Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Fri Jan 04 2019 Valid 12Z Mon Jan 07 2019 - 12Z Fri Jan 11 2019 ...Overview and Guidance Evaluation/Preferences... The latest collection of guidance continues to agree upon a fairly amplified but progressive series of upper waves progressing from the eastern Pacific across the CONUS next week. D+8 multi-day means are persistent in showing a strong core of positive height anomalies over the northeastern Atlantic while today's D+8 charts are more agreeable and better defined with a core of negative height anomalies centered a little south of the western Aleutians. Teleconnections relative to both suggest an increased tendency toward upper ridging from at least the northern half of the West Coast up through Alaska by late in the period and beyond, though individual progressive North Pacific systems could weaken it at times. Teleconnection-favored cyclonic flow over eastern North America may take until a little after day 7 Fri to become established. 00Z/06Z cycles of guidance have come in with significantly improved agreement for some aspects of the forecast compared to previous days. Now only 00Z CMC ensembles are holding back the southern part of the upper shortwave expected to be crossing the Plains as of day 3 Mon, while recent trends have essentially gone toward a compromise between GFS/ECMWF-based solutions for upstream energy over the Northwest U.S./Western Canada. The end result is some noticeable trending for the system affecting the northeast quadrant of the Lower 48 in the Mon-Tue time frame (farther north track of the MS Valley-Great Lakes wave, possibly merging with Canadian low pressure) but ultimately ending up with a low center closer to continuity once the system reaches near/offshore New England by Wed. Overall the ECMWF/ECMWF mean have been more consistent while the GEFS mean gravitated toward the current consensus earlier than GFS runs. Another amplifying upper trough will bring a strong storm into the picture off the West Coast during the first half of the week. There is decent model/ensemble agreement for the longitude of the system but much more latitude spread for one or more individual low centers. Such detail will be very sensitive to shortwave specifics aloft, which may not be easily resolved until the short range time frame. At the very least the 00Z GFS becomes noticeably farther south with its main surface low versus most other guidance from about early Tue onward. On the other hand the 00Z CMC/UKMET and latest GEFS/ECMWF means concentrate low pressure farther northward than the 06Z GFS/00Z ECMWF. Significant southward trend in the ensemble means over the last day (for forecasts valid 12Z Wed) and ECMWF continuity suggest leaning more toward the 06Z GFS/00Z ECMWF as a starting point. As the upper trough progresses across the western half of the Lower 48 during Thu-Fri, GFS/ECMWF runs and their means have come much closer together mostly by way of faster GFS/GEFS trends. The progressive nature of the pattern and specifically the rapid approach of the next upper trough off the West Coast--which shows better than average guidance agreement--by late in the week continue to support leaning away from the slowest side of the spread such as forecast by the 00Z CMC. The convergence among latest guidance allowed for primary emphasis on the 00Z ECMWF/06Z GFS and 00Z CMC for the first half of the period or so, in order from highest to lowest weight. By days 6-7 Thu-Fri CMC timing issues required phasing out its input while typical detail uncertainty led to raising total GEFS/ECMWF mean weight to 40-60 percent. ...Weather Highlights/Hazards... Across the West expect an episode of active weather to extend into Mon with heaviest rain/mountain snow likely to focus over northern California. Some moisture will extend eastward and bring areas of snow and low elevation rain across the Intermountain West into the northern-central Rockies. Moisture from the next Pacific storm should reach the West Coast by around Tue night-Wed, followed by rain/higher elevation snow progressing across the West and tapering off later in the week. Highest probability for heaviest precipitation again exists over northern California but significant totals are also possible along the remainder of the West Coast and over favored terrain inland through the Rockies. The deep low pressure offshore may bring a period of strong winds to coastal areas as well. Although some details have changed for the Midwest/Great Lakes system early in the week, an eventual track off the New England coast has kept the axis of highest snowfall potential fairly steady--from the far northern Great Lakes across northeastern New York state through New England. The Pacific origin of trailing air should keep temperatures from dropping too much below normal but will likely be cold enough to support a period of lake effect snow after the storm's departure. This system may bring some light-moderate rainfall farther southward. Southern Texas and vicinity may see some rainfall around midweek in association with southern stream moisture/shortwave energy aloft. Then expect precipitation to expand across the Plains/Mississippi Valley toward Thu-Fri as the system affecting the West through midweek emerges into the central U.S. Northern areas may see snow from this system while warm sector rainfall to the south could be locally heavy. The progression of Pacific-origin systems will keep temperatures mostly above normal over the Lower 48. The exceptions continue to be with highs over some areas from the West Coast into the Four Corners states through the period as well as over locations along the East Coast during the latter half of the week, in both cases generally below normal by only single digit anomalies. Expect warmest anomalies of plus 20F or greater for morning lows and generally plus 10-20F for highs over parts of the central/eastern U.S. during the first half of the week followed by another area of warmth reaching into the Plains late week. 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