Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1107 AM EST Mon Dec 17 2018 Valid 12Z Thu Dec 20 2018 - 12Z Mon Dec 24 2018 ...Periods of active weather expected over the Northwest... ...Widespread precipitation and wind with late week eastern system.... ...Overview... The forecast continues to start with an amplified flow with a digging trough over the Mississippi Valley, a strong upper ridge across the western U.S. and an incoming trough near the Pacific Northwest. The southern portion of the eastern trough is still expected to close off and will track northeastward over the Mid-Atlantic by the end of the week and traverse across the Northeast through the weekend. Meanwhile, the trough in the Pacific will cross over the Intermountain West on Thu/Fri, and will move over the northern tier states--which will cause a less amplified mean flow over the lower 48. By Monday, cyclonic flow can be expected over the northeastern U.S., with troughing just off the Pacific Northwest once again in addition to troughing across the Southwest. Overall, operational models agree with the synoptic pattern but differ within the detailing of disturbances from the Pacific. ...Guidance Evaluation/Preferences... Operational models were in fair agreement in the beginning of the medium range period--with the 00Z and 06Z GFS being more aggressive with the southern portion of the trough becoming a closed low. The 00Z ECMWF was quicker to having this low become an open wave by Friday. However, the 00Z CMC was more in line with the GFS--albeit faster with the progression of the closed low. The UKMet agreed with the timing of the trough, however remains an open low. Meanwhile, out west, all the operational models were close with the timing of the system moving across the Pacific Northwest and into the Intermountain West. Because there were not large discrepancies between the operational models, used a blend of the 06Z GFS/00Z ECMWF and to a lesser weight the 00Z CMC and UKMet. As previously stated, in regards to the later half of the medium range period, the overall pattern is established but the detailing is still uncertain. Specifically, the Pacific Northwest shortwaves are still difficult to resolve as the flow becomes more zonal/less amplified. Models are still showing short waves moving across the West Coast on Day 6/Sun and day 7/Mon. Within the operational models, the timing and evolution of these short waves have a large spread. The 06Z GEFS mean and 00Z ECMWF ensemble mean were used much more during the Day 5-7/Sat-Mon time frame since they were comparable but still used some of their respective operational models. ...Weather Highlights/Hazards... The system tracking from off the Pacific Northwest coast into Canada will bring a period of locally enhanced rain/higher elevation snow to northwestern areas on Thursday and into Friday. The Olympics/Vancouver Island and northern Cascades should see the highest totals with this system while still meaningful amounts extend as far south as northern California. Precipitation is expected to increase across the northern Rockies especially by Friday. Light precipitation may move through the Pacific Northwest region by the beginning of the weekend, but will increase significantly by Sunday and will continue into Monday from the Pacific Northwest to northern California in addition to the northern Rockies. Rain/snow will be of varying intensity with some locally enhanced areas possible depending on exact details of each system. Similar to earlier in the period, highest are most likely to be over the Pacific Northwest and northern California. The storm system tracking through the East will spread a broad area of precipitation over most of the eastern half of the U.S. during the latter half of the week. A majority of precipitation with this storm will be in the form of rain but chilly cyclonic flow in its wake will support areas of lake effect/terrain-enhanced snow during the weekend. The heaviest rainfall could potentially fall from the eastern Tennessee Valley/western North Carolina along with coastal North Carolina. Other areas of focus for significant totals will be over eastern New England in association with a period of Atlantic inflow, and locations northwest of the storm track. Because of saturated soil from the most recent multi-day rainfall event, additional rainfall from this system may be problematic for these areas. Some areas could see strong winds with this system as well. Expect very warm temperatures over and near the Plains late this week with some plus 20F or greater anomalies over northern areas especially for morning lows. The eastern U.S. system will also bring a brief surge of warmth into the Mid-Atlantic/New England with some plus 15-20F anomalies, while simultaneously pulling chilly air into the Southeast. From the weekend into early next week the trend toward less amplified mean flow aloft will promote highs within a few degrees of normal over most areas and morning lows may be slightly above normal. 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