Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EDT Sat Jun 23 2018 Valid 12Z Tue Jun 26 2018 - 12Z Sat Jun 30 2018 ...Overview and guidance/predictability assessment... Over the course of the period the majority of guidance is fairly consistent and agreeable in showing the establishment of a western U.S. trough while a mean ridge becomes more prominent over the eastern half of the country. After passage of a northern Plains/upper MS Valley through New England shortwave during the first half of the period, the expanding eastern ridge should deflect the westerlies well northward, into the Great Lakes/New England. At that time this area would also represent the southeast periphery of a broad cyclonic circulation covering the eastern half of Canada. The good continuity of the ensemble means thus far, and teleconnection support for the western trough (and to some degree flow downstream) relative to the building eastern Pacific ridge, suggest the pattern has above average predictability. However some individual model runs and ensemble members maintain the potential for a somewhat different outcome. The 12Z GFS seemed to provide one of the less likely solutions, with a faster late-period ejection of western trough energy into the Plains versus all other 12Z GEFS/ECMWF/CMC ensemble members at some locations. 18Z-00Z GFS runs have adjusted back to the consensus trough axis but with an embedded upper low/trough core somewhat deeper and/or south of the majority cluster by days 6-7 Fri-Sat. On the other hand CMC runs have tended to be somewhat flatter with the overall eastern Pacific/western U.S. pattern. The 00Z version compares better to other guidance than the prior run though. Uncertainty over the precise character of the western trough leads to some spread for downstream flow, with ensemble spread for 576-582dm height contours covering much of southeastern Canada and northeastern U.S. by late in the period. Questionable western trough progression in the 12Z GFS led to corresponding questionably high heights over eastern North America late in the forecast. Another forecast uncertainty during the period will involve the ultimate evolution of the closed low initially over western Canada and flow to its east/southeast. Solutions show meaningful divergence for details from about day 5 Thu onward. This will affect the handling of surface front(s) affecting the northern states. Ahead of this feature there is better than average agreement for the northern Plains/upper MS Valley system that opens up as it progresses east-northeastward. The updated forecast blend yielded only the typical run to run detail variability, with good continuity for significant features. The 18Z GFS and 12Z ECMWF/UKMET/CMC provided input for about the first half of the period. Then the forecast incorporated 50-60 percent total of the 18Z GEFS/12Z ECMWF means by days 6-7 Fri-Sat given their recent stability and to downplay lower confidence operational model specifics. Poor comparison to consensus led to removal of the CMC from the blend after day 5 Thu. ...Weather threats/highlights... The system tracking east-northeast from the northern Plains/upper MS Valley Tue onward will likely produce one or more episodes of locally heavy convection. Best potential for areas of enhanced rainfall should exist from the upper-mid MS Valley through the Great Lakes and Ohio Valley. Some pockets of moderate to heavy rainfall may extend into the Northeast but should generally be more scattered as the upper system weakens. The trailing front progressing across the northern tier states will stall over the Plains with at least a portion likely to return north as a warm front by Fri-Sat. The combination of this low level focus and increasing tendency for shortwave impulses to eject from the evolving western trough aloft may promote periods of convection/rainfall--some potentially heavy--from the northern Rockies eastward by late week/weekend. The western trough should also bring unsettled weather to other parts of the northern half of the West though with lighter amounts. Meanwhile expect periods of convection over the Southeast/Florida during the period with some heavy activity possible. A gradually weakening front over the region will provide some added low level focus during the first half of the period. The forecast is still on track for hot/humid conditions to become established over an increasingly large portion of the central/eastern states. Locations from the central Plains to Mid Atlantic/Northeast will have the best potential for seeing one or more days with min and/or max readings reaching as high as 10-15F above normal. Adding in the effects of humidity, highest heat index values should expand from the South early in the period to a large area centered over/near the mid MS Valley as well as southern Mid Atlantic/Southeast. The Great Basin/Four Corners region will see warm temperatures but with some gradual cooling later in the period with the arrival of the western trough aloft. This feature should bring coolest anomalies (5-15F below normal for highs) into the northern parts of the West by next Sat. Rausch WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities are found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4