Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Thu Aug 16 2018 Valid 12Z Sun Aug 19 2018 - 12Z Thu Aug 23 2018 ...Pattern Overview and Preferences... Guidance is consistent in showing an evolution toward an amplified eastern North American trough aloft while some degree of mean ridging persists over or near the Rockies/west-central U.S., anchored by an upper high tending to be over New Mexico or Arizona. Gradual opening/deamplification of the pattern over the northeastern Pacific should ultimately lead to shearing and ejection of a trough (likely with an embedded low) evolving near the West Coast from the weekend into early next week. One of the more intriguing aspects of the forecast continues to be the specifics of Plains into eastern Canada low pressure in response to the combination of the shortwave reaching the Plains as of Sun and possible interaction with upstream energy flowing from western Canada. Operational ECMWF runs have tended to be on the deeper side of the envelope with the surface low. The depth in some of those runs, as well as in a very small number of ECMWF ensemble members, by the time the low crosses the Great Lakes day 5 Tue would be sufficient to equal or exceed lowest sea level pressure records for the month of August. On the other extreme operational GFS runs have generally been on the weaker side of the spread. The 00Z GFS is particularly weak until after Tue due to being more sheared with the initial Plains energy and lack of influence from Canadian flow. Somewhat in deference to operational runs the GEFS mean has been fading in and out with its surface low depiction. ECMWF means have been more steady and generally trending a little stronger over the past couple days. 00Z UKMET/CMC runs are as deep or deeper than the 00Z ECMWF. Overall favor an operational model compromise as consensus suggests enough stream interaction to yield a fairly strong surface low relative to the time of year--but not as deep as the ECMWF/UKMET/CMC given the possibility that occasional warm-season biases of holding convection too close to surface lows versus extending out into the warm sector may be playing a role in the deepest surface solutions. The past two days of 00Z parallel (FV3) GFS runs support this intermediate approach as they have been near the ECMWF mean in depth/position over the Great Lakes as of early Tue. Regardless of surface low differences, there has been fairly good agreement/continuity regarding the trailing front reaching the East Coast/Gulf Coast by around midweek. Over the West Coast/Northwest there is still a moderate degree of uncertainty over the details of the upper trough that arrives around the start of the period, possibly develops a closed low by Mon, and eventually ejects/shears out with the approach of upstream trough energy. Multi-day means have been showing a core of negative height anomalies near the central Aleutians and positive height anomalies east of the Canadian Maritimes as the most stable and best defined features, with teleconnections relative to both favoring a mean trough axis near or a fraction offshore the West Coast and ridging somewhat inland as latest consensus shows. This relationship supports ensemble trends over the past day, trimming away prior members that had lower heights over portions of the West. Thus expect ejecting West Coast energy to be fairly weak by days 6-7 Wed-Thu. Approaching trough energy late in the period may be a little fast in the 00Z ECMWF, with remaining models/means a bit slower to varying degrees. A blend among the 06Z GFS and 00Z ECMWF/UKMET/CMC provided a good starting point for the desired intermediate evolution of the Plains-Great Lakes system for days 3-5 Sun-Tue. Favorable trends of the 06Z GEFS mean versus the 00Z cycle, relative agreement of the means for the overall pattern late in the period, and varying question marks with some operational details at that time frame, led to a transition toward even or greater weight of ensemble mean guidance by days 6-7 Wed-Thu. ...Weather Highlights and Hazards... The developing system tracking from the Plains through Great Lakes into eastern Canada will likely produce a broad area of active weather from Sun onward, with best potential for heavy rainfall and/or strong convection from the Plains into the Great Lakes/Ohio Valley. Expect the front trailing from the surface low to generate some showers/thunderstorms to the south and east as well. Meanwhile also expect rainfall of varying intensity near and south of a leading wavy front initially extending east from the Plains through the Mid-Atlantic. This front will lift north as a warm front ahead of the Plains/Great Lakes system. The Four Corners states will likely see diurnally favored convection with at least some pockets of moderate or heavier activity, while the upper trough evolving along the northern half of the West Coast and then eventually ejecting out could support some scattered rainfall over inland locations across northern parts of the West. Portions of the northern/central Plains should be within the core of coolest daytime temperatures versus normal (minus 10-15F anomalies) in the Sun-Tue time frame. Departure of Great Lakes low pressure after Tue and southward/eastward progression of the trailing cold front will bring a cooling trend into the eastern states but with less extreme anomalies. Expect warmest highs versus normal to be over parts of the Pacific Northwest with some readings locally 10-15F above normal Mon-Tue. Morning lows should be above normal most days over much of the West. 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