Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Thu Jan 03 2019 Valid 12Z Sun Jan 06 2019 - 12Z Thu Jan 10 2019 ...Overview... Latest guidance still generally shows a large scale pattern featuring an eastern Pacific mean trough, central U.S. ridge, and East Coast/western Atlantic mean trough with a moderate degree of progression for embedded features. Individual models and ensemble means still diverge significantly on some important details though. Also teleconnections relative to best-defined height anomaly centers suggest some potential conflicts, or at least some transitions, in favored flow over some areas. These issues temper confidence in some forecast specifics. The most common signals continue to be for a period of active weather along the West Coast during the period and some effects from a Great Lakes/New England system during the first half of next week. ...Guidance Evaluation and Preferences... This cycle's forecast emphasized the 00Z ECMWF/FV3 GFS and to some extent 00Z UKMET during the first half of the period, while slowly adding 06Z GEFS/00Z ECMWF mean input with time. Then the rest of the forecast phased out the FV3 GFS while further increasing ensemble mean weight and holding onto minority inclusion of the 00Z ECMWF. The most prominent issues that arise in the first three days of the period involve the character of shortwave energy progressing eastward from the West Coast followed by flow evolution over the northwestern U.S./southwestern Canada. There is still significant model/ensemble spread regarding the distribution of energy within the overall West Coast shortwave. GFS/ECMWF runs have tended to be fairly phased and progressive but the 00Z CMC and a number of 00Z GEFS/ECMWF/CMC ensemble members still show potential for more separation between northern and southern streams. At the same time GFS runs and to a slightly lesser extent CMC depict shortwave energy reaching the northern West Coast and British Columbia by Mon--leading to a surface low track farther southeast than the majority of other guidance. This GFS/CMC cluster seems less probable since this location appears to lie within an area more favorable for mean ridging, between an upstream Pacific trough and the leading trough heading into the Plains. The combination of above issues lead to either a fast progression of the northeastern system by day 5 Tue (GFS) or having it wiped out (CMC). Latest GEFS means favor a solution much closer to other ensemble means and ECMWF runs. The 00Z FV3 GFS is much closer to this preference in principle as well. By days 6-7 Wed-Thu the 00Z FV3 GFS becomes faster than consensus with trough energy nearing the West Coast as of early Wed. Among other guidance there is a split between faster ECMWF/ECMWF mean and slower GFS/GEFS mean solutions while the CMC/CMC mean offer a compromise. GEFS/ECMWF means have been fairly consistent with their respective ideas and are within typical error range for days 6-7 forecasts so confidence is not especially high in favoring one solution too much over the other. However the fairly rapid approach of another Pacific trough (reaching near 145W by early next Thu in most guidance) would seem to favor at least hedging a little faster than the GFS-based guidance along the West Coast late in the period. As for height anomaly centers and teleconnections... D+8 multi-day means reflect the consensus pattern of some eastern Pacific troughing/negative height anomalies with downstream east-central U.S. positive anomalies. However strong and persistent positive height anomalies over the far northeastern Atlantic as well as developing negative anomalies over the south-southwest of the western Aleutians both point to a West Coast/BC Coast ridge and eastern trough. Thus there is the risk that some aspects of flow may differ from current forecast or at least experience some transitions with time. ...Weather Highlights/Hazards... The West Coast will continue to be a focus for significant weather during the period, with northern California and the Pacific Northwest still seeing the greatest potential for highest 5-day totals (in some cases at least 5-10 inches liquid) of rainfall and mountain snow. Some of this moisture will extend into the northern Rockies. Leading system crossing the West early in the period and then another toward next Wed-Thu should spread areas of rain/snow across the southern half of the West as well. Early in the period there is still some uncertainty over the degree to which moisture streaming northeastward from lower latitudes of the eastern Pacific will enhance precipitation over Arizona and the southern Rockies. Confidence in specifics of individual Pacific systems is lower than desired but each one may have the potential of producing a period of strong winds in addition to enhancing precipitation. The anticipated Plains through Great Lakes/New England system from early Mon onward will likely produce meaningful precipitation over a decent portion of the northeast quadrant of the Lower 48, with highest totals likely over the Great Lakes/Northeast. Greatest snowfall potential continues to extend from the Upper Great Lakes into the Northeast, with accumulations of potentially heavy snow over the latter region particularly sensitive to system timing which is far from being decided in the guidance. Expect min temperatures to be above normal over nearly the entire Lower 48 during the period. Highest departures from normal may reach plus 20-30F over the central/east-central part of the country during the first half of the period. For high temperatures, the active pattern over the West will tend to keep readings near to somewhat below normal over a majority of the region. Above normal highs should prevail over most areas from the Rockies eastward, though the Northeast may see some slightly below normal highs after passage of a front and then the Great Lakes/Northeast storm. 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