Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM EDT Sat Aug 25 2018 Valid 12Z Tue Aug 28 2018 - 12Z Sat Sep 01 2018 ...Guidance Evaluation/Preferences... Guidance shows a similar larger scale pattern heading into midweek, but smaller scale differences that will drive convective evolutions will remain less resolved until the short term. A model and ensemble composite blend was suitable for Tuesday, but quickly growing system variance portends use of 12 UTC GEFS/ECMWF ensembles thereafter. These particular ensemble runs offer flow on the amplified side of recent full solution envelopes. Despite uncertainty with run to run solution flipping diurnally over the past few runs, amplification is favored downstream with ripple effects from the entrainment of extratropical lows associated with weakening west Pacific tropical systems Cimaron and Soulik. There is also some uncertainty with the southward extent of the front and associated convection over the central and eastern U.S. as it battles upper ridging. ...Pattern Overview and Weather Highlights/Hazards... A lead amplified mid-upper level trough is expected to progress east-northeastward over the north-central U.S. then through the Northeast and eastern Canada Tuesday-Thursday. This will support the sinking of a wavy frontal system across roughly the northern half of the lower 48 which will focus periods of organized convection. Potentially strong activity should occur from the south-central plains/mid-Mississippi Valley and Midwest/Great Lakes eastward. To the south, less predictable mesoscale processes within the hot/humid summertime airmass (temperatures well above average to near record levels: especially for minimums) and deep moisture/instability will also give rise to locally heavy rainfall which may produce local runoff issues with repeat/training of cells. Upstream, a strong upper high south of the Gulf of Alaska will retrograde westward and allow another cooling trough into the Pacific Northwest mid-late week with enough onshore flow to initiate modest precipitation. Schichtel 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