Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sat Oct 27 2018 Valid 12Z Tue Oct 30 2018 - 12Z Sat Nov 03 2018 ...Pattern Overview... A moderately progressive flow pattern across the U.S. continuing from the short range period into the start of the extended forecast will become increasingly dominated by larger scale/more amplified and slower to evolve features from the middle of next week onward. Guidance shows good agreement and continuity in depicting the establishment of a broad and deep North American trough between strong ridges over the eastern Pacific/western Atlantic. The Pacific ridge may weaken/broaden somewhat toward Fri-Sat. The expected flow pattern will set the stage for transport of chilly air southward into the central U.S. under the upper trough while wet conditions develop on the eastern side of the trough, especially from the Lower Mississippi Valley/Ohio Valley to the Eastern Seaboard. ...Guidance/Predictability Assessment... Early in the period the guidance seems to be in the process of becoming more stable/agreeable for southern Canada low pressure and trailing front that should extend into and just west of the southern Plains, while a weakening wavy warm front enters the Pacific Northwest. An operational model blend represents this part of the forecast well. From late Wed onward there is still a fair degree of uncertainty regarding the exact details of western U.S. shortwave energy that flows through the base of the developing mean trough and then progresses northeastward--likely generating a frontal wave that tracks somewhere between the Mississippi Valley and Appalachians. As was the case 24 hours ago the primary operational model cluster would favor a faster wave than most ensemble means, though as of 12Z Thu the ECMWF mean has gravitated toward the operational idea. The 00Z FV3 GFS was slower like the recent GEFS means but the recently arriving 06Z version adjusted to a compromise. Ensemble low plots continue to depict wide spread with little discernible clustering. By day 5 Thu predictability seems to be sufficiently low to recommend an intermediate approach by way of incorporating enough WPC continuity to nudge the surface wave a bit west of the 00Z ECMWF/UKMET/CMC and 06Z GFS, essentially a compromise between those runs and the 00Z GFS. Ensemble spread has narrowed somewhat for Pacific shortwave energy forecast to head into/around the eastern Pacific/West Coast ridge around day 6 Fri. A majority of this narrowing has been by trimming away the southern side of the spread (i.e. yesterday's members showing the strongest shortwaves). This trend and occasional tendency for ridges to hold on a little longer than forecast provide support for removing the 00Z CMC from the forecast during the latter half of the period as it has one of the stronger depictions of this shortwave energy. Otherwise a model/ensemble mean/continuity approach looks reasonable. By day 7 Sat enough flattening does occur to support a modest eastward nudge from continuity for the downstream trough aloft, which pushes at least the northern half of the East Coast front a little farther eastard. Trailing energy within the core of the upper trough may reflect as a couple frontal systems anchored by weak lows. ...Weather Highlights/Threats... The amplifying trough across the central U.S. will favor transport of Gulf of Mexico moisture and instability northward into portions of the central/eastern U.S., along/ahead of a wavy cold front. The result will be potentially widespread rain and thunderstorms, with the potential for areas of heavy rainfall from the Lower/Mid-Mississippi Valley and Ohio Valley Tue-Wed to the Mid-Atlantic and Northeast Thu-Fri. There is still a fairly strong signal in the guidance for this overall event but still significant uncertainty in specifics of the surface wave and supporting dynamics, so it will take additional time to resolve details such as exact axis of heaviest rainfall. Meanwhile the Pacific Northwest and northern Rockies will also see multiple rounds of precipitation (rain and high elevation snow) of varying intensity as weakening shortwaves/frontal systems traverse the northern periphery of the Pacific ridge aloft. With lighter amounts some of this moisture will likely extend southeastward into the central Rockies and vicinity. One or more weak systems/fronts crossing the central/east-central states may produce light to locally moderate precipitation. Some of this activity may fall as snow over northern areas. A fairly broad area of above average temperatures across much of the central U.S. as of early Tue will decrease in coverage and move into the East by mid-late week as the upper trough amplifies and a surface cold front spreads colder air across areas from the High Plains through the Mississippi Valley and eventually close to the Appalachians. Expect high temperatures from 5 to 15 deg F below average from the southern half of the Rockies through the central U.S. by mid-late week. The Rockies/High Plains should begin to moderate slightly by next weekend and the approach of upper ridging will promote above normal readings over the West Coast states by Thu-Sat. Meanwhile ahead of the primary front heading into the East, some areas will see high temperatures 5 to 10 degrees above average with greater anomalies likely for morning lows. Rausch/Ryan 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