Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Sun Jan 06 2019 Valid 12Z Wed Jan 09 2019 - 12Z Sun Jan 13 2019 ...Overview... Guidance is persistent in depicting a general trough-ridge-trough mean pattern aloft from the multi-day mean perspective, and containing multiple embedded progressive features. There will also be a tendency for some splitting of shortwave energy that digs into the eastern Pacific/West Coast mean trough, with some differences in exactly how the flow splits depending on the system. Latest D+8 means show an evolution toward stronger positive height anomalies centered near western Ontario. Corresponding teleconnections highlight the expected periods of active weather along the West Coast and rainfall potential over the southern Plains as well as potential for well above normal temperatures over the northern Plains. Signals for cooler temperatures over the East are not very prominent relative to the aforementioned feature but show up a little more when teleconnecting on northeastern Atlantic positive height anomalies and northern European negative height anomalies. ...Guidance Evaluation/Preferences... From the latest cycle of guidance, good model consensus exists into day 4 Thu. Over the past day or so solutions have been trending toward a more consolidated and deeper system tracking rather slowly from near the New England coast into the Canadian Maritimes. In some respects GFS runs have provided an extra day of lead time in depicting the current consensus evolution relative to the ECMWF. Meanwhile deep northward-tracking Pacific low pressure should remain 400-500 miles offshore the Pacific Northwest. No major trends are noted for this system. From late Thu or early day 5 Fri onward there is somewhat more spread along with trend in some cases. The 00Z UKMET becomes a pronounced weak/elongated extreme with the strong system forecast to come into the picture off the West Coast by Fri so it was removed from the blend at this time. The 00Z GFS also becomes somewhat weaker/elongated aloft versus the 06Z GFS and 00Z ECMWF/CMC with a surface low track to the south of those solutions and the ensemble means (but still within the full ensemble envelope). As the upper trough reaches the West Coast region by early Sat there is better than average consensus aided by faster/more open trends exhibited by the GFS starting with yesterday's 12Z run. The 06Z GEFS mean has also nudged closer to the model consensus versus the 00Z version. By day 7 Sun the 00Z CMC/FV3 GFS become questionably strong with remaining energy that passes through the long-term mean ridge position. In contrast to some recent cases from this pattern, GFS runs are a bit faster than other solutions to bring the next Pacific trough toward the West Coast by day 7 Sun. GEFS means thus far provide more support for the ECMWF/ECMWF mean/CMC cluster. Farther eastward continuity is good with southern stream shortwave energy crossing northern Mexico late in the week and supporting a western Gulf of Mexico surface wave by next Sat. On the other hand there is a pronounced trend in the latest guidance toward stronger high pressure over eastern North America by next weekend, ultimately suppressing and possibly dissipating what weak waviness emerges over the central U.S.. It may be reasonable to include a minority of continuity given the significant change from the prior cycle especially in the ECMWF. Typical difficulty of resolving shortwave details 6-7 days out in time lower confidence in specifics of exactly how one or more waves may evolve along a frontal boundary extending from the Gulf of Mexico into the western Atlantic just off the East Coast. Based on the above considerations, the forecast trends the early-period consensus increasingly toward the 06Z GEFS/00Z ECMWF means while holding onto a modest 00Z ECMWF/06Z GFS component (more of the former than the latter), incorporating 20 percent continuity, and removing the CMC after day 6 Sat. ...Weather Highlights/Hazards... Many locations along the West Coast should see multiple episodes of significant precipitation during the period. The first event extending into midweek should especially affect central/northern areas and taper off after Wed. Expect more moisture to arrive by Fri-Sat with meaningful rainfall/high elevation snow extending farther south across California than with the preceding event. Some of this latter system's moisture should then extend into the Great Basin/Southwest/Four Corners region. After only a very brief break, more potentially heavy rainfall/mountain snow will like reach California in particular around Sat night with some of this moisture eventually spreading farther inland. Currently expect northern to south-central California to see highest totals from this event. The system slowly tracking away from New England early in the period may produce heavy snow over northern New England through Wed and a period of lake effect/enhanced snow over parts of the Great Lakes/Appalachians. There should also be a couple days with fairly strong winds over portions of the eastern states. Southern stream energy will generate an area of locally heavy rainfall over/near the southern Plains late this week. Expect some of this moisture to progress eastward into next weekend. Northward extent of precipitation and what potential there may be for wintry precipitation types in the northern fringe of the moisture shield will depend on low confidence specifics of shortwave energy within the overall eastern U.S. trough aloft. Current guidance signals are for low coverage of any snow, perhaps with best potential over the Appalachians. At the same time an area of mostly light snow may cross parts of the Upper Midwest/Great Lakes. The northern-central High Plains and some areas extending to the east will be the focus for highest temperature anomalies during the period with highs and/or lows at least 10-20F above normal on multiple days. The active pattern over the West should continue to promote above normal morning lows but near to somewhat below normal highs over central/southern areas. Parts of the Pacific Northwest/northern Rockies may see somewhat above normal highs. The combination of initial upper troughing over the East and then trough re-amplification by next weekend will likely keep daytime temperatures near to somewhat below normal. The Northeast may see readings of at least 10-20F below normal with new snow cover followed by the approach of potentially strong high pressure. 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