifics of the weak trough/upper low initially over the Northwest. Latest consensus is generally east of the previous cycle at the start of the period early Wed. Still there is fairly good agreement as the ejecting shortwave reaches the northern Plains/upper MS Valley by Fri. Continuity/consensus remain good for the upper trough reaching western Canada and the Northwest U.S. by Thu-Fri. By days 6-7 Sat-Sun the primary forecast uncertainty continues to involve the specifics of how upstream energy rounding the Pacific ridge reinforces the western Canada/Northwest U.S. mean trough plus the shortwave details within the progressive west-southwesterly mean flow to the east of the trough. Thus in spite of above average confidence in the mean pattern, embedded features have sufficiently small scale to suggest below average predictability. As a result associated surface wave/frontal details across the northern U.S. and southern Canada will likely continue to vary in the guidance over the coming days. Overall an average of the latest GFS/ECMWF runs and their ensemble means provided a good balance of representing the mean pattern with a moderate degree of detail. Note that the 06Z GFS replaced some of the 00Z GFS input by day 7 as the 00Z run significantly flattened the northern side of the Pacific ridge with minimal support from other guidance. Also the 00Z CMC and a number of CMC ensembles flattened the overall pattern to a greater degree than supported by established consensus and teleconnections. ...Weather Highlights and Hazards... The cold front initially nearing the eastern and Gulf coasts will be accompanied by locally heavy showers/thunderstorms. Highest rainfall totals on Wed, at least on an areal average basis, should be over the Northeast with influence of strongest dynamics aloft. This front will stall near the Southeast/Gulf coasts and provide multiple days of diurnally favored showers/storms of varying intensity. Portions of the northern-central Plains into the Midwest/upper Great Lakes will likely see at least a couple episodes of heavy and/or strong convection during the late week/weekend time frame, focused by shortwaves emerging from the West and surface fronts with embedded waves. Persistent monsoonal moisture will continue to aid in daily convection over the Four Corners states through the period. Relatively pleasant temperatures at least several degrees F below normal in place from the Plains to just west of the Appalachians as of Wed will push eastward and then moderate as high pressure crosses the eastern half of the country. Strengthening southerly flow by the latter half of the period should bring warmest anomalies (5-10F above normal) into the central Plains/Midwest/Great Lakes regions. The extreme Northwest should see highs cooling to a few degrees F below normal by Fri-Sun as the mean trough aloft sets up over the region. 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