Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sun Oct 28 2018 Valid 12Z Wed Oct 31 2018 - 12Z Sun Nov 04 2018 ...Pattern Overview... The overwhelming majority of guidance maintains good agreement/continuity for evolution of the mean pattern but still displays meaningful disagreement for some important embedded details. An amplified mean trough will become established over the central U.S./southern Canada by mid-late week as a strong eastern Pacific ridge drifts toward the West Coast and another ridge builds over the western Atlantic. Incoming progressive northern Pacific flow will likely dampen the West Coast ridge somewhat after early Fri, which should lead to a broader mean trough over the lower 48 during next weekend. Prominent uncertainties within this large scale pattern involve the wave affecting the eastern half of the U.S. during the latter half of the week (plus any trailing frontal waves), a weak clipper dropping into the northern states/Midwest late week, and upstream energy/surface waves arriving from the Pacific. ...Guidance/Predictability Assessment... Starting with yesterday's (Oct 27) ECMWF run, there has been a pronounced slower shift in the guidance for the shortwave energy forecast to be near the southern Rockies as of early Wed and surface wave that should track northeastward from the southern Plains/lower Mississippi Valley during the latter half of the week. As of the 12Z Thu valid time, ensemble members show better clustering than 24 hours ago and add support for the slower trend. However only some CMC members and about 10 percent of ECMWF members argue for timing as slow as the 00Z CMC. Ensemble spread widens rapidly after 12Z Thu so confidence in any specific solution remains well below average. Overall preference incorporates ideas from non-00Z CMC guidance and a little continuity to reflect the slower trend but stay close to the middle of the spread given the diverse/inconsistent behavior of guidance thus far. There is also a fair amount of spread for the reaching the northern states/Midwest by Thu-Fri but the blend used for resolving the eastern system provides fairly good continuity into day 5 Fri. After that time the ensemble means and GFS runs indicate a slower trend corresponding to upstream agreement among this currently preferred cluster toward Pacific shortwave energy not being as amplified as the 00Z ECMWF/CMC. Regarding this incoming Pacific energy around Fri the 00Z ECMWF in particular is fairly extreme with its amplitude with some height contours falling near or just outside the full envelope of all 00Z GEFS/ECMWF/CMC ensemble members over parts of western Canada and northwestern U.S. Corresponding surface low pressure in the 00Z ECMWF also tracks farther south than most solutions. Then by day 7 Sun interaction with Canadian flow aloft results in a U.S./Canadian border system that is stronger/northward versus consensus and very different from the previous ECMWF run. Upstream Pacific energy is progressive with low amplitude, which lends itself to well below average predictability. Based on combined forecast considerations the updated forecast started with a consensus of 00-06Z GFS and 00Z ECMWF/UKMET on day 3 Wed but then reduced ECMWF weight while including a little continuity for days 4-5 Thu-Fri. By days 6-7 Fri-Sat the 06Z GEFS/00Z ECMWF means and continuity provided majority input. Some GFS details became a little questionable but overall evolution was close enough to the means to favor holding onto a modest component of their 00-06Z runs, and helped to provide a tad more detail for possible frontal waviness off the Mid-Atlantic/New England coast late in the period. ...Weather Highlights/Threats... The shortwave energy rounding the central U.S. mean trough aloft mid-late week and associated surface low pressure/frontal system will pull a significant amount of Gulf of Mexico moisture into the eastern half of the country. So far the guidance has been diverse and inconsistent for the precise strength/timing of the surface system, but there has been a more stable overall signal for heaviest rainfall to occur within an area from the mid-lower Mississippi Valley northeast into the Tennessee Valley/Ohio Valley and eastern Great Lakes. Across these regions expect potentially widespread rain and some thunderstorms, with multi-inch rainfall totals possible over the aforementioned regions. Some convection over southern areas may be on the strong side so consult SPC outlooks as specifics become more clear closer to the event. Some rain will likely extend to the East Coast but with lesser amounts than expected farther west. The Northeast may see somewhat greater rainfall than locations to the south, especially if trailing frontal waviness develops next weekend. The northern U.S./Midwest may see a period of rain/possibly northern snow late week with the clipper system tracking through the area. Meanwhile the Pacific Northwest and northern Rockies will see multiple periods of rain and high elevation snow as energy aloft flows around the northern periphery of the eastern Pacific ridge aloft. There is some uncertainty regarding precise amplitude of this energy and thus southward extent of precipitation. The developing eastern U.S. system will bring warm temperatures (especially for morning lows, with some plus 10-20F anomalies) into the East mid-late week while pulling below normal readings into the central U.S. Locations west of the Rockies will tend to see above normal temperatures. Broadening of the mean trough aloft by next Sun should lead to a moderating trend somewhat closer to normal over many areas at that time. 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