Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Dec 20 2018 Valid 12Z Sun Dec 23 2018 - 12Z Thu Dec 27 2018 ...Active weather over the West into early next week followed by significant central U.S. storm potential Wed-Thu... ...Overview... Guidance is consistent for the large scale forecast, showing that progressive/low amplitude flow aloft Sun-Mon will rapidly transition toward the establishment of an amplified western U.S. mean trough from about Christmas Day onward. Within this agreeable evolution there are significant embedded uncertainties though, both for individual shortwaves embedded in the progressive pattern early in the period and specifics of trough energy that deepens over the West and then likely ejects into the Plains. While the details await resolution, there are fairly good signals from the guidance in principle toward parts of the West receiving significant precipitation early-mid period while ejecting western energy then supports what could be a strong storm system across the central U.S. ...Guidance Evaluation/Preferences... The overall forecast philosophy continues to favor a multi-run/multi-model approach early in the period (with minority ensemble input) to deal with meaningful differences that arise from day 3 Sun onward. By the latter half of the forecast the blend still excludes GFS runs due to their straying on the fast side of guidance with ejecting western energy/surface low pressure into the Plains, favoring components of recent ECMWF/CMC runs and slightly more 12Z ECMWF mean input relative to the 18Z GEFS mean. Within the progressive flow in place early in the period, models/ensembles are still having some difficulty with a leading central-eastern U.S. shortwave during Sun-Mon. Fortunately any associated precipitation with this feature should be fairly light and a general blend accounts well for the detail uncertainty of this feature. Then off the northern Pacific Northwest coast there is still significant spread for approaching low pressure as of early Sun. Track preference based on guidance through 18Z data was for an average among the 18Z GEFS/12Z ECMWF means as a good intermediate starting point, with ECMWF runs in the southern 1/3 of the ensemble spread and trending northward over recent runs while GFS runs have been well northward. The new 00Z GFS has adjusted close to this scenario. Consult the Model Diagnostic Discussion PMDHMD for updated preferences based on the full suite of 00Z guidance. As associated shortwave energy moves inland there is still tremendous timing/amplitude spread. Thus far ECMWF runs have been remarkably consistent in being stronger than most other solutions all the way from the West across the Lower 48. The next closest ideas come from the past couple UKMET runs and 00Z CMC that become better defined once the energy reaches the Plains. The ECMWF's continuity and hint of the feature in recent ECMWF means required some accounting for this feature, but in more subdued fashion given typically lower predictability of shortwaves embedded in fast flow. Once this energy reaches the eastern U.S. the ECMWF/CMC have been suggesting possible interaction with eastern Canada energy. Again prefer to tone down this evolution somewhat by way of the means as the developing western trough should start to promote rising East Coast heights. By day 4 Mon expect another shortwave to approach the West Coast, ultimately amplifying into the well-advertised deep western trough. As of Mon the 12Z/18Z GFS runs were the least favored as they brought shortwave energy and surface low pressure farther east than most other guidance including the GEFS mean. The new 00Z GFS may be somewhat closer to consensus aloft but still has a questionably fast leading surface wave. The specifics of how much the western trough amplifies and then the timing of ejection of energy into the Plains continues to be complicated by an upstream North Pacific feature whose energy may feed down toward the overall mean trough. As has been the case over the past day, latest GFS runs are on the faster side of the spread with the western trough and ejection of its energy into the Plains in response to differences over the North Pacific. Continue to favor leaning closer to the slower cluster of guidance including the ECMWF/ECMWF mean and CMC/CMC mean given the strongly amplifying nature of the overall pattern. However it is still worth noting that any of these timing options would still fit within the expected mean pattern from a multi-day mean perspective. ...Weather Highlights/Hazards... Expect the two systems affecting the Northwest during the first half of the period to bring heaviest rain/higher elevation snow to favored terrain of western Oregon and northwestern California. Several inches liquid remain possible over some locations within this region. Western Washington will likely see significant precipitation as well but with somewhat less confidence that totals will reach levels that may be seen immediately to the south. Some of this moisture will spread inland to bring meaningful precipitation into the northern/central Intermountain West and Rockies. As upper troughing amplifies the Sierra Nevada range should see a period of enhanced rain/snow, and somewhat lighter activity may extend as far south as southern California and the Southwest. By Wed-Thu there is decent potential for a significant storm system to emerge over and track through the Plains as upper trough energy emerges from the West. Locations over the northern half or third of the Plains would see snow with this system based on the current moist likely scenario. Strong winds may add to the hazardous conditions. Meanwhile warm sector rainfall should expand an intensify within an area from the central Plains through the central Gulf Coast region. Ahead of this storm, forecasting precipitation coverage/timing over the central-eastern states is still difficult due to lack of agreement for progressive shortwaves. It is likely that most precipitation that falls will be on the light side, with the rain/snow line tending to lie near 40N latitude. The low amplitude pattern aloft from late weekend into Christmas Eve will promote near to above normal temperatures over most of the Lower 48. Warmest readings versus normal should be over the northern half of the Plains with some plus 10-15F anomalies for morning lows in particular. Then cooler air will begin to filter into the West as upper troughing starts to amplify over the region while the Plains remain warm. By Wed-Thu below normal temperatures will spread across the West into the High Plains with highs at least 5-10F below normal. At the same time central/east-central areas will trend warmer with expanding coverage of plus 10-20F anomalies. 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