Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Dec 18 2018 Valid 12Z Fri Dec 21 2018 - 12Z Tue Dec 25 2018 ...Active weather over the Northwest/central West Coast... ...Widespread precipitation and wind with late week eastern system.... ...Overview... During Fri-Sat deep troughing aloft with a likely embedded upper low will progress rapidly northward over the eastern U.S., pushing along a strong surface system that will reach eastern Canada by Sat. Incoming Pacific shortwave energy heading into a west-central U.S. ridge as of early Fri will usher in a short period of low amplitude flow containing multiple features affecting the West Coast and then continuing rapidly eastward--a pattern that typically has low predictability for specifics. With typical uncertainty in the details, guidance is consistent and fairly agreeable that the pattern will evolve toward amplified western U.S. mean troughing by around Christmas Day next Tue. Multi-day means by D+8 continue to show an area of positive height anomalies around 140-150W longitude with teleconnections supporting an amplified western trough. This enhances confidence in the general theme of most guidance at that time. ...Guidance Evaluation/Preferences... For the late week eastern system recent trends in the Thu-Thu night time frame (now in the short range) have been toward farther south formation/track of the upper low, leading to a slightly farther south position of the surface low at the start of the extended period early Fri. However with upstream flow rapidly flattening the system's northward progress seems to be fairly close to continuity by early Sat. Uncertainty persists with smaller scale details due in part to interaction with northern stream energy and effects of flow farther westward. Guidance is still having difficulty in resolving details of shortwave energy reaching western North America by Fri. A compromise among GFS/ECMWF runs and their ensemble means has provided the best stability over recent cycles. After many runs differing wildly from consensus, the past couple of UKMET runs have adjusted closer to the most likely scenario. On the other hand the past two CMC runs have trended much slower than most other solutions. After Fri confidence in any piece of guidance further decreases as amplitude/timing diverge due in part to weak low-predictability shortwaves streaming into the West through Sat. Regarding this trailing shortwave energy, ECMWF runs have toned it down a bit from 24 hours ago but still depict a shortwave reaching the Plains Sun and East Coast Mon with somewhat more surface reflection/precipitation coverage than some other guidance. Other recent models hint at this feature aloft Sun-Mon but with a variety of ideas for timing/amplitude. Most guidance still shows two well-defined systems nearing the Pacific Northwest mid-late period, the first approaching on Sun (GFS runs potentially a bit on the fast side and CMC plus ensemble means perhaps a little north) and the next on Mon (GFS and ECMWF in better agreement for this one). Then the majority cluster has the supporting shortwave for the Mon system amplifying as it heads into the West. By late in the period the 12Z/18Z GEFS means become the most questionable guidance as they dampen the ridge expected off the West Coast, thus pushing the western mean trough farther eastward. GFS runs may begin to exhibit somewhat fast progression of an upstream North Pacific system but still show better western trough amplification than the GEFS mean. Interestingly the 12Z CMC mean actually amplifies its trough a little west of the ECMWF mean, after CMC means have tended to be chronically under-amplified versus consensus in multiple cases over the Pacific/North America domain in recent weeks. Based on forecast considerations this cycle, the first part of the forecast emphasized the 18Z GFS/12Z ECMWF with lesser input from the 12Z UKMET/ECMWF mean. The latter half of the forecast favored the 18Z GFS and 12Z ECMWF/ECMWF mean, with GEFS mean input kept low enough not to have an adverse impact on the preferred pattern over the West. ...Weather Highlights/Hazards... Low amplitude shortwave energy crossing the West during Fri-Sat may produce some areas of light/scattered precipitation. Then expect a period of more significant weather near the West Coast from Sun into early next week. There is still considerably uncertainty in the specifics but the current most likely evolution would have one system bringing areas of enhanced rain/higher elevation snow to the central/northern West Coast by late Sat-Sun with additional moisture accompanying another system that arrives by Mon. After Mon the precipitation should extend farther southeastward than with preceding systems as upper troughing amplifies southeastward. One or both of these systems may produce strong winds over some areas. The strong storm affecting the East late this week into the weekend will be accompanied by areas of heavy rainfall on Fri, especially along the northeastern coast where a period of strong Atlantic inflow may enhance amounts. A trailing band of enhanced rainfall may extend over or just west of the Appalachians. The rainfall from this system may be problematic over some areas due to the soil being saturated from the recent multi-day event. As the system tracks farther northward expect some lake effect/terrain-enhanced snow during the weekend. Some areas could see strong winds with this system as well. Otherwise the central/eastern states will likely see modest coverage of precipitation, and fairly light intensity of what occurs. Specifics will take additional time to resolve given the low predictability of supporting shortwaves aloft. At least some of any moisture across the northern third to half of the area east of the Rockies may fall in the form of snow. Early in the period the eastern storm will bring a brief warm surge into the Mid-Atlantic/Northeast with some readings at least 10-20F above normal, while bringing less extreme below normal anomalies into the Southeast/Florida. The central U.S. will tend to see above normal temperatures during the period but with somewhat less extreme anomalies after Fri-Sat. Western systems should promote near to slightly below normal highs over the Northwest, with some expansion of cooler air into the Southwest toward the end of the period. 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