Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Wed Jan 02 2019 Valid 12Z Sat Jan 05 2019 - 12Z Wed Jan 09 2019 ...Overview... Guidance thus far has been consistent with the establishment of mean troughing just off the West Coast, a ridge over the central U.S., and eventually an amplifying mean trough near the East Coast. Details for some individual features continue to have a fair degree of uncertainty but there is better confidence in the theme of active weather along parts of the West Coast and some inland locations over the West. As the upper support for a leading eastern Pacific system progresses across the Lower 48 it may produce a storm system affecting the eastern U.S. by the early-middle part of next week. Also in that time frame another eastern Pacific system should approach western North America but with a farther north track than its Sat-Sun predecessor. ...Guidance Evaluation/Preferences... During the first half of the period a compromise among 00Z deterministic model ideas provided a good starting point for the overall forecast. The strong storm forecast to be off the West Coast as of early day 3 Sat shows better clustering than 24 hours ago but there is still a moderate degree of spread for intensity and timing. Small scale aspects of system evolution will continue to be important and require monitoring into the short range time frame. At the very least trends from yesterday suggest the models are trying to converge within the deeper half of the prior envelope, but opposing deeper ECMWF/weaker GFS trends are disconcerting. Prefer to lean away from the 06Z GFS as it is weaker/southeast versus other guidance with this leading storm and then introduces a trailing extraneous shortwave/surface system reaching the West late Sun-Mon. Meanwhile guidance appears to have stabilized for the depiction of the system departing from the East Coast this weekend. As the leading eastern Pacific trough progresses through the West into the Plains, the past couple ECMWF runs are closer to the ensemble means and recent GFS runs from 00Z/12Z cycles. Individual ensemble members still display meaningful spread for the degree of phasing between northern and southern streams, but faster/more phased trends seen in the UKMET/CMC toward the means provide some confidence improvement versus 24 hours ago. Over the eastern half of the Lower 48 by Tue-Wed the contrasting past two ECMWF runs provide a simple illustration of detail uncertainties that remain for the overall trough, beyond what the upstream shortwave differences in the 06Z GFS produce. However for the purposes of a deterministic forecast the ensemble means provide the most consistent starting point, with the 00Z GFS/ECMWF sufficiently close in principle to provide about half weight for enhancing the means. This solution continues recent slower trends for the system emerging from the Plains but otherwise by Tue-Wed maintains the idea of a defined storm system tracking from the Great Lakes to just off the New England coast Tue-Wed. The 00Z CMC was removed from the blend after day 5 Mon due to being notably slower than consensus. There is better than average agreement relative to the forecast projection for the next eastern Pacific upper trough as of day 6 Tue. Low confidence embedded details will determine specifics of the associated surface low but a sizable majority of guidance shows a surface low track farther northwest than for the system arriving during the weekend. By day 7 Wed the specifics of progressive North Pacific flow will likely begin to have some influence on the character of the trough nearing the West Coast. ...Weather Highlights/Hazards... Continue to expect periods of active weather over the West, with a strong system off the central West Coast as of early Sat likely bringing one episode of heavy rain/mountain snow and possibly strong winds followed by at least one or two other events. The central West Coast region, including northern California and southwestern Oregon, has the greatest probability of seeing the highest 5-day totals with some locations receiving 5-10 inches liquid. Significant precipitation is also likely over the Pacific Northwest and northern Rockies. Especially with the first system expect moisture to extend farther south through California and the Southwest into parts of the Rockies. The front trailing from the weekend storm may interact with lower latitude eastern Pacific moisture/waviness and enhance rain/higher elevation snow over parts of southern Arizona and the southern Rockies. The precise axis of this moisture will take some additional time to resolve though. As energy aloft progresses across the Lower 48, expect some degree of low pressure development from the Plains into Northeast/western Atlantic Mon-Wed. This system may incorporate some of the aforementioned Pacific moisture to increase precipitation amounts over the East, with highest totals currently indicated over the Great Lakes/New England with meaningful activity possibly extending farther south. The upper Great Lakes and New England should see the highest snowfall potential with this system. The system departing from the East Coast this weekend should bring mostly rain to parts of the Mid-Atlantic/New England with any wintry precipitation types likely confined to the far northern part of the moisture shield. The large scale pattern will support near to somewhat below normal high temperatures over the Southwest for most of the period. Coolest anomalies should be on Sun with some locations 5-10F below normal. Otherwise expect temperatures to be above normal over a majority of the Lower 48, though the East Coast may trend down at least to normal by next Wed as upper troughing amplifies over the region. Warm anomalies to the east of the Rockies will be most pronounced for morning lows, with plus 20F or greater departures from normal likely on multiple days from the northern Plains through the Upper Mississippi Valley and from the southern Plains into Ohio Valley around Mon-Tue. 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