Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 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 period will start with very amplified flow, featuring a western ridge and sharpening Mississippi Valley trough. Within the southern part of the trough an upper low should close off and then track northward over the eastern U.S. into Canada--possibly opening up at some point due to interaction with northern stream energy. Associated strengthening low pressure should track from the lower Mississippi Valley through the upper Ohio Valley and eastern Great Lakes. Meanwhile a fairly strong system tracking into western Canada from just off the Pacific Northwest coast will usher in a transition toward less amplified mean flow over the Lower 48. With a fair degree of uncertainty in shortwave details the overall mean flow over the West should trend toward some degree of mean troughing from Sun onward in response to positive height anomalies seen over the east-central Pacific in most D+8 multi-day mean charts from the past couple days. ...Guidance Evaluation/Preferences... Through the 12Z/18Z cycles there was a solid model/ensemble mean clustering for most aspects of the eastern U.S. storm aside from the CMC beginning to stray on the fast side after early Fri, while the CMC mean still looks excessively fast in general. New 00Z solutions appear to add some finer-scale detail uncertainties from late Thu onward. On the opposite side of the country guidance is taking a long time to resolve details of the system initially just off the Pacific Northwest coast and quickly tracking into southern Canada, due to sensitivity in shortwave specifics aloft. UKMET runs have yet to be remotely close to consensus, with most being extremely fast/suppressed and the new 00Z run now on the slow side. Based on guidance comparisons early in the period, days 3-4 Thu-Fri placed primary emphasis on the 18Z GFS/12Z ECMWF with less weight on the CMC and the least on the UKMET. By mid-late period the most prominent forecast issue for sensible weather becomes the progression of Pacific shortwaves which will be difficult to resolve several days out in time due to being embedded within fairly fast and low amplitude flow. In the Sat-Mon period the 12Z ECMWF does not compare particularly well to other guidance for a leading shortwave that it has over the Plains by Sun, so prefer to downplay that aspect of its forecast with some input from the prior 00Z/16 run. For trailing shortwaves/frontal systems reaching the West Coast, the most common theme in the guidance is for one to arrive by day 6 Sun and then another to approach on day 7 Mon. Recent GFS runs have been either weak or slow with the latter while the FV3 GFS compares better to the majority cluster. This distribution of guidance favors emphasizing the GEFS/ECMWF means and ECMWF runs for this aspect of the forecast. ...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 late this week. 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. Some moisture will reach into the northern Rockies. After a brief break another couple systems should bring periods of precipitation to the northern half or more of the West Coast from the weekend into early next week with lighter activity streaming into the Interior West and 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 moisture over most of the eastern half of the Lower 48 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. Guidance is stable in depicting best potential for heaviest rainfall over the Florida Peninsula and North Carolina coast. 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. Rainfall from this system may be problematic for areas that experienced heavy rain in this past weekend's event. 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 15-25F anomalies, while at the same time 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 generally somewhat above normal. 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