Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1107 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 overall pattern is still expected to transition from a fairly zonal/low amplitude pattern through the weekend to a deepening trough by the end of the medium range period. Through the beginning of the forecast period, the large digging trough will exit into the Northeast/Atlantic Ocean while a short wave skirts across the northern tier states. Another short wave will move across the Pacific Northwest and into the Intermountain West. These features will move quickly eastward with more incoming short waves approaching the Pacific Northwest. By Monday, guidance still have not converged on the energy approaching the Pacific Northwest and continues to have significant spread upstream. The energy upstream will impact the trough across the western U.S. as it likely will eject into the Plains. ...Guidance Evaluation/Preferences... On Days 3 and 4 (Saturday/Sunday), operational models are overall in good agreement but the small scale details of the short waves. To account for the uncertainty, followed suit with the previous shift and took a blend of the 06Z GFS, 00Z ECMWF and 00Z UKMET. By Sunday, the short wave forecast to reach the Pacific Northwest still has spread with respect to the amplitude and timing. Once again, the 00Z ECMWF is still farther south with the surface low compared to other guidance. By Monday, the next system approaching the West Coast will strongly dig across the western U.S. and northern Mexico. The GFS and ECMWF have been consistent within themselves in terms of how this trough evolves but not with each other. The GFS continues to be much faster--and consequently does not dig the trough as far south as the ECMWF. It is also faster with upstream energy in the Pacific. The 00Z ECMWF ensemble mean, 06Z GEFS mean, and 00Z NAEFS lean toward the 00Z ECMWF with the trough being slower and stretching toward Mexico. Thus, the blend used increased the ensemble mean usage by Day 5/Monday (Christmas Eve) with some 00Z ECMWF and 06Z GFS and discounted the 06Z GFS on Day 7/Wednesday. It is important to note that other scenarios may still be plausible when viewed from a multi-day mean perspective. ...Weather Highlights/Hazards... The first round of heavy precipitation (rain in lower elevations/higher elevation snow) will start late Saturday night and into Sunday morning for the Pacific Northwest. Precipitation will also spread into the Intermountain West and northern California on Sunday. By Monday, as this activity begins to taper off for northern California through the northern Rockies, the next system will approach the Pacific Northwest once again. Rain and higher elevation snow will impact the Pacific Northwest, Intermountain West, and southward into southern California and the Southwest through the first half of the week. The 5-day totals for the period will likely reach at least several inches liquid at some locations over western Oregon and extreme northwest California. Additionally, 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. The Plains will see an increase in precipitation by midweek as the amplifying western trough approaches the region. For now, there is low confidence in the details 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 and eastern U.S. during the period--with some high temperatures for the far northern portions of New England reaching to 25 degrees above normal. The Southeast and Florida will be below normal for most of the weekend but will return to near normal by Monday. On Tuesday (Christmas), temperatures will be slightly above normal for the central and eastern U.S. with slightly below normal for the Northeast. The West will begin trending cooler/near normal through the first half of the week. Reinhart/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