Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sun Sep 02 2018 Valid 12Z Wed Sep 05 2018 - 12Z Sun Sep 09 2018 ...Pattern Overview and Guidance/Predictability Assessment... Guidance continues to be in the process of refining the details within an agreed-upon northeastern Pacific mean trough, to the southeast of a strong ridge extending through the Bering Sea. There is still a general signal that downstream amplification of the pattern in response to the extratropical evolution of current Typhoon Jebi may serve to reinforce or further strengthen the Bering Sea ridge late in the week. Tendency for guidance to take some time to account properly for such strong extratropical transitions provides one source of uncertainty in the forecast. As fairly progressive mean flow becomes established over southern Canada and northern U.S. expect the initially strong eastern U.S. ridge aloft to weaken with time. To the south of the ridge a wave currently over the eastern Bahamas should reach the vicinity of the central Gulf Coast by the start of the extended period on Wed. However there is considerable spread regarding the strength and path of this feature after midweek. A deep layer inverted trough may reach Florida from the Bahamas toward Fri-Sat. Over the past 1-2 days guidance clustering has improved somewhat for what had appeared to be a very uncertain forecast within and downstream from the northeastern Pacific mean trough. Solutions have converged/stabilized fairly well for a leading shortwave energy crossing central/eastern Canada, ultimately pushing a Great Lakes-Plains front into the Mid-Atlantic by late week/weekend. Meanwhile short range energy closing off an upper low over the northeast Pacific should be approaching the Pacific Northwest coast by day 5 Fri with continued east-northeast ejecting as upstream energy dropping south over Alaska ultimately forms another northeastern Pacific upper low. During the latter half of the period GFS/ECMWF and ensemble mean consensus has been fairly stable so far with the southern Canada-northern U.S. low pressure/frontal system associated with the ejecting shortwave. The upper low which guidance forms over the northeast Pacific next weekend is a notable trend from 24 hours ago when there was less separation relative to flow over northern-western Canada. The 18Z GFS pulled this late-period upper low farther west than most other guidance by day 7 Sun, discouraging its use in the blend at that time. Farther east the greatest uncertainty exists from day 4 Thu onward with the ultimate evolution/path of the feature which is most likely to be over/near southeastern Louisiana as of early Wed. Recent GFS runs have been weakest at the surface and take the mid level energy/surface reflection quickly northward around the western periphery of the eastern ridge aloft. In contrast ECMWF runs have held onto enough of a westward extension of the ridge aloft to yield a track into southeastern Texas. CMC runs are between the two extremes while the UKMET is close to the ECMWF scenario. Not surprisingly the ensembles show an equally divergent array of possibilities. Important details are small enough in scale as to have lower than desired predictability, keeping confidence low in any specific solution. There is some agreement on a general weakness aloft over the southern Plains by mid-late period so at least having some surface troughing as a weather-focusing feature seems like a reasonable starting point. Assuming less recurvature of energy/low pressure than depicted in the GFS, remaining consensus holds onto more high pressure near the East Coast toward the end of the period. The updated forecast started with a blend consisting of mostly the 18Z GFS/12Z ECMWF early in the period, with gradually increasing 18Z GEFS/12Z ECMWF mean input with time so that day 7 Sun reached 75 percent total ensemble mean weight. The forecast gradually phased out GFS input toward the latter part of the period. ...Weather Highlights/Hazards... Locations along and inland from the central-western Gulf Coast will need to monitor the potential for heavy rainfall in association with the feature currently forecast to be near southeast Louisiana as of early Wed. The spread in possible solutions after Wed leads to low confidence in specifics for location/intensity of rainfall during the rest of the period but considerable moisture will promote areas of heavy rain. Meanwhile the central Plains through the Midwest/Great Lakes and perhaps even locations farther to the east will be another area at risk for heavy rainfall, as a wavy front stalls over central latitudes of the central-eastern U.S.. Even if GFS specifics are not especially favored, there remains the potential that some Gulf moisture may eventually feed into the activity near the front. Farther to the southwest expect periods of convection over the southern Rockies and High Plains. The Pacific Northwest may see some precipitation with the shortwave approaching late in the week but with relatively light amounts at most locations. During mid-late week the front initially from the Great Lakes into central Plains will serve as a divide between very warm/hot and humid conditions over the East and cooler readings over parts of the Plains. Some locations from the Great Lakes/Ohio Valley into Mid-Atlantic/Northeast should see plus 10F or greater temperature anomalies. Generally expect min temps to be farther above normal with some record warm lows most likely on Wed-Thu. The northern-central Plains may see one or more days with highs 10F or more below normal. High pressure behind the front will bring a pronounced cooling trend into at least the northern half of the East by next weekend. Passage of shortwave energy across the Northwest will lead to a cooling trend over that region by Sat. Rausch 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