Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1032 AM EDT Sat Jun 23 2018 Valid 12Z Tue Jun 26 2018 - 12Z Sat Jun 30 2018 ...Overview and guidance/predictability assessment... The model guidance shows good agreement on a shift towards increasing western troughing/increased ridging near the Mid-Atlantic states by the end of the period. There are some detail issues in regards to how strong the western trough gets and how closed the system moving through the Great Lakes on Wednesday might be as it moves over the top of the Southeastern ridge, but overall agreement is so good that confidence is above average in the forecast. The days 3-7 pressures, winds, and 500 hPa heights are based on a deterministic blend of the 00z Canadian, 00z ECMWF, 00z UKMET, and 06z GFS through Thursday. For Friday into next Saturday, slowly increased the contribution of the 00z NAEFS mean and 00z ECMWF ensemble mean to compromise 40-45% of the distribution by the end of the period. The remainder of the weather grids are based on a more even blend of the National Blend of Guidance, WPC continuity, and the NAEFS ensemble mean guidance. The plan for the days 4-7 QPF is to use an even blend of the 12z National Blend of Models, 06z GFS, 00z ECMWF, and 00z Canadian guidance. Should the 12z GFS prove useful, it could be included. ...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 rainfall. Best potential for areas active convection 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 system weakens. The trailing front progressing across the Northern Tier will stall over the Plains with at least a portion likely to return north as a warm front by Friday into next Saturday. 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, with possible records appearing to be confined to warm overnight lows from Wednesday onward. Due to increased low-level moisture within a more summer-like regime, 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. This feature should bring coolest anomalies (5-15F below normal for highs) into the northern parts of the West by next Saturday. Roth/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