Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Dec 19 2018 Valid 12Z Sat Dec 22 2018 - 12Z Wed Dec 26 2018 ...Active weather over the West, possibly reaching the central U.S. next Wed... ...Wind and some lingering precipitation as strong system departs from the East.... ...Overview... The one aspect of the forecast that has reasonable confidence is the expected pattern transition from one of fairly low amplitude at the start of the weekend toward a deepening mean trough over the West. Guidance has been consistent in showing the development of mean ridging/positive height anomalies over the Pacific around 140W or so with teleconnections supporting the downstream trough. However beyond that the confidence in specifics is much lower. From the start of the period guidance differs for details of individual shortwaves progressing from the eastern Pacific across the Lower 48. By day 5 Mon even latest 00Z guidance becomes 180 degrees out of phase with some of these shortwaves. There is some semblance of consensus that a shortwave forecast to approach the West Coast by Mon will be the one that establishes the western U.S. mean trough. However there are differences in timing of this energy's amplification, and beyond that, significant spread in upstream North pacific energy that may influence how the early week western trough may ultimately eject into the Plains. ...Guidance Evaluation/Preferences... In order to emphasize operational guidance during the first half of the period while downplaying lower-confidence shortwave details, the forecast blend was mostly weighted toward the last two available GFS (12Z-18Z/18) and ECMWF (00Z-12Z/18) runs along with a little 12Z UKMET for day 3 Sat into day 5 Mon. After that time the forecast trended toward more 12Z NAEFS/ECMWF mean input while maintaining some operational ECMWF weight as the average of the past two runs aligned more closely to the means for the western trough versus the faster-ejecting GFS. Behind the rapidly departing deep East Coast upper trough, guidance valid this Sat generally shows one area of shortwave energy concentrated over southern Canada/northern Plains/Upper Mississippi Valley and more diffuse energy farther south followed closely by western U.S. energy reaching the central states by early Sun. There has been some degree of clustering toward this trailing shortwave nearing the East Coast by Mon. However the 18Z GEFS mean becomes curiously slow with this feature while the new 00Z CMC becomes slow/amplified and the 00Z UKMET trends slower as well. The next defined shortwave is forecast to reach near the West Coast by early Sun. There is still significant spread for shortwave amplitude and resulting track/depth of surface low pressure. ECMWF runs are fairly far southward with the surface low relative to the majority of ensemble members, favoring at least some degree of compromise with other guidance. Differences for this shortwave energy persist/widen as it continues eastward, complicating the sensible weather forecast. About 24-36 hours later the next system should be nearing the West Coast, with the upper trough then strongly digging over the western states and northern Mexico. Over the past 1-2 days the ECMWF mean has been the most consistent guidance source for the overall trough through day 7 Wed. The GEFS mean has trended slower over the past day toward the ECMWF mean. By the last couple days of the forecast one big question will be the strength/speed of upstream energy rounding the eastern Pacific ridge. Latest GFS/CMC runs are strong/fast enough with the upstream energy for it to cause a quicker ejection of the western trough into the Plains. 12Z/18Z FV3 GFS runs are slower than the operational GFS, plus the CMC mean (which has had a pronounced flat/progressive tendency in amplified regimes over recent weeks) continues to be even a little slower with the western trough than the ECMWF mean. These considerations lead to favoring the NAEFS/ECMWF means and lagged-average ECMWF approach. It is important to note that other scenarios may still be plausible when viewed from a multi-day mean perspective. ...Weather Highlights/Hazards... The two main systems likely to affect the West Coast should focus the heaviest rain and higher elevation snow from the Pacific Northwest into northwestern California, then eventually the Sierra Nevada range as upper troughing begins to amplify southeastward. 5-day totals for the period will likely reach at least several inches liquid at some locations over western Oregon and extreme northwest California. Moisture/shortwave energy spreading inland will bring periods of rain/higher elevation snow through at least the northern 2/3 of the Interior West/Rockies. Some precipitation may reach into southern California and the Southwest but with amounts very sensitive to uncertain specifics aloft. Depending on evolution of the systems, there may be one or more periods of strong winds especially along the West Coast. Farther east, the Plains may start to see more organized precipitation around the middle of next week in advance of the amplifying western trough. Before then leading shortwaves will likely produce some areas of precipitation but poor guidance agreement for details will keep confidence low for specific coverage/timing/amounts. Any moisture reaching the northern half or third of the area east of the Rockies may be in the form of snow. Over the East expect brisk winds and lake effect/terrain-enhanced snow to linger into the weekend behind the strong storm tracking into eastern Canada. The general pattern evolution from low amplitude flow aloft to a western mean trough will promote above normal temperatures over much of the central U.S. during the period, with highest anomalies likely to be for morning lows on Sat and Wed. Meanwhile the West will trend from having temperatures a few degrees F on either side of normal for highs (generally more above normal for mins) over the weekend toward below normal readings next week. The extreme Southeast/Florida will moderate after a chilly day on 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