Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Tue Jan 01 2019 Valid 12Z Fri Jan 04 2019 - 12Z Tue Jan 08 2019 ...Overview... The majority of guidance generally shows a trough-ridge-trough pattern aloft from the multi-day mean perspective, with embedded features showing at least a moderate degree of progression. The flow will be sufficiently progressive to lower predictability of individual features and uncertainty over interaction among separate streams will add complexity to the forecast. This pattern should promote multiple episodes of significant precipitation over parts of the West Coast while a leading system tracking northeast from the Lower Mississippi Valley late this week into the weekend should produce another area of meaningful precipitation to portions of the East. ...Guidance Evaluation/Preferences... For the system tracking northeast from the Lower Mississippi Valley day 3 Fri onward, guidance has displayed a pronounced slower trend over the past couple days with ECMWF/ECMWF mean runs catching onto this trend a bit sooner than the GFS and especially GEFS mean. The slower trends favor leaning at least 2/3 toward the GFS/ECMWF whose latest runs as of 00Z/06Z are a bit slower than the 00Z UKMET/CMC. Farther westward, already by day 4 Sat there is significant guidance spread with a system that most guidance brings close to the West Coast. Rapid development from initially low amplitude/progressive energy suggests it may take well into the short range time frame to resolve. North-south spread among ensemble members is very wide and run-run model/ensemble spread for depth is considerable as well. At the moment the operational GFS runs are deepest while the 00Z CMC detaches a low from the westerlies leading to a completely different evolution offshore California and extreme northwest Mexico. An intermediate solution closest to the 00Z ECMWF appears to be the best starting point given the full range of guidance, but again with fairly low confidence. As the overall trough progresses eastward, there has been a lot of spread and run-run variability with timing and degree of phasing. The past couple ECMWF runs race ahead of latest GEFS/ECMWF mean runs. The 00Z GFS matches up with the means better until becoming a little faster with the next upper trough nearing the West Coast toward the end of the period next Tue. On the other hand the 06Z GFS brings a fairly strong trough into the Northwest on day 6 Mon, a scenario different from most other solutions. The CMC/CMC mean show greater separation and thus much slower southern stream timing, but GEFS mean trends toward the ECMWF mean (plus superior verification for some features within Pacific/western North American flow over recent weeks) suggest greater probability of the current GEFS/ECMWF means. Based on these considerations a general model blend on day 3 Fri removes the CMC thereafter, then the UKMET after day 4 Sat due to becoming slower than consensus with the West Coast trough, the ECMWF after day 5 Sat given its faster western trough progression. Then 00Z GFS weight decreased day 7 Tue. 06Z GEFS/00Z ECMWF mean weight increased with the gradual reduction of operational model input. ...Weather Highlights/Hazards... While specifics remain uncertain, the overall pattern should be favorable for significant rainfall and high elevation snow near the West Coast during the period. The best potential for highest 5-day totals will be centered over or near northern California with some locations possibly receiving at least 5-10 inches liquid during Fri-Tue. Favored locations over western Washington/Oregon and northern Rockies will likely see significant totals as well. In less extreme fashion some moisture should extend farther southward over California and the Interior West/Rockies. Depending on how the storm forecast to be near the West Coast on Sat evolves there could be areas with strong winds. Over the East, the system tracking northeastward from the Lower Mississippi Valley late this week into the weekend should produce some areas of locally moderate to heavy rainfall. The Mid-Mississippi Valley through Ohio Valley into Mid-Atlantic/Southern New England may be one favored area for enhanced rainfall, just north of the upper low track. Briefly intense rainfall could cross the Southeast ahead of the cold front trailing from the surface low. The majority of precipitation with this system should be in the form of rain. Any wintry precip types should be confined to the very northern/northwestern fringe of the moisture shield. A wavy frontal system emerging from the Plains may bring an episode of light to moderate precipitation to parts of the eastern half of the Lower 48 during the latter half of the period. Wintry precip types should again be confined to far northern latitudes. The expected pattern aloft will tend to favor below normal high temperatures over the Southwest/Four Corners region, with the southern Rockies and vicinity also on the cool side for mins late this week. Otherwise expect fairly mild temperatures for the time of year, especially for morning lows. Some locations over the extreme Northern Tier may see lows at least 20-30F above normal Fri-Sat. 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