Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Fri Aug 31 2018 Valid 12Z Mon Sep 03 2018 - 12Z Fri Sep 07 2018 ...Pattern Overview and Guidance/Predictability Assessment... The main fixture in the synoptic-scale pattern will be an expansive mid/upper-level ridge extending from the Mississippi River eastward to the western Atlantic. Expected to be around 2 to 2.5 sigma above climatology, its presence should dominate the pattern with some weakening by Day 7/September 7 as a backdoor front drops southward across New England. Generally speaking, would say the models are in decent agreement with its position and strength. To the south of the positive height anomaly, some sort of tropical disturbance is primed to slide westward across the Florida Peninsula into the Gulf of Mexico. While it may feature a rather weak area of surface low pressure, the ECMWF has been trending downward with its strength in recent cycles. Ensemble low plots still suggest its presence with the GEFS members finally showing up in the 00Z cycle. The National Hurricane Center indicates a 10 percent chance for tropical cyclone genesis to occur during the next 5 days with strong vertical shear being a main inhibiting factor. Across the western/central portions of the continent, a conveyor belt of shortwaves should continue to skirt the international border with Canada while amplifying downstream across Saskatchewan/Manitoba. It appears the 06Z/00Z GFS and 00Z GFS-parallel have joined recent runs of the ECMWF being more amplified by mid-week. The guidance has been trending toward the allowance of more denting in the upper ridge. While this shortwave lifts into the higher latitudes of Canada, a digging longwave trough takes shape over the eastern Pacific during the Day 5-7, September 5-7 period. Ensemble spaghetti plots depict building spread with this system although recent guidance suggests a greater level of amplification. This seems to be largely driven by upstream influences from the extratropical transition of Super Typhoon Jebi. One notable difference is the operational solutions, primarily the GFS, are quicker than the more clustered ensemble means late in the forecast. With reasonable model agreement through Wednesday, generally leaned on a combination of the 00Z GFS/GFS-parallel, ECMWF, and CMC solutions with a bit of ensemble means incorporated by mid-week. Thereafter, attempted to build the forecast around the 00Z GEFS/00Z ECMWF ensemble means to ultimately hold the upper trough back a bit along the West Coast. ...Weather Highlights/Hazards... The corridor of warmer temperatures relative to early September climatology will reside from the Great Lakes eastward into the northeastern U.S. in response to the strong ridge aloft. High temperatures should generally be around 5 to 10 degrees above average although slightly higher numbers can be expected over the upper Mid-Atlantic/lower New England. Similar departures are likely during the overnight hours as well which may lead to a slew of daily records being broken, especially on Tuesday and Wednesday mornings. Meanwhile, the cooler spot will likely be over the Upper Intermountain West and Northern Rockies, particularly on Tuesday with highs only in the 60s. Regarding precipitation, multiple rounds of convection are in the forecast along the slow moving frontal zone stretching from the central U.S. into the Great Lakes region. While 24-hour amounts do not appear to be excessive in nature, the multi-day nature of these events could produce hydrologic issues at some point during the period. Otherwise, expect widespread warm sector convection south of the boundary with a better focus along the Gulf Coast. This is in response to better moisture quality as well as the mentioned tropical wave tracking westward across the Gulf of Mexico. Farther west, some convective chances will return to the Four Corners region although most solutions favor higher amounts eastward into the adjacent High Plains. Rubin-Oster 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