Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1123 AM EDT Sun Aug 26 2018 Valid 12Z Wed Aug 29 2018 - 12Z Sun Sep 02 2018 ...Late summer heat to exit the Northeast later this week... ...Weather Pattern Highlights and Hazards... A series of shortwave troughs will dig over the northeastern Pacific to split off the west coast, with energetic flow and modest precipitation potential working into the Pacific Northwest/West before progressing east-northeastward through southern Canada and the U.S. northern tier. Wavy frontal systems underneath will combine with deepened moisture and instability to fuel periods of heavy convection, focused from the eastern Plains/Mississippi Valley to the Midwest/Mid-South and Northeast/Mid-Atlantic. Expect a threat for locally excessive, but less organized, thunderstorms/heavy downpours within the hot/humid sector in the southern/eastern U.S. which may align with meso-boundaries and local convergence effects. This potent late summer hot sector under ridging aloft will offer spotty record maximum temperatures over the Northeast/Mid-Atlantic Wednesday with numerous record high minimum temperatures mainly Wednesday into Thursday with high dew points into the upper 60s and low 70s. Cold front will push through the Northeast early Thursday and the mid-Atlantic thereafter, settling into the Carolinas where relief will be much more marginal. Hot sector over the U.S. southern tier/Plains will generally hold firm for the next week. ...Guidance and Uncertainty Assessment... The larger scale flow pattern is reasonably well-depicted within the guidance over the lower 48 but was much more uncertain upstream over Alaska and the North Pacific. See the PMDAK discussion for more details. That uncertainty translates into western Canada and the Pac NW by next weekend with large timing differences among the ensembles. ECMWF ensembles have trended toward increased troughing south of Alaska whereas the GEFS/Canadian members maintain stronger ridging through next weekend, which modulates the longitude of downstream troughing in the West. Teleconnections support a trough axis farther west than the ECMWF ensemble mean along 120W rather than 110W given the maintenance of upper ridging in the east. Preferred a blend that trended toward the 00Z NAEFS and 06Z GEFS mean but still with some minority weighting of the 00Z ECMWF ensemble mean as it was the better performer in recent days. Fracasso/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